AMUSE-VIRGO. II. DOWN-SIZING IN BLACK HOLE ACCRETION

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 (3 sigma) limiting luminosity of 3.7 x 10(38) erg s(-1) over 0.5-7 keV. The stellar mass distribution of the targeted sample, which is mostly composed of formally inactive galaxies, peaks below 10(10) M-circle dot, 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(-0.4+/-0.1) up to 10(42) erg s(-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(-0.62-0.12+0.13), with an intrinsic scatter of 0.46(-0.06)(+0.08) 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.