COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS

The ¹²CO (J = 1–0) observations for 20 galaxies (see Table 2) were carried out during the ALMA Cycle 0 period. All galaxies except for NGC 455 are located in the southern sky (Decl. < 0°). We used the compact configuration for the three nearby galaxies (Arp 230, AM 0956-282, and NGC 7727) and the extended configuration for the other 17 galaxies to achieve ≲1 kpc spatial resolution. The number of 12 m antennas was 16–24 depending on observation. The primary beam of the 12 m antenna is ∼50'' at 115 GHz. The correlator was configured to have four spectral windows, each with 1.875 GHz bandwidth and 488 kHz frequency resolution.

We imaged the calibrated visibility data using the Common Astronomy Software Applications package. The basic properties of the data are summarized in Table 2. The synthesized beam sizes range between 12 and 41 by adopting Briggs weighting of the visibilities (robust = 0.5). This weighting is recommended as it offers a good compromise between sensitivity and resolution. The achieved rms noise levels in the velocity resolution of 20 km s⁻¹ are 1.78–5.26 mJy beam⁻¹. In addition, we made the continuum maps using the line free channels. The uncertainty of flux calibration is 5%.

The ¹²CO (J = 2–1) observations for five galaxies (NGC 828, UGC 2238, UGC 9829, NGC 6052, and UGC 10675) were carried out using the SMA in 2011 June and October. The data were obtained using the compact configuration. The primary beam is ∼50'' at 230 GHz. The correlator was configured to have two sidebands, each with 2 GHz bandwidth and 3.25 MHz frequency resolution.

Data inspection was carried out using the IDL-based SMA calibration tool MIR and imaging was done using the MIRIAD package. The basic properties of the data are summarized in Table 2. The synthesized beam sizes range between 26 and 42 by adopting natural weighting of the visibilities, which gives maximum sensitivity. The achieved rms noise levels in the velocity resolution of 20 km s⁻¹ are 18.2–27.3 mJy beam⁻¹. The uncertainty of flux calibration is 20%.

The ¹²CO (J = 1–0) observations for two galaxies (UGC 6 and UGC 4079) were carried out using the CARMA in 2011 January and February. The data were obtained using the C configuration. The primary beam of the 10.4 m antenna is ∼60'' at 115 GHz. The correlator was composed of 10 bands, and each band consisted of 15 channels with 31.25 MHz frequency resolution.

Data inspection and imaging were carried out using the MIRIAD package. The synthesized beam sizes are 193 × 144 for UGC 6 and 174 × 153 for UGC 4079 by adopting natural weighting of the visibilities. The achieved rms noise levels in the velocity resolution of 100 km s⁻¹ are 3.72 mJy beam⁻¹ for UGC 6 and 2.05 mJy beam⁻¹ for UGC 4079. The uncertainty of flux calibration is 20%.

We further obtained published and archival interferometric CO maps for 10 galaxies, out of which 7 galaxies (see Table 3) were observed using the SMA (e.g., Wilson et al. 2008), 2 galaxies (NGC 2782 and NGC 4441) were observed using the PdBI (Hunt et al. 2008; Jütte et al. 2010), and 1 galaxy (NGC 3256) was observed using ALMA for science verification (SV). The basic properties of the data are summarized in Table 3. The observed CO transitions are J = 1–0 for three galaxies, J = 2–1 for five galaxies, and J = 3–2 for two galaxies. The synthesized beam sizes range between 07 and 76, which correspond to ≲1 kpc for each source. The achieved rms noise levels in the velocity resolution of 20 km s⁻¹ are 1.37–37.2 mJy beam⁻¹. The uncertainty of flux calibration is 5% for NGC 3256, which was observed using ALMA, and 20% for the rest.

The ¹²CO (J = 1–0) observations were carried out using the NRO 45 m telescope for 10 galaxies (see Table 5) during 2011 and 2012. The single-beam, two sideband, and dual-polarization receiver (T100; Nakajima et al. 2008) was used as the receiver front end. The analog signal from T100 is downconverted to 2–4 GHz and digitized to 3 bits before being transferred to the digital FX-type spectrometer SAM45. The 488 kHz resolution mode (2 GHz bandwidth) of SAM45 was used for the frequency resolution. Typical system temperatures were 150–200 K.

Data reduction was carried out using the AIPS-based NRO calibration tool NEWSTAR. After flagging scans with large ripples, the final spectra were made by fitting a first-order baseline. The antennae temperature ($T_{\rm A}^{*}$) was converted to the main beam temperature (T_mb) using a main beam efficiency of η_mb = 0.4 and $T_{\rm mb} = T_{\rm A}^{*}/\eta _{\rm mb}$. The final spectra are converted to Jansky using a Kelvin-to-Jansky conversion factor of 2.4 Jy K⁻¹. The primary beam size is ∼15'' at 115 GHz. The rms noise levels in the velocity resolution of 30 km s⁻¹ range between 1 mK and 13 mK.

In addition, we gather archival single-dish CO (1–0) data for 16 galaxies. The basic properties of the archival data are summarized in Table 5. Eight galaxies were observed using the SEST 15 m telescope (Mirabel et al. 1990; Casoli et al. 1992; Huchtmeier & Tammann 1992; Andreani et al. 1995; Wiklind et al. 1995; Horellou & Booth 1997). Five galaxies were observed using the NRAO 12 m telescope (Sanders et al. 1991; Maiolino et al. 1997; Evans et al. 2005). Two galaxies were obtained with the IRAM 30 m telescope (Zhu et al. 1999; Bertram et al. 2006), and one galaxy was obtained with the FCRAO 14 m telescope (Young et al. 1995).

The CO emission was newly detected in the interferometric maps of 20 out of 27 galaxies. The final number of our sample is 37 including 7 galaxies after combining with archival data. The integrated line intensities are summarized in Table 4. The integrated intensity maps and velocity fields of 30 galaxies are presented in Figure 1. They were made by smoothing and clipping the intensity using the AIPS task, MOMNT. The cleaned image cube was smoothed both spatially and in velocity and then this cube was clipped at the 1.5σ level per channel for 20 galaxies observed with ALMA (Cycle 0), 5σ level per channel for NGC 3256 because of high side-lobe levels that cannot be eliminated, and 2σ level per channel for the others. The molecular gas in 20 out of 30 galaxies detected in the CO line is distributed in the nuclear regions and the CO emission peaks roughly correspond to the peak in the K-band images (RJ04). In the other 10 galaxies, the CO distribution show multiple components such as arms, bars, and other structures which are not associated to the main body. Detailed description of the individual galaxies are provided in the Appendix.

Table 4. The CO and 3 mm Continuum Properties

ID	Name	Transition	Sν, codv	3 mm	Recovered Flux	log $M_{\rm H_{2}, INT}$
			(Jy km s−1)	(mJy)	(%)	(M☉)
(1)	(2)	(3)	(4)	(5)	(6)	(7)
1	UGC 6	CO (1–0)	15 ± 3	...	32 (56'', 2)	9.17
2	NGC 34	CO (2–1)	386 ± 77	...	122 (11'', 3)	9.40
3	Arp 230	CO (1–0)	25 ± 1	1.88 ± 0.09	93 (15'', 1)	8.23
4	NGC 455	CO (1–0)	<0.53	<0.16	...	<7.51
5	NGC 828	CO (2–1)	1001 ± 200	...	...	9.73
6	UGC 2238	CO (2–1)	492 ± 98	...	...	9.58
7	NGC 1210	CO (1–0)	<0.50	0.54 ± 0.03	...	<7.23
8	AM 0318-230	CO (1–0)	<0.84	<0.26	...	<8.33
9	NGC 1614	CO (2–1)	722 ± 144	...	59 (22'', 3)	9.48
10	Arp 187	CO (1–0)	63 ± 3	24.3 ± 1.2	116 (57'', 4)	10.34
11	AM 0612-373	CO (1–0)	24 ± 1	<0.27	...	9.71
12	UGC 4079	CO (1–0)	<1.12	...	...	<7.97
13	NGC 2623	CO (2–1)	256 ± 51	...	...	9.16
14	NGC 2782	CO (1–0)	124 ± 25	...	54 (45'', 5)	9.24
15	UGC 5101	CO (2–1)	246 ± 49	...	71 (23'', 6)	9.81
16	AM 0956-282	CO (1–0)	25 ± 1	2.27 ± 0.11	...	7.73
17	NGC 3256	CO (1–0)	1664 ± 83	27.5 ± 1.4	80 (44'', 7)	9.66
18	NGC 3597	CO (1–0)	141 ± 7	5.78 ± 0.29	76 (44'', 8)	9.58
19	AM 1158-333	CO (1–0)	3 ± 1	<0.32	...	7.71
20	NGC 4194	CO (2–1)	84 ± 17	...	23 (12'', 3)	8.77
21	NGC 4441	CO (1–0)	43 ± 9	...	68 (15'', 1)	8.83
22	UGC 8058	CO (3–2)	392 ± 78	...	97 (23'', 9)	9.72
23	AM 1255-430	CO (1–0)	33 ± 2	<0.11	...	9.78
24	AM 1300-233	CO (1–0)	96 ± 5	8.40 ± 0.42	43 (45'', 10)	9.17
25	NGC 5018	CO (1–0)	<1.31	0.72 ± 0.04	...	<7.36
26	Arp 193	CO (3–2)	910 ± 182	...	89 (22'', 9)	9.57
27	AM 1419-263	CO (1–0)	<4.38	1.93 ± 0.10	...	<8.64
28	UGC 9829	CO (2–1)	63 ± 13	...	...	9.70
29	NGC 6052	CO (2–1)	162 ± 32	...	58 (11'', 3)	9.60
30	UGC 10675	CO (2–1)	44 ± 9	...	...	8.93
31	AM 2038-382	CO (1–0)	33 ± 2	<0.24	53 (45'', 11)	9.43
32	AM 2055-425	CO (1–0)	51 ± 3	3.85 ± 0.19	43 (46'', 10)	9.50
33	NGC 7135	CO (1–0)	4 ± 1	7.03 ± 0.35	...	7.73
34	NGC 7252	CO (1–0)	87 ± 4	1.99 ± 0.10	55 (45'', 12)	9.63
35	AM 2246-490	CO (1–0)	39 ± 2	1.91 ± 0.10	46 (46'', 10)	9.38
36	NGC 7585	CO (1–0)	<1.19	<0.28	...	<7.50
37	NGC 7727	CO (1–0)	14 ± 1	0.58 ± 0.03	...	8.02

Notes. Column 1: ID number. Column 2: source name. Column 3: the CO transition. Column 4: the CO integrated intensity. Column 5: the peak flux of the 3 mm continuum emission. Column 6: the recovered flux derived by comparing the CO flux in the same circular region using the interferometric and single-dish measurements. The diameter of the circular region and reference of the single-dish measurements are noted in brackets. Column 7: the molecular gas mass estimated using the interferometric map. References. (1) This work; (2) Maiolino et al. 1997; (3) Albrecht et al. 2007; (4) Evans et al. 2005; (5)Young et al. 1995; (6) Papadopoulos et al. 2012b; (7) Casoli et al. 1992; (8) Wiklind et al. 1995; (9) Mao et al. 2010; (10) Mirabel et al. 1990; (11) Horellou & Booth 1997; (12) Andreani et al. 1995.

