Observational Evidence for Enhanced Black Hole Accretion in Giant
  Elliptical Galaxies
  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.
