Authors: Giulia A. D. Savorgnan, Alister W. Graham, Alessandro Marconi, Eleonora Sani
In our first paper, we performed a detailed (i.e. bulge, disks, bars, spiral arms, rings, halo, nucleus, etc.) decomposition of 66 galaxies, with directly measured black hole masses, MBH, that had been imaged at 3.6 μm with Spitzer. Our sample is the largest to date and, for the first time, the decompositions were checked for consistency with the galaxy kinematics. We present correlations between MBH and the host spheroid (and galaxy) luminosity, Lsph (and Lgal), and also stellar mass, M∗,sph. While most previous studies have used galaxy samples that were overwhelmingly dominated by high-mass, early-type galaxies, our sample includes 17 spiral galaxies, half of which have MBH < 107 MSun, and allows us to better investigate the poorly studied low-mass end of the MBH—M∗,sph correlation. The bulges of early-type galaxies follow MBH ∝ M1.04±0.10∗,sph and define a tight "red sequence" with intrinsic scatter ε(MBH|M∗,sph) = 0.43 ± 0.06 dex and a median MBH/M∗,sph ratio of 0.68 ± 0.04%, i.e. a ±2σ range of 0.1—5%. At the low-mass end, the bulges of late-type galaxies define a much steeper "blue sequence", with MBH ∝ M2—3∗,sph and MBH/M∗,sph equal to 0.02% at MBH ≈ 106 MSun, indicating that gas-rich processes feed the black hole more efficiently than the host bulge as they coevolve. We additionally report that: i) our Sérsic galaxy sample follows MBH ∝ M1.48±0.20∗,sph, a less steep sequence than previously reported; ii) bulges with Sérsic index nsph < 2, argued by some to be pseudo-bulges, are not offset to lower MBH from the correlation defined by the current bulge sample with nsph > 2; and iii) Lsph and Lgal correlate equally well with MBH, in terms of intrinsic scatter, only for early-type galaxies – once reasonable numbers of spiral galaxies are included, the correlation with Lsph is better than that with Lgal.