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Six galaxies (NGC 455, NGC 1210, AM 0318-230, UGC 4079, NGC 5018, and NGC 7585) were not detected in the CO (1–0) line. The observations of all these galaxies except for UGC 4079 have been conducted at ALMA. The 3σ upper limits of the molecular gas mass are 2 × 10^{7 − 8} M_☉, which are estimated from the integrated intensity maps and assuming a velocity width of 300 km s⁻¹. AM 1419-263 is tentatively detected in the CO (1–0) line, but there is no robust emission (>1.5σ). Thus we classify AM 1419-263 as a non-detection because the emission is too faint to discuss the distribution and kinematics of the molecular gas. The CO (1–0) emission is not seen in the interferometric map of UGC 4079 due to limited sensitivity, while the CO (1–0) spectra was clearly obtained using the single-dish measurements (Figure 3). The rms noise level in the interferometric maps is about 10 times larger than that in the single-dish spectra, assuming a beam filling factor of unity and a point source unresolved with the beam size. There are two similarities among these seven galaxies. One similarity is low FIR luminosities (L_FIR < 3 × 10¹⁰ L_☉). The FIR luminosities of five galaxies are upper limits because they are not detected in the IRAS maps. The other is that the Sérsic indices estimated by RJ04 are distributed around n = 4, which suggests a de Vaucouleurs profile. The K-band images (RJ04) of these galaxies are presented in Figure 2. These galaxies show relatively smooth and featureless stellar spheroidal structures.

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The 3 mm continuum emission was detected in 14 out of 21 galaxies observed using ALMA. While the CO emission was not detected in NGC 1210, NGC 5018, and AM 1419-263, the unresolved 3 mm continuum emission was clearly seen in the nuclear regions. The 3 mm continuum contour maps of 11 galaxies detected in both CO (1–0) line and 3 mm continuum emission are presented in Figure 1 (middle) using red color contours. The total flux of the 3 mm continuum is summarized in Table 4 and ranges from 0.54 mJy to 27.5 mJy. The total flux was estimated by integrating the extended emission if the 3 mm continuum emission was resolved. If not, the total flux corresponds to the peak flux of the 3 mm continuum. For seven galaxies undetected in the 3 mm continuum, we estimated 3σ upper limits, assuming an unresolved point source of the nucleus. The 3σ upper limits are below 0.4 mJy. The continuum emission was unresolved in nine galaxies and associated with the nuclei defined by the K-band images. In the rest, the continuum emission is resolved. We remark the 3 mm continuum emission detected on the east of AM 0956-282. The peak flux is 0.24 mJy (>3σ detection). The 3 mm continuum emission is very faint, but this is a robust detection. Detailed description of the individual galaxies are provided in the Appendix.

The new CO (1–0) spectra of 16 galaxies observed using the NRO 45 m telescope are presented in Figure 3 and the properties of the data are summarized in Table 5. We report the first detections of the CO line in seven galaxies. The integrated line intensity of Arp 230 is 4.1 ± 0.8 K km s⁻¹, which corresponds to 10% of the previous measurement using the NRAO 12 m telescope (Galletta et al. 1997), the difference of which is likely attributed to the large CO extent. The integrated line intensities of NGC 2623 and NGC 2782 are 37.4 ± 7.5 K km s⁻¹ and 23.5 ± 4.7 K km s⁻¹, respectively. These correspond to 53% and 25% of the previous measurements using the FCRAO 14 m telescope (Young et al. 1995). The integrated line intensities of UGC 5101, NGC 3656, NGC 4194, UGC 8058, and NGC 6052 are 17.4 ± 3.5 K km s⁻¹, 51.6 ± 10.3 K km s⁻¹, 47.7 ± 9.5 K km s⁻¹, 13.0 ± 2.6 K km s⁻¹, and 16.9 ± 3.4 K km s⁻¹, respectively. These five galaxies were observed using the IRAM 30 m telescope, and the integrated line intensities are 15.6 ± 3.1 K km s⁻¹ for UGC 5101 and 22.0 ± 4.4 K km s⁻¹ for UGC 8058 (Solomon et al. 1997), 23.4 K km s⁻¹ for NGC 3656 (Wiklind et al. 1995), 29.26 ± 0.26 K km s⁻¹ for NGC 4194 (Albrecht et al. 2004), and 16.4 K km s⁻¹ for NGC 6052 (Albrecht et al. 2007). Using the Kelvin-to-Jansky conversion factor of 4.95 for the IRAM telescope, the ratios of the intensities of the NRO to the IRAM single-dish measurements are 60% for UGC 5101, 118% for NGC 3656, 87% for NGC 4194, 32% for UGC 8058, and 50% for NGC 6052. The ratios are quite different, which may result from multiple factors such as pointing accuracy. The total flux of NGC 4441 is 65.5 ± 13.1 Jy km s⁻¹, which is 1.6 larger than that estimated from the previous observations using the Onsala Space Observatory (OSO) 20 m telescope (Jütte et al. 2010). The total flux of the OSO single-dish measurements is estimated to be ∼40 Jy km s⁻¹ from the molecular gas mass reported by Jütte et al. (2010). We also use literature data of single-dish CO (1–0) measurements for 16 galaxies to discuss the molecular gas mass in a later section and the CO properties of these sources are summarized in Table 5.

Table 5. Properties of Single-dish Observations and Archival Data

ID	Name	Telescope	Beam Size	rms	Tmb	ICO(1-0)	log $M_{\rm H_{2}, SD}$	Reference
			(arcsec)	(mK)	(mK)	(K km s−1)	(M☉)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
1	UGC 6	NRAO 12 m	56	...	...	1.6 ± 0.2	9.66	2
2	NGC 34	NRAO 12 m	56	...	...	3.9 ± 0.3	9.19	2
3	Arp 230	NRO 45 m	15	4.2 (20)	29.5 ± 10.6	4.1 ± 0.8	7.82	1
5	NGC 828	IRAM 30 m	22	...	...	71.1 ± 0.7	9.58	3
6	UGC 2238	NRAO 12 m	56	...	...	6.0 ± 1.2:	9.45	4
9	NGC 1614	FCRAO 14 m	46	...	...	5.8 ± 0.8	9.05	5
10	Arp 187	NRAO 12 m	57	...	...	1.8 ± 0.1	10.27	6
12	UGC 4079	NRO 45 m	15	1.2 (30)	19.7 ± 3.1	4.0 ± 0.8	8.90	1
13	NGC 2623	NRO 45 m	15	5.5 (30)	126.2 ± 13.7	37.4 ± 7.5	9.01	1
14	NGC 2782	NRO 45 m	15	7.6 (30)	121.1 ± 19.0	23.5 ± 4.7	8.91	1
15	UGC 5101	NRO 45 m	16	2.8 (30)	37.5 ± 7.1	17.4 ± 3.5	9.34	1
17	NGC 3256	SEST 15 m	44	...	...	71 ± 2	9.72	7
18	NGC 3597	SEST 15 m	44	...	...	6.9 ± 1.4	9.71	8
20	NGC 4194	NRO 45 m	15	7.7 (30)	262.6 ± 19.2	47.7 ± 9.5	9.21	1
21	NGC 4441	NRO 45 m	15	8.8 (30)	131.6 ± 21.9	22.0 ± 4.4	8.92	1
22	UGC 8058	NRO 45 m	16	3.1 (30)	60.6 ± 7.7	13.0 ± 2.6	9.27	1
24	AM 1300-233	SEST 15 m	43	...	...	8.2 ± 1.6:	9.53	9
25	NGC 5018	SEST 15 m	44	...	...	<1.4 ± 0.3:	<8.80	10
26	Arp 193	NRAO 12 m	56	...	...	7.0 ± 0.3	9.59	6
28	UGC 9829	NRO 45 m	15	1.3 (20)	46.1 ± 3.2	11.0 ± 2.2	9.63	1
29	NGC 6052	NRO 45 m	15	2.9 (10)	160.2 ± 7.1	16.9 ± 3.4	9.30	1
30	UGC 10675	IRAM 30 m	23	...	...	3.7 ± 0.2	8.99	11
31	AM 2038-382	SEST 15 m	43	...	...	2.3 ± 0.4	9.71	12
32	AM 2055-425	SEST 15 m	42	...	...	4.4 ± 0.9:	9.87	9
34	NGC 7252	SEST 15 m	43	...	...	5.8 ± 0.5	9.88	13
35	AM 2246-490	SEST 15 m	42	...	...	3.1 ± 0.6:	9.72	9
38	NGC 2655	NRO 45 m	15	6.3 (30)	53.7 ± 15.8	12.3 ± 2.5	8.11	1
39	Arp 156	NRO 45 m	15	2.9 (30)	23.5 ± 7.1	10.2 ± 2.0	9.80	1
40	NGC 3656	NRO 45 m	15	12.5 (30)	165.5 ± 31.2	51.6 ± 10.3	9.36	1
41	NGC 3921	NRO 45 m	15	1.0 (40)	25.7 ± 2.6	7.7 ± 1.5	9.30	1
42	UGC 11905	NRO 45 m	15	3.8 (30)	21.7 ± 9.5	10.1 ± 2.0	9.49	1
43	IC 5298	NRO 45 m	15	5.4 (30)	48.0 ± 13.5	11.3 ± 2.3	8.83	1

Notes. Column 1: ID number. Column 2: source name. Column 3: telescope. Column 4: the noise level in the velocity resolution shown in (). Column 5: the main beam temperature corrected for atmospheric attenuation. Column 6: the CO (1–0) line integrated intensity. Double colon means that the flux error is unknown. The error is assumed to be 20% in this paper. Column 8: the molecular gas mass estimated using the single-dish spectra. Column 7: reference. References. (1) This work; (2) Maiolino et al. 1997; (3) Bertram et al. 2006; (4) Sanders et al. 1991; (5) Young et al. 1995; (6) Evans et al. 2005; (7) Casoli et al. 1992; (8) Wiklind et al. 1995; (9) Mirabel et al. 1990; (10) Huchtmeier & Tammann 1992; (11) Zhu et al. 1999; (12) Horellou & Booth 1997; (13) Andreani et al. 1995.

