The connection between star formation and supermassive black hole activity in the local Universe

We study the nuclear (AGN) activity in the local Universe (z < 0.33) and its correlation with the host galaxy properties, derived from a Sloan Digital Sky Survey sample with spectroscopic star-formation rate (SFR) and stellar mass determination. To quantify the level of AGN activity we used the XMM-Newton Serendipitous Source Catalogue. Applying multiwavelength selection criteria (optical BPT-diagrams, X-ray/optical ratio etc), we found that 24 per cent of the detected sources are efficiently-accreting AGN with moderate-to-high X-ray luminosity, twice as likely to be hosted by star-forming galaxies than by quiescent ones. The distribution of the specific Black Hole accretion rate (λ_sBHAR) shows that nuclear activity in local, non-AGN dominated galaxies peaks at very low accretion rates (-4 ≲ log λ_sBHAR ≲ -3) in all stellar mass ranges. We observe systematically larger values of λ_sBHAR for galaxies with active star formation than for quiescent ones, and an increase of the mean λ_sBHAR with SFR for both star-forming and quiescent galaxies. These finding confirm the decrease in AGN activity with cosmic time and are consistent with a scenario where both star-formation and AGN activity are fuelled by a common gas reservoir.