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[*] The black hole mass distribution in early-type galaxies: cusps in HST photometry interpreted through adiabatic black hole growth
van der Marel R.P.
AJ, 117, 744-763, 1998
© 1998. The American Astronomical Society. All Rights Reserved.

[*] Citations to this paper in the ADS

HST observations show that the surface brightness profiles of early-type galaxies have central cusps. I show that the observed characteristics of these cusps are consistent with the hypothesis that: (i) all early-type galaxies have central BHs that grew adiabatically in homogeneous isothermal cores; and (ii) these `progenitor' cores followed scaling relations similar to those of the fundamental plane.

The models studied here are the ones first proposed by Young. They predict I = r^{-1/2} at asymptotically small radii, but I = r^{-gamma} at the radii observable with HST. The slope gamma can take on all observed values, and increases monotonically with mu = M_bh / M_core. `Core' profiles (which have a break at a resolved radius and a shallow slope inside that radius) and `power-law' profiles (which have a steep slope down to the resolution limit and no clear break) can both be reproduced.

Observations show that with few exceptions, galaxies with M_V < -22 have core profiles, and galaxies with M_V > -20.5 have power-law profiles; both profile types occur in galaxies with -22 < M_V < -20.5. For the models, the scaling relations for early-type galaxies imply that the progenitor core mass scales with luminosity as M_core = L^{1.5}. If, as suggested by various arguments, the black hole (BH) mass M_bh scales roughly linearly with luminosity, then mu = L^{-0.5}. This yields larger cusp slopes in lower-luminosity galaxies. Models with BH masses and progenitor cores that obey established scaling relations predict (at the distance of the Virgo cluster) that galaxies with M_V < -21.2 have core profiles and galaxies with M_V > -21.2 have power-law profiles. This reproduces both the sense and the absolute magnitude of the observed transition. Intrinsic scatter in BH and galaxy properties can explain why both types of galaxies are observed around the transition magnitude. The observed bimodality in cusp slopes may be due to a bimodality in M_bh/L, with rapidly rotating disky galaxies having larger M_bh/L than slowly rotating boxy galaxies.

I apply the models to 46 galaxies with published HST photometry. Both core and power-law galaxies are well fitted. The models suggest a roughly linear correlation between BH mass and V-band galaxy luminosity, log M_bh = -1.83 + log L in solar units (RMS scatter 0.33 dex). This agrees with the average relation for nearby galaxies with kinematically determined BH masses. Photometrically and kinematically determined BH masses agree to within 0.25 dex RMS for galaxies that have both. These results provide additional support to the hypothesis that every galaxy (spheroid) has a central BH. The BH mass distribution inferred here is consistent with quasar statistics for a BH accretion efficiency of 4%.

The proposed scenario is not a unique way to interpret the observed surface brightness cusps of galaxies, but it explains observational correlations that are otherwise unexplained, and it yields BH masses that agree with those determined kinematically.

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Last modified March 2, 1999.
Roeland van der Marel, marel@stsci.edu.
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