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We estimate the recovered flux for 18 galaxies, comparing the flux of the same CO transition in the same region measured from the interferometric maps with the single-dish spectra. For six galaxies that were observed in the CO (2–1) and CO (3–2) lines with interferometers, we obtain the CO (2–1) and CO (3–2) single-dish data from literature. The integrated line intensity I_CO in K km s⁻¹ is converted into the integrated flux S_COΔv in Jy km s⁻¹ using Kelvin-to-Jansky conversion factors of 23.1 Jy K⁻¹ for the FCRAO 14 m telescope, 4.95 Jy K⁻¹ for the IRAM 30 m telescope, 30.4 Jy K⁻¹ for the NRAO 12 m telescope, 2.4 Jy K⁻¹ for the NRO 45 m telescope, and 27 Jy K⁻¹ for the SEST. The recovered flux is shown in Table 4.

The molecular gas mass ($M_{\rm H_{2}}$) is estimated from the CO integrated intensity using the following two equations (Carilli & Walter 2013; Casey et al. 2014):

$$\begin{eqnarray} \frac{L^{\prime }_{\rm CO}}{\rm K\, km\, s^{-1}} &=& 3.25 \times 10^{7} \left(\frac{S_{\rm CO}\Delta v}{\rm Jy\, km\, s^{-1}} \right) \nonumber \\ &&\times \left(\frac{\nu _{\rm obs}}{\rm GHz}\right)^{-2}\left(\frac{D_{\rm L}}{\rm Mpc}\right)^{2} (1+z)^{-3}, \end{eqnarray} \tag{ 1 }$$

where L_CO is the CO luminosity, S_COΔv is the CO integrated intensity, ν_obs is the observing frequency, D_L is the luminosity distance, and z is the redshift. Then the molecular gas mass is derived by

$$\begin{eqnarray} \frac{M_{\rm H_{2}}}{ M_{\odot }} &=& \alpha _{\rm CO} \frac{L_{\rm CO}}{ L_{\odot }}, \end{eqnarray} \tag{ 2 }$$

where α_CO is the CO luminosity-to-H₂ mass conversion factor in M_☉ pc⁻² (K km s⁻¹)⁻¹. The conversion factor has been a controversial issue for a long time. Solomon & Barrett (1991) find that the conversion factor is roughly constant in the Galaxy and nearby spiral galaxies and suggest that the conversion factor is X_CO = 2.2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹, corresponding to α_CO = 4.8 M_☉ pc⁻² (K km s⁻¹)⁻¹. However, the conversion factor in gas-rich galaxies at high redshift and local U/LIRGs is lower than the standard value in the Galaxy (e.g., α_CO ∼ 0.6–0.8; Downes & Solomon 1998; Papadopoulos et al. 2012a). It is suggested that the conversion factor in mergers, on average, are lower values than those in high-redshift disk galaxies which may have gravitationally unstable clumps (Narayanan et al. 2012). Indeed, the low α_CO values are found in mergers such as the Antennae galaxies (α_CO ∼ 0.2–1; Zhu et al. 2003) and Arp 299 (α_CO ∼ 0.2–0.6; Sliwa et al. 2012).

We estimate the molecular gas mass of a merger remnant with a FIR luminosity less than 10¹¹ L_☉ using the conversion factor of α_CO = 4.8 (Solomon & Barrett 1991), and we use α_CO = 0.8 (Downes & Solomon 1998) for the 14 U/LIRGs. In estimating the molecular gas mass from the CO (3–2) and the CO (2–1) luminosities, we convert the high-J CO luminosity into the CO (1–0) luminosity, assuming that the CO (3–2)/CO (1–0) and the CO (2–1)/CO (1–0) brightness temperature ratio is 0.5, which is similar to the average CO (3–2)/CO (1–0) ratio in local U/LIRGs estimated by Iono et al. (2009). We estimate the molecular gas mass using the single-dish CO data ($M_{\rm H_{2}, SD}$) and also using the interferometric CO maps ($M_{\rm H_{2}, INT}$), and summarize $M_{\rm H_{2}, SD}$ and $M_{\rm H_{2}, INT}$ in Table 4 and Table 5, respectively. The molecular gas mass in the sample of merger remnants ranges between 10⁷ M_☉ and 10¹¹ M_☉.

Although $M_{\rm H_{2}, SD}$ should in general be larger than $M_{\rm H_{2}, INT}$ as it retrieves extended flux that is not recovered by the interferometer, some sources in Tables 4 and 5 have $M_{\rm H_{2}, INT}$ that is larger than the single-dish flux. Several factors may contribute to this, such as the uncertainty in calibration, and possibilities that the single-dish beam did not cover all of the CO distribution for extended sources. The most likely explanation is the assumption of our CO line ratio where we use CO (3–2) or CO (2–1) for interferometric data, and CO (1–0) for single dish. We use a uniform line ratio for all sources, but there is an uncertainty in this assumed line ratio because the line ratios vary among the sources (e.g., Iono et al. 2009) due to the different physical conditions in the interstellar matter. We note, however, that the uncertainty in the relative mass between that obtained from interferometric and single-dish data does not affect the results presented in this paper. A full discussion of the molecular mass will be provided in a forthcoming paper (J. Ueda et al., in preparation; Paper II).

It is clear from Figure 1 that most of the merger remnants show disk-like rotation in their CO velocity fields. In order to quantify the kinematics and the geometry of the rotating molecular gas, we apply a fitting program (AIPS's task GAL) to the CO velocity fields of the 30 galaxies detected in the CO line. The galactic center is defined by the emission peak of the K-band image (RJ04), and we assume that the molecular gas rotates around the galactic center. We fit tilted concentric ring models to the velocity field using least-squares fitting routine for the kinematical parameters and a default convergence condition.

The estimated position angle, inclination, and systemic velocity are summarized in Table 6. The fitting routine converged for 24 out of the 30 galaxies. The fitting procedure failed for the remaining six galaxies (AM 1158-333, NGC 4194, AM 1255-430, UGC 9829, NGC 6052, and NGC 7135) due to the clumpy CO distribution or the complex velocity distribution that is not characterized by a simple gas disk model. The CO distribution is strongly distorted in these six galaxies, and the CO emission is not associated with the galactic center in NGC 6052 and NGC 7135.

Table 6. Properties of the Molecular Gas Disk

ID	Name	Position Angle	Inclination	V0	RCO	R80	$\frac{R_{80}}{R_{\rm eff}}$	Ring
		(deg)	(deg)	(km s−1)	(kpc)	(kpc)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
1	UGC 6	306 ± 1	21 ± 9	6473 ± 1	1.2 ± 0.2	0.5 ± 0.2	1.1 ± 0.4	Y
2	NGC 34	346 ± 1	26 ± 1	5701 ± 1	3.1 ± 0.3	1.7 ± 0.3	4.5 ± 0.9	Y
3	Arp 230	113 ± 1	65 ± 1	1694 ± 1	3.2 ± 0.1	1.8 ± 0.1	2.6 ± 0.2	Y
5	NGC 828	306 ± 1	60 ± 1	5229 ± 1	7.9 ± 0.3	3.5 ± 0.3	1.3 ± 0.1	Y
6	UGC 2238	138 ± 1	61 ± 1	6361 ± 1	8.5 ± 0.4	2.6 ± 0.4	1.2 ± 0.2	Y
9	NGC 1614	352 ± 1	36 ± 1	4744 ± 1	4.6 ± 0.3	1.4 ± 0.3	0.40 ± 0.08	Y
10	Arp 187	165 ± 2	53 ± 5	11441 ± 4	9.3 ± 0.3	3.5 ± 0.3	0.50 ± 0.05	N
11	AM 0612-373	123 ± 1	45 ± 1	9465 ± 1	5.0 ± 0.3	1.3 ± 0.3	0.19 ± 0.04	Y
13	NGC 2623	76 ± 1	18 ± 4	5532 ± 1	1.1 ± 0.1	0.5 ± 0.1	0.18 ± 0.04	Y
14	NGC 2782	260 ± 1	35 ± 1	2573 ± 1	2.3 ± 0.1a	1.1 ± 0.1a	0.18 ± 0.01	Y
15	UGC 5101	256 ± 1	25 ± 2	11799 ± 6	3.0 ± 0.2	1.0 ± 0.2	4.0 ± 0.8	Y
16	AM 0956-282	353 ± 2	34 ± 7	986 ± 1	1.6 ± 0.1	0.4 ± 0.1	0.18 ± 0.03	N
17	NGC 3256	84 ± 1	43 ± 1	2760 ± 1	6.6 ± 0.3a	3.2 ± 0.3a	1.7 ± 0.2	N
18	NGC 3597	264 ± 1	55 ± 1	3475 ± 1	2.3 ± 0.1	1.5 ± 0.1	1.2 ± 0.1	Y
19	AM 1158-333	...	...	...	1.0 ± 0.1b	0.8 ± 0.1b	0.48 ± 0.04	...
20	NGC 4194	...	...	...	2.2 ± 0.1b	1.3 ± 0.1b	1.3 ± 0.1	...
21	NGC 4441	30 ± 1	31 ± 3	2716 ± 1	2.2 ± 0.1a	0.9 ± 0.1a	0.24 ± 0.03	Y
22	UGC 8058	98 ± 1	17 ± 9	12641 ± 5	1.5 ± 0.2	0.8 ± 0.2	16 ± 3	N
23	AM 1255-430	...	...	...	3.7 ± 0.2b	2.0 ± 0.2b	0.66 ± 0.07	N
24	AM 1300-233	233 ± 1	23 ± 3	6298 ± 1	2.4 ± 0.2	1.1 ± 0.2	0.08 ± 0.01	Y
26	Arp 193	331 ± 1	26 ± 2	6993 ± 1	2.6 ± 0.2	1.2 ± 0.2	0.40 ± 0.08	Y
28	UGC 9829	...	...	...	5.1 ± 0.5b	4.2 ± 0.5b	0.49 ± 0.05	...
29	NGC 6052	...	...	...	3.8 ± 0.2b	2.0 ± 0.2b	0.49 ± 0.05	...
30	UGC 10675	175 ± 1	31 ± 4	9810 ± 1	2.6 ± 0.5	1.6 ± 0.5	3 ± 1	Y
31	AM 2038-382	330 ± 1	23 ± 5	5935 ± 1	2.2 ± 0.1	1.2 ± 0.1	1.2 ± 0.1	Y
32	AM 2055-425	41 ± 2	24 ± 10	12357 ± 7	3.6 ± 0.2	1.7 ± 0.2	0.57 ± 0.08	N
33	NGC 7135	...	...	...	0.4 ± 0.1b	0.3 ± 0.1b	0.01 ± 0.01	...
34	NGC 7252	118 ± 1	23 ± 3	4684 ± 1	2.6 ± 0.1	1.5 ± 0.1	0.60 ± 0.05	Y
35	AM 2246-490	156 ± 1	30 ± 3	12439 ± 8	2.8 ± 0.3	1.3 ± 0.3	0.15 ± 0.03	Y
37	NGC 7727	113 ± 1	62 ± 2	1815 ± 1	1.6 ± 0.1	0.8 ± 0.1	0.35 ± 0.05	N

