Observational Evidence for Enhanced Black Hole Accretion in Giant Elliptical Galaxies

Abstract

We present a study of the relationship between black hole accretion rate (BHAR) and star formation rate (SFR) in a sample of giant elliptical galaxies. These galaxies, which live at the centers of galaxy groups and clusters, have star formation and black hole activity that is primarily fueled by gas condensing out of the hot intracluster medium. For a sample of 46 galaxies spanning 5 orders of magnitude in BHAR and SFR, we find a mean ratio of log(BHAR/SFR) = -1.45 +/- 0.2, independent of the methodology used to constrain both SFR and BHAR. This ratio is significantly higher than most previously-published values for field galaxies. We investigate whether these high BHAR/SFR ratios are driven by high BHAR, low SFR, or a different accretion efficiency in radio galaxies. The data suggest that the high BHAR/SFR ratios are primarily driven by boosted black hole accretion in spheroidal galaxies compared to their disk counterparts. We propose that angular momentum of the cool gas is the primary driver in suppressing BHAR in lower mass galaxies, with massive galaxies accreting gas that has condensed out of the hot phase on nearly radial trajectories. Additionally, we demonstrate that the relationship between specific BHAR and SFR has much less scatter over 6 orders of magnitude in both parameters, due to competing dependence on morphology between the M_BH--M_* and BHAR--SFR relations. In general, active galaxies selected by typical techniques have sBHAR/sSFR ~ 10, while galactic nuclei with no clear AGN signatures have sBHAR/sSFR ~ 1, consistent with a universal M_BH--M_spheroid relation.