[Surveys -- Oral ]

AMUSE-Virgo: the complete nuclear X-ray census

Elena Gallo, MIT
T. Treu, C. Leipski, R. Antonucci (UCSB), J.-H. Woo (UCLA)

We completed the census of nuclear X-ray activity in 100 early type Virgo galaxies observed by Chandra as part of the AMUSE-Virgo survey. The stellar mass distribution of the sample peaks below 10E+10 M_Sun, in a regime where nuclear super-massive black holes (SMBHs) may be difficult to retain by bulges due to their shallow potential wells, and could even be replaced by massive nuclear star clusters. Out of these 100 objects, 32 show a nuclear X-ray source, 6 of which also host a massive nuclear cluster as visible from high-resolution, archival HST images. As the sensitivity threshold of our Chandra survey is close to the Eddington limit for a solar mass object, low-mass X-ray binaries (LMXBs) within the inner few arcsec are a likely source of contamination to the nuclear X-ray emission. This is assessed by assigning to each X-ray nucleus a weight which corresponds to (1-P_X), where P_X is the probability of hosting a LMXB of luminosity L\_X as evaluated from the known shape and normalization of the X-ray luminosity function of LMXBs in globular clusters/the field, respectively in the presence/absence of a nuclear star cluster. We construct the mass-dependent distribution of active SMBHs for these 100 early types, the majority of which are deemed formally `inactive'. We confirm that the fraction f_X of active nuclear SMBHs is an increasing function of the host stellar mass. The differential logarithmic X-ray luminosity function of active SMBHs in our sample scales with the X-ray luminosity as L\_X\^(0.5+/- 0.1) between 3E+38 and 10E+42 erg/s; the fitted slope is much shallower than for LMXBs, confirming the different nature of the nuclear X-ray sources' population.