Notes. Column 1: ID number. Column 2: source name. Columns 3–5: the position angle, inclination, and systemic velocity of the molecular gas disk. These parameters are estimated by fitting the CO velocity field. Column 6: the radius enclosing the maximum extent of the molecular gas disk and the CO distribution. Column 7: the radius enclosing 80% of the total CO flux. Column 8: the ratio of R₈₀ to the K-band effective radius (R_eff; RJ04). Column 9: "Y" means that a ring-like structure is seen in the PV diagram along the kinematical major axis. ^aThe radii are estimated after excluding the CO extensions which clearly exist outside the molecular gas disk. ^bThe radii are estimated without correcting for the geometry of the galaxy.

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In addition, we investigate the position–velocity (PV) diagrams along the kinematical major axes of 24 galaxies with molecular gas disks (Figure 4). The kinematical major axes are estimated by fitting the CO velocity fields. The PV diagrams reveal that the emission peaks are clearly shifted from the galactic centers in 18/24 galaxies (see Table 6), suggesting the presence of a ring and/or a bar. The PV diagrams of NGC 828, UGC 2238, and NGC 7252 show a double peak located at the transition point between rigid rotation and flat rotation, which may signify a spiral and/or a bar structure rather than a ring structure.

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In order to quantify the extent of the molecular gas disk, we estimate two types of radii (R_CO and R₈₀) using the integrated intensity maps. R_CO is the radius from the galactic center that encloses the maximum extent of the molecular gas disk, and R₈₀ is the radius from the galactic center, which contains 80% of the total CO flux. We chose this method because most of our attempts to fit an exponential disk were unsuccessful. In particular, the radial profiles of molecular gas deviate from an exponential profile in the outer regions. For the 24 galaxies with robust disk models in Section 5.1, we take into account the derived position angle and the inclination of the gas disk (Table 6). We define these 24 galaxies as "Type A." For six galaxies in which the CO velocity field cannot be modeled by circular motion, we define them as "Type B" and estimate R_CO and R₈₀ without correcting for the geometry of the galaxy. The R_CO and R₈₀ of 30 galaxies are summarized in Table 6. The histogram of R_CO is shown in Figure 5 (left). The dark and light gray bars show Type A and Type B, respectively. For three galaxies (NGC 2782, NGC 3256, and NGC 4441), we exclude the CO extensions which clearly exist outside the molecular gas disk (see Figure 1).

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The histogram of R₈₀ is shown in Figure 5 (right). The representation of the color is the same as in Figure 5 (left). In order to examine the effect of the detection limit on the estimated radii, we investigate the relation between 1σ mass sensitivity and R₈₀. The absence of an obvious systematic correlation allows us to argue that the detection limit does not strongly affect the observed size of the molecular gas. In addition, there is no significant effect of the different low J rotational transitions (J = 1–0, 2–1, 3–2) on the estimated radii, because the distributions of the CO (1–0), CO (2–1), and CO (3–2) are not different in local galaxies such as M51 (Vlahakis et al. 2013), although the distribution may vary in different population of galaxies with extreme CO excitation conditions. The average R_CO of Type A galaxies is 3.5 ± 2.3 kpc and the average R₈₀ of the molecular gas disks is 1.5 ± 0.8 kpc. Arp 187 has a molecular gas disk with the largest R₈₀ of 3.5 ± 0.3 kpc. Its maximum extent is also the largest in our sample. On the other hand, AM 0956-282 has a molecular gas disk with the smallest R₈₀ of 0.4 ± 0.1 kpc. The R_CO of AM 0956-282 is smaller than the average, but it is not the smallest in our sample. We estimate R_{80, MW} of the molecular gas disk in the Milky Way using the radially averaged surface brightness (Nakanishi & Sofue 2006). As a result, we find that R_{80, MW} is 8.5 ± 0.5 kpc, which is about five times larger than the average R₈₀ in our sample. There is no merger remnant in the present sample, which has a less concentrated molecular gas disk than the gas disk in the Milky Way.

We estimate the ratio of R₈₀ to the K-band effective radius (R_eff; Table 1) for our sample of merger remnants to investigate the relative size of the molecular gas disk to the stellar component. R_eff is the radius of the isophote containing half of the total K-band luminosity. The ratio between R₈₀ and R_eff (R_ratio, hereafter) is summarized in Table 6. The histogram of R_ratio of the 29 sources except for UGC 8058 is shown in Figure 6. The R_ratio of UGC 8058 is extremely large (R_ratio = 16). UGC 8058 is the most distant source (D_L ≃ 180 Mpc) in our sample, and the CO distribution is only marginally resolved and R₈₀ is likely overestimated due to the large beam, leading to an overestimation of R_ratio. Another possibility of the large ratio is that R_eff is underestimated due to an excess light in the galactic center.

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The R_ratio of Type A galaxies except for UGC 8058 range between 0.08 and 4.5. The source with the largest R_ratio is NGC 34 (R_ratio = 4.5 ± 0.9), followed by UGC 5101 (R_ratio = 4.0 ± 0.8) and UGC 10675 (R_ratio = 3 ± 1). These three sources have relatively high FIR luminosities (L_FIR > 10¹¹ L_☉), suggesting that the molecular gas may eventually become the stellar disk. However, the timescale for stellar disk formation is related to the complex interplay among gravitational compression, residual gas inflow, shear motion, stellar feedback, and the gas dynamics in the non-axisymmetric potential, and it may not be easy to determine. In order to confirm the formation of stellar disks, we will have to observe these sources using dense gas and ionized gas tracers at high angular resolution. In addition, there is no relation between the relative size of the molecular gas disk and FIR luminosity for the whole sample (Figure 7). While half of the U/LIRGs in our sample have compact disks that are consistent with previous studies (Downes & Solomon 1998), the rest of U/LIRGs have extended molecular gas disks. Therefore the formation of extended molecular gas disks is not related to the physical activity that increases the FIR luminosity, namely, the starburst/AGN.

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We find that the molecular gas disks in 54% (13/24) of the sources are more compact than the K-band effective radius. These gas disks may have formed by past gas inflow that was triggered by dynamical instability during the merging event (Barnes 2002) or dry mergers. Since the molecular gas is already concentrated in the nuclear regions as seen in present day early-type galaxies (e.g., Crocker et al. 2011; Alatalo et al. 2013; Davis et al. 2013), these sources are candidates which could become early-type galaxies in the future. On the other hand, the molecular gas disks in 46% (11/24) of the sources are larger than the K-band effective radius. The frequent occurrence of large gas disks suggest the possibility that merging galaxies can evolve into bulge-dominated late-type galaxies unless there are further mechanisms to transport the molecular gas toward the central region (e.g., nuclear bar; Bournaud & Combes 2002) and decrease the size of the molecular gas disk. The ubiquitous presence of both compact and extended gas disks is consistent with the numerical predictions that mergers of gas-rich galaxies can produce both early- and late-type galaxy population (Springel & Hernquist 2005).

Understanding the process of disk formation is particularly important at high-redshift when morphological segregation into different Hubble types begins. However, the exact formation mechanism of disk galaxies is not well understood. Two commonly adopted scenarios are major mergers of gas-rich progenitor disks (e.g., Robertson & Bullock 2008), or cosmological inflow of low-angular momentum cold gas that eventually settles onto the galactic disks (e.g., Kereš et al. 2005; Dekel et al. 2009). Recent high-sensitivity submillimeter observations and optical integral field spectroscopy reveal the presence of molecular and ionized gas disks in star-forming galaxies at high redshift (e.g., Förster Schreiber et al. 2006; Tacconi et al. 2013). Tacconi et al. (2013) classify 50%–75% of their sample of z = 1–3 galaxies as disk/spiral galaxies based on the CO kinematics and stellar morphology.

An important finding from our CO study of merger remnants is the high occurrence rate of molecular gas disks with various sizes, some of which are approaching the size of the Milky Way disk. While the surrounding environment and the properties of the progenitor disks may be different at low and high redshifts, this suggests the possibility that a good fraction of what are identified as disk galaxies (i.e., favoring the cold accretion model) at high redshifts through rotational kinematics may in fact contain a substantial subset of merger remnants. Deep- and high-resolution imaging of the rest-frame K-band emission is needed in order to understand the true nature of these sources. Finally, we note that the initial collision of our sample sources probably occurred a few billion years ago (∼typical merger timescale), and the observational evidence of high (>30%) gas mass fraction even at z = 0.3 (3 Gyr ago) (e.g., Daddi et al. 2010; Combes et al. 2013) is consistent with the theoretical prediction that disks with higher gas mass fraction are more likely to survive the collision (see discussion in Introduction, and in Hopkins et al. 2009).

Several loops and shells are identified in this galaxy in optical wavelengths. The Hubble Space Telescope (HST)/Wide Field and Planetary Camera 2 (WFPC2) V-band image clearly shows circumnuclear spiral structures (Martini et al. 2003). The Hα velocity field reveals a rotating disk whose rotational velocity is almost constant (V_rot = 210–220 km s⁻¹) over a radial distance range of 4''–12'' (Smirnova & Moiseev 2010). Optical spectroscopy classifies the galaxy as a Seyfert 1.8 (e.g., Dahari & De Robertis 1988; Osterbrock & Martel 1993). The H i spectra show a double-horn profile (Mirabel & Sanders 1988), which is characteristic of a rotating disk.

