The Subaru/XMM-Newton Deep Survey (SXDS). VI. Properties of Active Galactic Nuclei Selected by Optical Variability

We present the properties of active galactic nuclei (AGN) selected by optical variability in the Subaru/XMM-Newton Deep Field (SXDF). Based on the locations of variable components and light curves, 211 optically variable AGNs were reliably selected. We made three AGN samples; X-ray-detected optically nonvariable AGNs (XAs), X-ray-detected optically variable AGNs (XVAs), and X-ray-undetected optically variable AGNs (VAs). In the VA sample, we found a bimodal distribution of the ratio between the variable component flux and the host flux. One of these two components in the distribution, a class of AGNs with a faint variable component <IMG SRC="eq-00001.gif" ALT="i^{\prime }_{\mathrm{vari}\,}\sim 25"/> i'vari [?] 25 mag in bright host galaxies <IMG SRC="eq-00002.gif" ALT="i^{\prime }\sim 21"/> i' [?] 21 mag, is not seen in the XVA sample. These AGNs are expected to have low Eddington ratios if we naively consider a correlation between bulge luminosity and black hole mass. These galaxies have photometric redshifts <IMG SRC="eq-00003.gif" ALT="z_{\mathrm{photo}\,}\sim 0.5"/> zphoto [?] 0.5 and we infer that they are low-luminosity AGNs with radiatively inefficient accretion flows (RIAFs). The properties of the XVA and VA objects and the differences from those of the XA objects can be explained within the unified scheme for AGNs. Optical variability selection for AGNs is an independent method and could provide a complementary AGN sample which even deep X-ray surveys have not found.