Amuse-virgo. II. Down-sizing in black hole accretion

Abstract

We complete the census of nuclear X-ray activity in 100 early-type Virgo galaxies observed by the Chandra X-ray Telescope as part of the AMUSE-Virgo survey, down to a (3σ) limiting luminosity of 3.7 × 10 38ergs-1 over 0.5-7 keV. The stellar mass distribution of the targeted sample, which is mostly composed of formally "inactive" galaxies, peaks below 1010 M ⊙, a regime where the very existence of nuclear supermassive black holes (SMBHs) is debated. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid nuclei which also host a massive nuclear cluster as visible from archival Hubble Space Telescope images. After carefully accounting for contamination from nuclear low-mass X-ray binaries based on the shape and normalization of their X-ray luminosity function (XLF), we conclude that between 24% and 34% of the galaxies in our sample host an X-ray active SMBH (at the 95% confidence level). This sets a firm lower limit to the black hole (BH) occupation fraction in nearby bulges within a cluster environment. The differential logarithmic XLF of active SMBHs scales with the X-ray luminosity as L X-0.40.1 up to 1042ergs-1. At face value, the active fraction - down to our luminosity limit - is found to increase with host stellar mass. However, taking into account selection effects, we find that the average Eddington-scaled X-ray luminosity scales with BH mass as M BH , with an intrinsic scatter of 0.46+0.08-0.06 dex. This finding can be interpreted as observational evidence for "down-sizing" of BH accretion in local early types, that is, low-mass BHs shine relatively closer to their Eddington limit than higher mass objects. As a consequence, the fraction of active galaxies, defined as those above a fixed X-ray Eddington ratio, decreases with increasing BH mass.