The CO (1–0) emitting gas is distributed within a radius of 3'' (≃1.3 kpc) from the galactic center. The detailed distribution and kinematics is not clear due to the limited velocity resolution (ΔV = 100 km s⁻¹), but the velocity field reveals the presence of a rotating disk. The maximum rotational velocity is estimated to be ∼280 km s⁻¹. This molecular gas disk is about four times smaller than the ionized gas disk studied by Smirnova & Moiseev (2010).

This galaxy is classified as an LIRG. The galaxy has several ripple-like patterns and two linear tidal tails extending toward the northeast and the southwest seen in the optical. The projected length of the longer tail is over 20 kpc (Schweizer & Seitzer 2007). The HST/Advanced Camera for Surveys (ACS) B-band image clearly shows a bluish central disk with spirals and its optical light is dominated by a ∼400 Myr old post-starburst population (Schweizer & Seitzer 2007). The nature of the activity in this galaxy based on the optical spectra is controversial. Some authors classify the galaxy as a Seyfert 2 (e.g., Veilleux et al. 1995; Yuan et al. 2010; Brightman & Nandra 2011), while others suggest that the galaxy is classified as a H ii galaxy rather than a Seyfert galaxy (e.g., Mulchaey et al. 1996; Gonçalves et al. 1999). Fernández et al. (2010) find two diffuse radio lobes, spanning 390 kpc, which could be a signature of an AGN or a starburst-driven superwind. Fernández et al. (2010) also suggest that an atomic gas disk is forming from the gas of the northern tail. Xu et al. (2014) present ALMA Cycle 0 observations of the CO (6–5) line and find a compact gas disk with a size of 200 pc.

This galaxy has a molecular gas disk extended in comparison with the stellar structure (R₈₀/R_eff = 4.54), but the absolute size of the molecular gas disk (R_CO ≃ 3 kpc) is approximately equal to the average size in our sample. The PV diagram shows nearly symmetric isophotes with respect to the systemic velocity and two emission peaks on opposite sides of the nucleus, indicating the presence of a ring or a disk with non-uniform distribution.

This galaxy is well known as a shell galaxy and listed as a candidate polar ring galaxy (Whitmore et al. 1990). The HST/WFPC2 V-band image shows strong dust lanes along the polar ring. Stellar shells are clearly visible in the northeast and southeast. McGaugh & Bothun (1990) estimate the timescale for shell formation based on the optical color of the shells. The color implies an age of 1–2 Gyr for the outermost shell, an age of 0.5–0.7 Gyr for the intermediate shell, and a young age (⩽0.3 Gyr) for the inner shells. The majority of the 6 cm radio continuum and H i emission is aligned with the optical dust lane (Cox & Sparke 2004; Schiminovich et al. 2013). From the H i velocity field, Schiminovich et al. (2013) find the presence of an outer H i disk and a dense inner H i disk/ring.

The CO (1–0) emitting gas is seen along the optical dust lane and distributed in three main components. One component is associated with the nucleus and the others are located at the tangential points of the polar ring. The velocity field shows a shallow velocity gradient of ∼75 km s⁻¹ kpc⁻¹, which has been corrected using the estimated inclination of 65°. These signatures suggest the presence of a slow-rotating ring/disk of the molecular gas, which appears to coincide with the inner H i disk/ring (Schiminovich et al. 2013). In addition, we find three 3 mm continuum components, whose peaks roughly correspond to the CO emission peak of the three main components.

There are few studies focusing on this galaxy. Optical imaging reveals two diffuse tails extending toward the northwest and the southeast. The galaxy is undetected in the IRAS Sky Survey Atlas, but detected in the AKARI All-Sky Survey. We estimate a FIR luminosity of 3.0 × 10⁹ L_☉ using AKARI data at 65 μm, 90 μm, and 140 μm (Yamamura et al. 2010) and the formula defined by Takeuchi et al. (2010).

This galaxy is undetected in the CO (1–0) line. The 3σ upper limit on the molecular gas mass is 3.3 × 10⁷ M_☉.

This galaxy is classified as an LIRG. The NIR image shows several filaments surrounding the main body and spiral features within 10'' from the galactic center (Smith et al. 1996). The small 3.3 μm polycyclic aromatic hydrocarbon (PAH) equivalent width (EW ⩽ 20 nm) suggests that a powerful buried AGN is present in the galaxy (Imanishi 2006). In the Hα emission line, there are three sources with comparable surface brightness (Hattori et al. 2004). One diffuse source is associated with the nucleus and the others are located nearly symmetrically along the major axis of the galaxy. The rotation curve measured from the Hα emission shows flat rotation with a constant velocity (V_rot sin i) of 200 km s⁻¹ outside ≳8'' from the galactic center (Márquez et al. 2002). Multiple CO line spectra show double-horn profiles, suggesting the presence of a rotating disk (Sanders et al. 1991; Casoli et al. 1992; Narayanan et al. 2005). Previous CO (1–0) interferometric observations provide a rotation curve of the molecular gas disk (Wang et al. 1991), but the observed disk size was limited to a 3.5 kpc radius.

Our new CO (2–1) observations confirm the presence of a large molecular gas disk with a radius of 8 kpc. The rotation curve of the molecular gas disk shows flat rotation with a velocity of ∼270 km s⁻¹ outside a radius of 8''. This rotational velocity is larger than that measured form the Hα emission (Márquez et al. 2002) by 70 km s⁻¹. The PV diagram shows a symmetric distribution and a double peak located at the transition point between rigid rotation and flat rotation, which may signify a spiral and/or a bar structure rather than a ring structure.

This galaxy is classified as an LIRG. The Hα emission is distributed across the galaxy in several knots (Hattori et al. 2004). According to optical spectroscopy, Veilleux et al. (1995) classify the galaxy as a Low Ionization Nuclear Emission Region galaxy (LINER), while Yuan et al. (2010) suggest that this is a starburst–AGN composite. However, there is no sign of an AGN or an obscured AGN in the NIR spectra (Imanishi et al. 2010). The radio continuum emission is elongated in the northeast and southwest, corresponding to the optical morphology (Condon et al. 1990).

We find a large molecular gas disk in this galaxy for the first time with a radius of 8.5 kpc. The CO (2–1) emitting gas is elongated in the direction of the morphological major axis of the stellar component. The rotation curve of the molecular gas disk shows flat rotation with a velocity of ∼200 km s⁻¹ at ≳6'' from the galactic center. The PV diagram reveals nearly symmetric isophotes and a double peak located at the transition point between rigid rotation and flat rotation.

This galaxy presents shells in optical images (Malin & Carter 1983). The K-band surface brightness is approximately a de Vaucouleurs profile with a the Sérsic index of 4.08 (RJ04). The UV imaging reveals a tidal tail extending toward the north and a diffuse debris structure on the south region (Marino et al. 2009). Furthermore, several UV knots with a luminosity similar to that of the nucleus are seen along the tidal tail. The H i emission observed by Schiminovich et al. (2001) corresponds to the tidal tail visible in the UV images. The PV diagram measured from the [O ii] line at 4959 Å shows rigid rotation, suggesting a rotating ionized gas component (Longhetti et al. 1998).

This galaxy is undetected in the CO (1–0) line. The 3σ upper limit on the molecular gas mass is 1.7 × 10⁷ M_☉.

There are few studies focusing on this galaxy. The K-band isophotal shape clearly shows a featureless rectangular core, even though the overall shape is dominated by disky isophotes (Chitre & Jog 2002, RJ04).

This galaxy is undetected in the CO (1–0) line. The 3σ upper limit on the molecular gas mass is 2.2 × 10⁸ M_☉.

This galaxy is classified as an LIRG. Optical imaging shows a bright center with two spiral arms at scales of few kiloparsecs, a liner tail extending over 20 kpc toward the southwest (Mullan et al. 2011), and a large curved extension to the southeast of the nucleus. The H i map shows a single tail (Hibbard & Yun 1996), which does not follow the optical tail. Several knots and condensations traced by the Hα and the Pα emission are distributed along the eastern spiral arm (Alonso-Herrero et al. 2001; Rodríguez-Zaurín et al. 2011). A star-forming ring with a radius of ∼300 pc is observed at the Pα emission (Alonso-Herrero et al. 2001) and radio (e.g., Neff et al. 1990; Olsson et al. 2010). Haan et al. (2011) suggest that the galaxy built up a central stellar cusp due to nuclear starbursts, judging from the significant core light excess in the H-band image and relatively large 6.2 μm PAH EW. Soifer et al. (2001) find that 87% of the 12 μm flux come from within a radius of 4'' (1.2 kpc) from the nucleus. This galaxy is classified as a starburst-dominated galaxy with no AGN signature through optical and infrared spectroscopy (e.g., Veilleux et al. 1995; Imanishi et al. 2010). Small HCN/HCO⁺ line ratios are further evidence for dominance of starburst activity (Costagliola et al. 2011; Imanishi & Nakanishi 2013). High-resolution CO (2–1) observations at 05 resolution reveals a nuclear ring of molecular gas (König et al. 2013) fed by dust lanes.

We use the CO (2–1) data obtained by Wilson et al. (2008). The PV diagram shows rigid rotation and a double peak located at a distance of 300–400 pc from the galactic center, which is consistent with the CO (1–0) data obtained at the Owens Valley Radio Observatory (OVRO; Olsson et al. 2010). This CO (2–1) data is unable to spatially resolve the nuclear molecular ring presented by König et al. (2013).

This is a well-known radio galaxy. Many radio continuum surveys including Arp 187 have been conducted, but there are few studies focusing on this galaxy using all available wavelengths. Optical imaging shows two diffuse tails running in the north and south directions. Evans et al. (2005) suggest that the star formation rate (SFR) is low because the galaxy has a low L_IR/L_CO, which is similar to the global L_IR/L_CO of local spiral galaxies, and the dust temperature is lower (30–45 K) than the typical value of radio galaxies. Assuming that the IR luminosity is entirely due to star formation, the SFR derived from the FIR luminosity is a few solar masses per year.

