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Title:
Quantifying the (X/peanut)-shaped structure of the Milky Way - new constraints on the bar geometry
Authors:
Ciambur, Bogdan C.; Graham, Alister W.; Bland-Hawthorn, Joss
Affiliation:
AA(Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia), AB(Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia), AC(Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 471, Issue 4, p.3988-4004 (MNRAS Homepage)
Publication Date:
11/2017
Origin:
OUP
Astronomy Keywords:
Galaxy: bulge, Galaxy: disc, Galaxy: fundamental parameters, Galaxy: structure
Abstract Copyright:
2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
DOI:
10.1093/mnras/stx1823
Bibliographic Code:
2017MNRAS.471.3988C

Abstract

The nature, size and orientation of the Milky Way's bar and `bulge' have been the subject of conflicting interpretations in the literature. Here, we present a novel approach to inferring the properties of the long bar, which extends beyond the inner `bulge', by using information encoded in the Galaxy's X/peanut (X/P)-shaped structure. We perform a quantitative analysis of the X/P feature seen in WISE wide-field images, at 3.4 and 4.6 μm, by measuring the deviations of the isophotes from pure ellipses and using the radial profile of their sixth-order Fourier harmonic (cosine term, B6). In addition to the vertical height and integrated `strength' of the observed X/P instability, we report an intrinsic radius RΠ,int = 1.67 ± 0.27 kpc, and an orientation angle of α = 37°+7°-10° with respect to our line of sight to the Galactic Centre. Based on X/P structures observed in other galaxies, we assume that the Milky Way's X/P structure is intrinsically symmetric, aligned with the long Galactic bar, and that its size is correlated with this bar. The implications for the Galactic bar are that it is oriented at a 37° angle and has a radius of ≈4.2 kpc, but possibly as low as ≈3.2 kpc. We have investigated how the Milky Way's X/P structure compares with analogues in other galaxies, and find that it is consistent with recently established scaling relations, though with a marginally stronger X/P instability than expected. We additionally perform a photometric decomposition of the Milky Way's major axis surface brightness profile, accounting for spiral structure, and determine an average disc scalelength of h = 2.54 ± 0.16 kpc.
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