Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX

Abstract

We analyze a volume-limited sample of massive bulge-dominated galaxies with data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX) satellite. The galaxies have central velocity dispersions greater than 100 km s^(−1) and stellar surface mass densities that lie above the value where galaxies transition from actively star-forming to passive systems. The sample is limited to redshifts 0.03 < z < 0.07. At these distances, the SDSS spectra sample the light from the bulge-dominated central regions of the galaxies. The GALEX NUV data provide high sensitivity to low rates of global star formation in these systems. Our sample of bulge-dominated galaxies exhibits a much larger dispersion in NUV − r color than in optical g − r color. The dispersion increases for galaxies with smaller central velocity dispersions, and nearly all of the galaxies with bluer NUV − r colors are active galactic nuclei (AGNs). Both GALEX images and SDSS color profiles demonstrate that the excess UV light is nearly always associated with an extended disk. When comparing fiber-based estimates of stellar age to global ones, we find that galaxies with red outer regions almost never have a young bulge or a strong AGN. Galaxies with blue outer regions have bulges and black holes that span a wide range in age and accretion rate. Galaxies with young bulges and strongly accreting black holes almost always have blue outer disks. The black hole growth rate correlates much more strongly with the age of the stars in the bulge than in the disk. Our suggested scenario is one in which the source of gas that builds the bulge and black hole is a low-mass reservoir of cold gas in the disk. The presence of this gas is a necessary but not sufficient condition for bulge and black hole growth. Some mechanism must transport this gas inward in a time variable way. The disk gas itself is likely to be the result of the accretion of gas from an external source. As the gas in the disk is converted into stars, galaxies will turn red, but further inflow can bring them back into the blue NUV − r sequence.