The CO (1–0) emitting gas appears to form a disk overall, but the distribution is disturbed and warped in the nuclear region. We detect two 3 mm continuum components located at both sides of the nucleus, spanning 4 kpc. The direction connecting these two components is different from the kinematical major axis of the molecular gas. These components can be small radio lobes, if an AGN exists. In order to check for the presence of an AGN, we estimate the q value of the radio–FIR correlation (Helou et al. 1985) using the 1.4 GHz radio continuum data (Condon et al. 1998). Arp 187 shows a radio-excess (q = −0.1), suggesting an AGN activity. We also investigate the radio spectral index at a few GHz using the 5 GHz and 8.4 GHz radio data from the literature (Wright & Otrupcek 1992). We find a spectral index of α = −1.0, which also indicates the presence of an AGN.

There are few studies focusing on this galaxy. Optical imaging shows a long tail extending over 30 kpc toward the south and a dust lane running along the morphological minor axis of the stellar body (Smith & Hintzen 1991). The rotation curve measured from the Ca ii triplet absorption line (λ ∼ 0.85 μm) reveals a velocity gradient (V_rot sin i = ± 160 km s⁻¹; Rothberg & Fischer 2010), which cannot be fully explained by either rigid or flat rotation.

Most of the CO (1–0) emitting gas is distributed within a radius of 2.5 kpc from the galactic center. We note that the kinematical major axis of the CO distribution is perpendicular to the apparent morphological major axis of the stellar body. The PV diagram shows rigid rotation and a multi-peak. The maximum rotational velocity is estimated to be ∼380 km s⁻¹.

There are few studies focusing on this galaxy. The HST/WFPC2 B-band imaging shows clumpy structures in the main body and a faint tail with several blobs, extending toward the northeast. The H i spectra show a double-horn profile with a FWHM of 234 km s⁻¹ (Springob et al. 2005).

The new CO (1–0) spectra obtained using the NRO 45 m telescope show a double-horn profile with a FWHM of 240 km s⁻¹, suggesting the presence of a rotating gas disk. However, the galaxy is undetected in the interferometric maps due to the limited sensitivity. Thus the rotating molecular gas disk is not confirmed.

This galaxy is classified as an LIRG. The optical image reveals two prominent tails extending over 120'' (∼43 kpc; 1'' = 0.36 kpc) toward the northeast and the southwest (Soifer et al. 2001). High-resolution HST/ACS images reveal ∼100 star clusters in the southern region of the nucleus. Optical colors of the clusters are consistent with ages of ∼1–100 Myr (Evans et al. 2008). Soifer et al. (2001) find that 80% of the 12 μm flux comes from within a radius of 2'' (∼0.7 kpc) from the galactic center. The rotation curves measured form the Hα and the [N ii] emission show flat rotation with a constant velocity (V_rot sin i) of ∼100 ± 50 km s⁻¹ (Smith et al. 1996). While Risaliti et al. (2000) find no X-ray evidence for the presence of an AGN, later work shows the signature of a Compton-thick AGN based on the X-ray data (Maiolino et al. 2003). A high HCN/HCO⁺ line ratio is further evidence for an AGN (HCN/HCO⁺(1–0) = 1.5; Imanishi et al. 2009). The H i spectra show both emission and absorption, and two distinct absorption features are observed at velocities below and above the systemic velocity of the galaxy (e.g., Biermann et al. 1978; Mirabel & Sanders 1988). The Very Large Array (VLA) H i map shows two tidal tail, which follow the optical tails (Hibbard & Yun 1996).

We use the CO (2–1) data obtained by Wilson et al. (2008). The CO (2–1) emitting gas is distributed within a radius of 3'' (≃1.1 kpc) from the galactic center. This R_CO is the smallest of our sample sources with molecular gas disks. The CO (2–1) maps are similar to the CO (1–0) map obtained at the OVRO by Bryant & Scoville (1999). The PV diagram shows rigid rotation and the maximum rotational velocity is estimated to be V_rot sin i = 180 km s⁻¹.

This galaxy has a long H i tail (∼5') extending toward the northwest and a short H i tail (∼2') extending toward the east, which is associated with a stellar tail (Smith 1991). The H i velocity field shows the presence of an inner disk counter-rotating with respect to the gas motions in the outer region (Smith 1991). Smith (1994) conclude using a three-body dynamical model that the galaxy is a merger of two disk galaxies with a mass ratio of ∼4:1. The HST/WFPC2 images reveals a nuclear stellar bar within a radius of 75, which appears to be fueling gas into the nuclear starburst (Jogee et al. 1999). Mullan et al. (2011) identify 87 star cluster candidates in the eastern tail and 10 candidates in the northwestern tail using the HSTWFPC2 V- and I-band images. Previous studies considered its nuclear activity to be dominated by a nuclear starburst (e.g., Kinney et al. 1984; Boer et al. 1992), although recent radio and X-ray observations suggest the presence of a hidden AGN (Braatz et al. 2004; Zhang et al. 2006; Krips et al. 2007).

Hunt et al. (2008) investigate the CO (1–0) and CO (2–1) distribution and kinematics using high spatial and velocity resolution maps obtained at the PdBI. The CO emission is aligned along the stellar nuclear bar with two emission peaks on opposite side of the nucleus, spanning ∼6'', and distributed in an elongated structure with two spiral arms within a radius of 8''. We estimate the rotation curve of the molecular gas disk and find flat rotation with a velocity of ∼260 km s⁻¹ outside a radius of 4''. In addition, there are two more extended spiral features extending toward the north and the south. Additional CO interferometric maps were obtained using the OVRO by Jogee et al. (1999). They find that the bar-like gaseous feature is offset in a leading sense with respect to the stellar bar and shows non-circular motion, which suggests gas inflow into the nucleus.

This galaxy is classified as an LIRG or a ULIRG depending on the assumed distance, and is well known as an object with a buried AGN based on multi-wavelength observations. Imanishi et al. (2003) detect an absorbed hard X-ray component with a 6.4 keV Fe Kα line, while Armus et al. (2004) detect the 14.3 μm [Ne v] line. These detections indicate the presence of a buried AGN. A high HCN/HCO⁺ line ratio (= 2.5) is further evidence for dominance of AGN activity (Imanishi 2006). Optical imaging shows a 38 kpc linear tail extending toward the west and a large ring (Surace et al. 2000). Windhorst et al. (2002) suggest the presence of an inclined dusty disk using the HST/WFPC2 I-band image. The rotation curves measured from the Hα and [N ii] emission show flat rotation with a constant velocity (V_rot sin i) of ∼200 km s⁻¹ (Smith et al. 1996).

We use the CO (2–1) data obtained by Wilson et al. (2008). The PV diagram shows rigid rotation and a double peak located at a distance of 400–500 pc from the galactic center. The PdBI CO (1–0) map (Genzel et al. 1998) is similar to the CO (2–1) map (Wilson et al. 2008), but the CO (1–0) emission is more extended than the CO (2–1). Thus the radius of the molecular gas disk which we estimate in this study is underestimated.

There are few studies focusing on this galaxy. Optical imaging shows a diffuse core with patchy structures. RJ04 could not determine structural parameters of the K-band surface brightness due to its disturbed structure. It is possible that the uncertainty of the position of the nucleus defined in the K-band peak is a large. This galaxy is detected in the AKARI All-Sky Survey, and the FIR luminosity is estimated to be 7.0 × 10⁸ L_☉ using the three bands (65 μm, 90 μm, and 140 μm; Yamamura et al. 2010).

The CO (1–0) emitting gas is distributed in three main components. The molecular gas masses of these components are 4.8 × 10⁷ M_☉, 4.2 × 10⁶ M_☉, and 1.3 × 10⁶ M_☉. The largest mass component is associated with the main body of the galaxy. The position angle of the kinematic major axis is almost 0°, which is not aligned with the apparent isophotal major axis of the stellar body. The PV diagram reveals a velocity gradient, which is steeper in the northern region than the southern region. The intermediate mass component located in the southeast of the largest component shares the same velocity distribution. A diffuse optical counterpart, possibly a short tidal tail is seen in the same region. Thus this component is likely to have been ejected from the main body due to the interaction. The smallest mass component is located in the west of the main body. This component has a stellar counterpart with a weak 3 mm continuum emission associated with it. This component may be a star cluster or a dwarf galaxy.

This galaxy is classified as an LIRG. The galaxy has two prominent tidal tails seen in the optical and the H i emission (English et al. 2003). The projected length of the tails are longer than 40 kpc (Mullan et al. 2011). The K-band image (RJ04) shows a single nucleus, but a double nucleus separated by ∼5'' (∼0.9 kpc) along the north–south direction has been detected in the radio, NIR, and X-ray (Norris & Forbes 1995; Kotilainen et al. 1996; Lira et al. 2002). The SMA CO (2–1) observations also reveal the presence of the double nucleus (Sakamoto et al. 2006). The northern nucleus has been identified as a intense star-forming region with a powerful outflowing galactic wind (Lípari et al. 2000; Lira et al. 2002). While Neff et al. (2003) suggest that the both nuclei may host low-luminosity AGNs using radio and X-ray observations, Jenkins et al. (2004) find no strong evidence for AGNs based on XMM-Newton X-ray observations. The HST/WFPC2 images shows spiral structures (Laine et al. 2003). Zepf et al. (1999) find more than 1000 young stellar clusters with ages ranging from a few to several hundred Myrs in the inner region (7 kpc × 7 kpc) using the HST/WFPC2 images.

We use the ALMA SV data. The velocity field and PV diagram reveals the presence of a disturbed disk. The rotation curve of the molecular gas disk shows rigid rotation within a radius of 20''. The strongest 3 mm continuum emission in our sample is detected in this galaxy. The SV data was published by Sakamoto et al. (2014), which, combined with their own higher spatial resolution Cycle 0 data, show several arm-like structures (Sakamoto et al. 2014). The SV data by itself is unable to spatially resolve the double nucleus presented by Sakamoto et al. (2006, 2014).

Optical imaging shows plumes elongated in the west of the main body (Lutz 1991). van Driel et al. (1991) detected two radio components at opposite sides of the galactic center. Optical spectroscopy classifies the galaxy as a H ii galaxy (e.g., Veilleux et al. 1995; Kewley et al. 2000). Lutz (1991) find a population of 14 blue stellar clusters and later work by Carlson et al. (1999) also find ∼700 compact objects using the HST/WFPC2 images, which suggests that the progenitor galaxies were gas-rich.

