The Evolutionary History of Galactic Bulges: Photometric and Spectroscopic Studies of Distant Spheroids in the GOODS Fields

We report on the first results of a new study aimed at understanding the diversity and evolutionary history of distant galactic bulges in the context of now well-established trends for pure spheroidal galaxies. To this end, bulges have been isolated for a sample of 137 spiral galaxies within the redshift range 0.1 < z < 1.2 in the GOODS fields. Using proven photometric techniques, we determine the characteristic parameters (size, surface brightness, profile shape) of both the disk and bulge components in our sample. In agreement with earlier work that utilized aperture colors, distant bulges show a broader range of optical colors than would be the case for passively evolving populations. To quantify the amount of recent star formation necessary to explain this result, we used DEIMOS to secure stellar velocity dispersions for a sizeable fraction of our sample. This has enabled us to compare the fundamental plane of our distant bulges with that for spheroidal galaxies in a similar redshift range. Bulges of spiral galaxies with a bulge-to-total luminosity ratio (B/T) >0.2 show similar patterns of evolution to those seen for pure spheroidals such that the stellar populations of all spheroids with M > 10¹¹ M_☉ are consistent with a single major burst of star formation at high redshift (z_f≳ 2), while bulges with M < 10¹¹ M_☉ must have had more recent stellar mass growth (~10% in mass since z ∼ 1). Although further data spanning a wider range of redshift and mass are desirable, the similarity between the assembly histories of bulges and low-mass spheroidals seems difficult to reconcile with the picture whereby the majority of large bulges form primarily via secular processes within spiral galaxies.