Bursty stellar populations and obscured active galactic nuclei in galaxy bulges

We investigate trends between the recent star formation history and black hole growth in galaxy bulges in the Sloan Digital Sky Survey. The galaxies lie at 0.01 < z < 0.07 where the fibre aperture covers only the central 0.6-4.0 kpc diameter of the galaxy. We find strong trends between black hole growth, as measured by dust-attenuation-corrected [O III] luminosity, and the recent star formation history of the bulges. 56 per cent of the bulges are quiescent with no signs of recent or ongoing star formation and, while almost half of all active galactic nuclei (AGN) lie within these bulges, they contribute only ~10 per cent to the total black hole growth in the local Universe. At the other extreme, the AGN contained within the ~4 per cent of galaxy bulges that are undergoing or have recently undergone the strongest starbursts, contribute at least 10-20 per cent of the total black hole growth. Much of this growth occurs in AGN with high amounts of dust extinction and thus the precise numbers remain uncertain. The remainder of the black hole growth (>60 per cent) is contributed by bulges with more moderate recent or ongoing star formation. The strongest accreting black holes reside in bulges with a wide range in recent star formation history. We conclude that our results support the popular hypothesis for black hole growth occurring through gas inflow into the central regions of galaxies, followed by a starburst and triggering of the AGN. However, while this is a significant pathway for the growth of black holes, it is not the dominant one in the present-day Universe. More unspectacular processes are apparently responsible for the majority of this growth.

In order to arrive at these conclusions we have developed a set of new high signal-to-noise ratio (S/N) optical spectral indicators, designed to allow a detailed study of stellar populations which have undergone recent enhanced star formation. Working in the rest-frame wavelength range 3750-4150 Å, ideally suited to many recent and ongoing spectroscopic surveys at low and high redshift, the first two indices are equivalent to the previously well-studied 4000-Å break strength and Hδ equivalent width. The primary advantage of this new method is a greatly improved S/N for the latter index, allowing the present study to use spectra with S/N per pixel as low as 8. The third index measures the excess strength of Ca II (H&K), which is particularly sensitive to the transition of a post-starburst spectrum from A to F stars, and allows the degeneracy between time of burst and strength of burst to be broken.