The CO (1–0) integrated intensity map shows a bar-like structure. The velocity field shows a smooth velocity gradient and the rotation curve of the molecular gas disk shows flat rotation with a velocity of ∼155 km s⁻¹ outside a radius of 1 kpc. The 3 mm continuum is roughly distributed along the bar-like CO structure. The strongest peak of the 3 mm continuum is located at a distance of 37 from the galactic center and corresponds to one of the CO peaks.

There are few studies focusing on this galaxy. Optical imaging shows a diffuse extension toward the south of the main stellar body. The main body is elongated in the northeast and southwest. The brightest peak, which we define as the nucleus, is located at the northeast edge. Arp & Madore (1987) classify the galaxy as an elliptical galaxy with jets. The H i spectra show a double-peaked profile with a FWHM of ∼100 km s⁻¹ (Theureau et al. 1998).

We detect two molecular gas components traced by the CO (1–0) line. One component is associated with the nucleus. This cannot be resolved due to the limited spatial resolution. The other is distributed below the main body blue-shifted 80 km s⁻¹ from the systemic velocity (V_sys = 3027 km s⁻¹). The molecular gas mass of the component associated with the nucleus is 1.2 × 10⁷ M_☉, which is 2.8 times smaller than that of the other component.

This galaxy is known as the "Medusa Merger." The optical morphology is characterized by a diffuse ("hair-like") tail extending over 10 kpc toward the north of the main body. The H i map shows a single ∼50 kpc tail extending toward the opposite side of the optical tail (Manthey et al. 2008). This galaxy is a candidate of a minor merger between a large elliptical and a small spiral galaxy (Manthey et al. 2008). Weistrop et al. (2004) identify 48 UV-bright knots, two-thirds of which are younger than 20 Myr, and suggest that the UV knots will become globular clusters in several Gyr. The HCN/HCO⁺(1–0) line ratio is less than unity, suggesting a starburst-dominated galaxy (Costagliola et al. 2011). The ionized gas traced by the 12.8 μm [N ii] line shows a smooth velocity gradient in the north–south direction through the galactic enter (Beck et al. 2014).

The galaxy is unusual in that the CO (2–1) emission is distributed along the optical tail. As seen in the velocity field, the diffuse gas forms three tails in the northern region of the main body. In addition, the CO (2–1) emitting gas appears to be forming a rotating disk in the main body, although the kinematical parameters for the molecular gas cannot be determined because it is difficult to exclude the gaseous tails from the component associated with the main body. These kinematic features are consistent with the OVRO CO (1–0) map (Aalto & Hüttemeister 2000).

Optical imaging shows one tidal tail extending toward the north and two shells to the southwest of the main body. This galaxy is a candidate of a merger remnant between a spiral and an elliptical galaxy (Manthey et al. 2008; Jütte et al. 2010). The large-scale H i distribution and kinematics is studied by Manthey et al. (2008) using the Westerbork Radio Synthesis Telescope. The total mass of the atomic gas is 1.46 × 10⁹ M_☉. The H i distribution shows two prominent tidal tails extending more than 40 kpc toward the north and south. The northern tail follows the optical tail closely. The H i velocity field shows simple rotation in the inner region, indicating that the gas has settle down. Its velocity gradient continues into the tidal tails.

Jütte et al. (2010) investigate the CO (1–0) distribution and kinematics using high spatial and velocity resolution maps obtained at the PdBI. Most of the CO emission comes from a central rotating disk with a radius of about 2 kpc. The kinematic major axis of the molecular gas disk is nearly perpendicular to the rotation of large-scale H i structure (Manthey et al. 2008), indicating a kinematically decoupled core. The integrated intensity map we obtained is slightly different from Jütte et al. (2010) due to different clipping levels. In our map, there are CO extensions on the outer side of the molecular gas disk, which appear to be kinematically decoupled from the disk.

This galaxy is the only object in our sample to be classified as a ULIRG, and well known as an object which harbors both intense starburst and powerful AGN activities. The fractional contribution of the AGN and the starburst to the bolometric IR luminosity is uncertain. Farrah et al. (2003) suggest that the AGN contributes ∼30% to the IR luminosity, and Veilleux et al. (2009) estimate that the AGN contribution is ∼70%. This galaxy is also characterized by jets and kiloparsec-scale outflows, which are likely the result of the negative feedback from the AGN. The jets and outflows have been detected at radio wavelengths (e.g., Carilli et al. 1998; Lonsdale et al. 2003), and recently detected at millimeter wavelengths (e.g., Feruglio et al. 2010; Aalto et al. 2012). The outflow rate of molecular gas is estimated to be ∼700 M_☉ yr⁻¹, which exceeds the ongoing SFR in the host galaxy (Feruglio et al. 2010). Optical imaging shows two tidal tails extending toward the north and south, whose combined projected length is over 70 kpc (Koda & Subaru Cosmos Team 2009). Multi-wavelength observations (at NIR, millimeter, and radio) suggest the presence of a nearly face-on disk (Carilli et al. 1998; Downes & Solomon 1998; Taylor et al. 1999).

We use the CO (3–2) data obtained by Wilson et al. (2008). The CO (3–2) map reveals the presence of a molecular gas disk as previously presented by Downes & Solomon (1998). The maximum extent of the gas disk estimated using the CO (3–2) map is similar to that seen in the PdBI CO (1–0) map (Downes & Solomon 1998). The PV diagram shows a main component which appears to be a gas disk and a high-velocity component located along the line of sight in the direction of the galactic center. This high-velocity component may be part of molecular outflows.

There are few studies focusing on this galaxy. The optical morphology is X-shaped with tails and plumes, the longest of which extends over 20 kpc toward the southeast (Smith & Hintzen 1991).

The distribution of the CO (1–0) emitting gas is significantly disturbed. The kinematical parameters for the molecular gas cannot be determined. The channel map (V = 8560–8840 km s⁻¹) shows an arc-like distribution in the northeast region. The molecular gas moves along the arc and merges into a gas component associated with the nucleus.

This galaxy is classified as an LIRG. The HST/ACS B-band image show a disrupted nuclear region, a small tail extending toward the northeast, and a bright region located at ∼4 kpc southwest of the galactic center. Although this region is identified as a secondary nucleus (Miralles-Caballero et al. 2011), it is likely to be a massive star-forming region (Monreal-Ibero et al. 2010; Rodríguez-Zaurín et al. 2011). Monreal-Ibero et al. (2010) suggest that the mechanism of ionization in the nuclear region is shocks (100–150 km s⁻¹), using the [O i] λ6300/Hα ratio, which is a good tracer of shock-induced ionization. Optical spectroscopy classify the galaxy as an LINER Corbett et al. (2003). There is no sign of either an AGN or an obscured AGN in the NIR spectra (Imanishi et al. 2010).

The CO (1–0) emitting gas appears to form a disk overall, but the distribution is disturbed and warped in the nuclear region. The kinematic major axis of the molecular gas is inconsistent with the morphological major axis of the stellar body. The 3 mm continuum emission is elongated along the kinematic minor axis of the molecular gas.

The galaxy has several shells seen in the NUV (Rampazzo et al. 2007) and the optical (Malin & Carter 1983), and it presents a prominent linear dust lane (Fort et al. 1986). Goudfrooij et al. (1994) find extended Hα+[N ii] emission. The projected size of the Hα+[N ii] emitting region is larger than 40'' (≃7.3 kpc). The rotation curves measured from the Ca ii triplet absorption line (λ ∼ 0.85 μm) and the CO stellar absorption line (λ = 2.29 μm) show flat rotation (Rothberg & Fischer 2010), suggesting the presence of a rotating disk. The H i emission is not associated with the nuclear region; however, it is distributed along a filamentary structure connecting between NGC 5018 and its two neighbors, NGC 5022 and MCG 03-34-013 (Kim et al. 1988). This H i filament is a sign of interaction between these galaxies.

This galaxy is undetected in the CO (1–0) line. The 3σ upper limit on the molecular gas mass is 2.3 × 10⁷ M_☉. Lees et al. (1991) and Huchtmeier & Tammann (1992) also report a non-detection in their single-dish CO measurements.

This galaxy is classified as an LIRG. The HST/ACS B-band image shows two straight tails emerging almost at 90° angles, and dust lanes across the nuclear region. There are two emission peaks in the nuclear region, spanning ∼340 pc, traced by the IR (Scoville et al. 2000) and the radio (Condon et al. 1991), but the K-band image (RJ04) shows a single peak. The rotation curves measured form the Hα and the [N ii] emission show flat rotation with a constant velocity (V_rot sin i) of 150 km s⁻¹ (Smith et al. 1996). The soft X-ray emission is elongated along the morphological minor axis of the stellar body, which is likely associated with outflow driven by a nuclear starburst or AGN (Iwasawa et al. 2011). Optical spectroscopy classify the galaxy as a LINER (Veilleux et al. 1995), while Yuan et al. (2010) suggest that this is a starburst–AGN composite. However, the small HCN/HCO⁺(1–0) line ratio of 0.9 and the strong 3.3 μm are evidence for starburst activity with no significant AGN contribution (Imanishi et al. 2009).

Three CO interferometric maps were published (Downes & Solomon 1998; Bryant & Scoville 1999; Wilson et al. 2008). We use the CO (3–2) data obtained by Wilson et al. (2008). The PV diagram shows an asymmetric distribution and the presence of a rotating disk. Other CO (1–0) and CO (2–1) interferometric data also shows a nuclear ring or disk of molecular gas (Downes & Solomon 1998; Bryant & Scoville 1999). The CO (3–2) emission is detected outside the extent of the CO (1–0) and CO (2–1) emission, due to the high sensitivity in CO (3–2) measurements.

There are few studies focusing on this galaxy. Optical imaging shows a ∼20 kpc plume elongated in the southwest and northeast and a 4 kpc tail extending toward the south (Smith & Hintzen 1991). Even though the optical structure is strongly distorted, the K-band surface brightness is almost perfectly fit by the de Vaucouleurs profile (RJ04). The rotation curve measured from the Ca ii triplet absorption line (λ ∼ 0.85 μm) shows a shallow velocity gradient (Rothberg & Fischer 2010), which cannot be fully explained by rigid rotation or flat rotation.

This galaxy is tentatively detected in the CO (1–0) line, but there is no robust emission (>1.5σ) continuing in velocity. The detailed distribution and kinematics of the molecular gas is not clear due to the limited sensitivity. The 3 mm continuum emission is clearly detected at the ∼20σ level in the nucleus.

