AMUSE-Virgo. I. Supermassive black holes in low-mass spheroids

We present the first results from the AGN Multiwavelength Survey of Early-Type Galaxies in the Virgo Cluster (AMUSE-Virgo). This large program targets 100 early-type galaxies with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory and the Multiband Imaging Photometer on board the Spitzer Space Telescope, with the aim of providing an unbiased census of low-level supermassive black hole activity in the local universe. Here we report on the Chandra observations of the first 16 targets, and combine them with results from archival data of another, typically more massive, 16 targets. Pointlike X-ray emission from a position coincident with the optical nucleus is detected in 50% of the galaxies (down to our completeness limit of similar to 4 x 10(38) ergs s(-1)). Two of the X-ray nuclei are hosted by galaxies (VCC 1178 [N4464] and VCC 1297 [N4486B]) with absolute B magnitudes fainter than -18, where nuclear star clusters are known to become increasingly common. After carefully accounting for possible contamination from low-mass X-ray binaries, we argue that the detected nuclear X-ray sources are most likely powered by low-level accretion on to a supermassive black hole, with a less than or similar to 11% chance contamination in VCC 1178, where a star cluster is barely resolvable in archival Hubble Space Telescope images. Based on black hole mass estimates from the global properties of the host galaxies, all the detected nuclei are highly sub-Eddington, with luminosities in the range -8.4 < log(L0.3-10 keV/ L-Edd) < -5.9. The incidence of nuclear X-ray activity increases with the stellar mass M-star of the host galaxy: only between 3% and 44% of the galaxies with M-star < 10(10) M-circle dot harbor an X-ray active supermassive black hole. The fraction rises to between 49% and 87% in galaxies with stellar mass above 1010 M-circle dot (at the 95% confidence level).