There are few studies focusing on this galaxy. Optical imaging shows two long tidal tails running north–south. The projected length of the northern tail is longer than 30 kpc. The K-band image shows a bar structure across the nucleus (RJ04).

This galaxy is an unusual case in that most of the CO (2–1) emission comes from the root of the northern tail, with only weak emission seen in the nucleus. The molecular gas shows a kinematic signature of gas streaming along the tail, but it does not appear to inflow into the nuclear region because the velocity of the molecular gas in the nucleus is about 250 km s⁻¹ smaller than that in the tail leading to the nucleus.

The optical morphology is dominated by a large number of clumpy features, which appear to be young massive clusters according to their color and brightness (Holtzman et al. 1996). The H i map shows a tidal tail extending toward the south (Garland et al. 2007). Alloin & Duflot (1979) interpret the galaxy as a merger of two late-type galaxies, which might have produced the burst of star formation in clumps. Taniguchi & Noguchi (1991) suggest from numerical N-body simulations of the collision of two disk galaxies that the evolutionary phase of this galaxy corresponds to about 150 Myr after the first impact. The peak of the radio continuum emission is shifted from the emission peak in optical images, suggesting the presence of off-nuclear starbursts (e.g., Condon et al. 1990; Deeg et al. 1993). The OVRO CO (1–0) emission is barely resolved and the size of the CO (1–0) source is 75 × 48 (Garland et al. 2007).

The CO (2–1) emitting gas comes from the northern part of the galaxy and is not associated with the nucleus nor bright knots in the optical. The velocity field of the molecular gas as well as the ionized gas (García-Lorenzo et al. 2008) are significantly disturbed and shows complex structures. The majority of the molecular gas appears not to rotate in circular motion around the nucleus. It is likely that the galaxy has not undergone violent relaxation, according to the kinematics of the molecular gas and the apparent morphology.

Optical imaging shows a plume extending toward the southwest of the main body. Optical spectroscopy classifies the galaxy as a Seyfert 1 (Denisiuk et al. 1976; Gallego et al. 1996). However, Gonçalves et al. (1999) suggest that the galaxy is an LINER because the line strengths are weaker than those in classical AGNs. Furthermore, there is no evidence of an AGN from the radio–FIR correlation (Ji et al. 2000).

The CO (2–1) emitting gas appears to form a disk overall, but it is extremely disturbed. The PV diagram reveals three peaks, two of which overlap along the line of sight. There are only three resolution elements along the radial direction due to the limited sensitivity and the low angular resolution. Thus it is possible that derived radial properties have large uncertainties.

Optical imaging shows a plume extending toward the east and a prominent southwest loop. Optical spectroscopy reveals the dominance of A–F-type stars while lacking strong H ii region type emission lines, suggesting that the burst of star formation ended less than a few ×10⁸ yr ago (Sekiguchi & Wolstencroft 1993). The H i spectrum is roughly a double-horn profile (Richter et al. 1994). The rotation curve measured from the CO stellar absorption line at 2.29 μm, however, shows no sign of a rotating disk (Rothberg & Fischer 2010).

The CO (2–1) integrated intensity map is shaped like a peanut. A peanut-shaped gas distribution is often seen in barred galaxies because gas piles up at the bar ends. The size of the northern concentration is larger than the southern one. The PV diagram shows a velocity gradient of ∼260 km s⁻¹ kpc⁻¹ and two emission peaks, which are offset by 2'' (∼0.8 kpc) from the galactic center. These signatures imply the presence of a ring or a bar, but a bar structure is not identified in optical images.

This galaxy is classified as an LIRG or a ULIRG depending on the assumed distance. The HST/ACS B-band image shows two tidal tails and knotty structures in the main body. The velocity field of the Hα emission shows rotational motion with a velocity width of 150 km s⁻¹ without inclination correction (Mihos & Bothun 1998). Optical spectroscopy classifies the galaxy as a starburst–AGN composite (Yuan et al. 2010; Brightman & Nandra 2011). The IR spectroscopy reveals the presence of a buried AGN (Risaliti et al. 2006; Imanishi et al. 2010) and the X-ray emission is clearly dominated by a hidden AGN (Franceschini et al. 2003).

Most of the CO (1–0) emission comes from the southeast region of the main body, but the emission peaks seen in all channel maps are located at the nucleus. The PV diagram also shows gas concentration in the nucleus. In addition, an extension of the molecular gas is distributed in the southwest region of the main body, which does not correspond to the stellar arm structure. The velocity field in the molecular gas disk shows a less distinct velocity gradient due to the low inclination of the molecular gas disk (i = 10°) and non-circular motion. The 3 mm continuum emission is associated with the nucleus and the peak corresponds to the CO peak seen in the integrated intensity map.

This galaxy has shell structures seen in the optical (Malin & Carter 1983) as well as in the UV (Rampazzo et al. 2007). The distribution of warm ionized gas shows an asymmetric structure relative to the stellar body, elongated in the southwest direction (Rampazzo et al. 2003), which corresponds to the FUV distribution. The velocity field of warm ionized gas indicates rotational motion with a maximum rotation velocity (V_rot sin i) of 78 km s⁻¹ (Rampazzo et al. 2003). Rampazzo et al. (2007) suggest, using the relation between the optical line-strength indices and UV colors, that the galaxy experienced recent nuclear star formation likely triggered by the interaction/accretion that formed the shells. The VLA H i emission is mostly detected outside the main body (Schiminovich et al. 2001).

The CO (1–0) emitting gas is distributed in the southern region of the main body and not associated with the nucleus. The detailed structure and kinematics of the molecular gas are not clear due to the weak CO emission. The 3 mm continuum emission is robustly detected and the peak of the continuum emission corresponds to the nucleus.

This galaxy is a well-known merger, called the "Atoms for Peace." The galaxy has remarkable loops and two long tidal tails seen in the optical. High-resolution optical imaging carried out using the HST/WFPC reveals spiral arms within a radius of 35 from the galactic center and weak spiral features out to about 9'' (Whitmore et al. 1993). Miller et al. (1997) detect 499 cluster candidates, whose mean age is estimated to be 0.6 ± 0.2 Gyr by Whitmore et al. (1997). The H i emission is associated with the optical tidal tails, whose projected length are 520'' and 270'' (Hibbard et al. 1994), corresponding to 160 kpc and 82 kpc, respectively. Schweizer et al. (2013) suggest that the atomic gas is falling back to the nucleus. Previous CO (1–0) interferometric observations show a rotating molecular gas disk (Wang et al. 1992), but the observed disk size was limited to a ∼1.2 kpc radius.

Numerical simulations focusing on this galaxy have been conducted (e.g., Borne & Richstone 1991; Hibbard & Mihos 1995; Chien & Barnes 2010). They conclude that the galaxy is a merger between two disk galaxies and resulting in an elliptical galaxy. The merger age is estimated to be about 0.5–1 Gyr from numerical simulations (Hibbard & Mihos 1995; Chien & Barnes 2010) as well as observations (Schweizer 1982). This is in good agreement with the mean age of the star clusters (Whitmore et al. 1997). Mihos et al. (1993) suggest that the SFR has fallen to one-third of the value that it would have been at its peak according their simulation.

Our new CO (1–0) observations confirm the presence of a molecular gas disk two times larger (R_CO = 2.6 kpc) than the previous results (Wang et al. 1992). The velocity field shows a smooth velocity gradient, and the rotation curve of the molecular gas disk shows flat rotation with a velocity of ∼260 km s⁻¹ outside a radius of 1 kpc. The PV diagram shows a double peak located at the transition point between rigid rotation and flat rotation. The 3 mm continuum emission is detected around the nucleus at the 4σ level.

This galaxy is classified as an LIRG or a ULIRG depending on the assumed distance. The HST/ACS B-band image shows a single nucleus with long prominent tails. Haan et al. (2011) suggest that the galaxy built up a central stellar cusp due to nuclear starbursts, judging from the significant core light excess in the H-band image and relatively large 6.2 μm PAH EW. While optical spectroscopy classifies the galaxy as a Seyfert 2 (Yuan et al. 2010), there is no signature of an AGN according to a selection by hard X-ray color and the 6.4 keV iron line (Iwasawa et al. 2011).

The velocity field shows the presence of a warped gas disk. The PV diagram reveals rigid rotation and a double peak located off-center, suggesting the possibility of a ring structure, but a non-uniform distribution of molecular gas may also explain the observed diagram. The 3 mm continuum emission is associated with the nucleus and the peak corresponds to the CO peak seen in the integrated intensity map.

This galaxy has shells seen in the optical (Malin & Carter 1983). Multicolor optical imaging reveals that the shells are bluer than the main body, and that the timescale from the interaction, by which the shells were formed, is typically ∼1 Gyr (McGaugh & Bothun 1990). The upper limit of the atomic gas mass is 7.5 × 10⁹ M_☉ (Balick et al. 1976).

The galaxy is undetected in the CO (1–0) line. The 3σ upper limit on the molecular gas mass is 3.1 × 10⁷ M_☉. Georgakakis et al. (2001) also report non-detection in their CO measurements obtained at the OSO 20 m telescope.

The HST/WFPC2 B-band image reveals spiral structures (Peeples & Martini 2006), while the K-band surface brightness profile roughly follows the de Vaucouleurs law (Chitre & Jog 2002, RJ04). The Hα emission is associated with the nuclear region (Knapen 2005), where the mean stellar color is slightly bluer than the outer envelopes (Schombert et al. 1990). The rotation curve measured from the Mg_b absorption line (λ ∼ 0.52 μm) reveals a velocity gradient (V_rot sin i = ± 150 km s⁻¹; Simien & Prugniel 1997). This galaxy is detected in the AKARI All-Sky Survey, and the FIR luminosity is estimated to be 4.5 × 10⁸ L_☉ using three bands (65 μm, 90 μm, and 140 μm; Yamamura et al. 2010).

The PV digram shows rigid rotation, suggesting the presence of a rotating gas disk. The maximum rotational velocity is estimated to be ∼150 km s⁻¹. A close inspection of the channel maps reveals two different components. The low-velocity component (V = 1665–1735 km s⁻¹) is associated with the nucleus and the high-velocity component (V = 1740–1990 km s⁻¹) is distributed along the optical dust lane. The molecular gas mass in the high-velocity component is nearly eight times larger than that in the low-velocity component. The 3 mm continuum emission is associated with the nucleus and the peak corresponds to the CO peak seen in the integrated intensity map.