Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. II.   Evidence for Tidally Enhanced Star Formation and Black Hole Accretion
  Active galactic nuclei (AGNs) are occasionally seen in pairs, suggesting that tidal encounters are responsible for the accretion of material by both central supermassive black holes (BHs). In Paper I of this series, we selected a sample of AGN pairs with projected separations r_p < 100 kpc and velocity offsets < 600 km/s from the SDSS DR7 and quantified their frequency. In this paper, we address the BH accretion and recent star formation properties in their host galaxies. AGN pairs experience stronger BH accretion, as measured by their [O III]5007 luminosities (corrected for contribution from star formation) and Eddington ratios, than do control samples of single AGNs matched in redshift and host stellar mass. Their host galaxies have stronger post-starburst activity and younger mean stellar ages, as indicated by stronger H-delta absorption and smaller 4000 A break in their spectra. The BH accretion and recent star formation in the host galaxies both increase with decreasing projected separation in AGN pairs, for r_p ~< 10-30 kpc. The intensity of BH accretion, the post-starburst strength, and the mean stellar ages are correlated between the two AGNs in a pair. The luminosities and Eddington ratios of AGN pairs are correlated with recent star formation in their host galaxies, with a scaling relation consistent with that observed in single AGNs. Our results suggest that galaxy tidal interactions enhance both BH accretion and star formation in AGN pairs, even though the majority of low redshift AGNs is not coincident with on-going interactions. 
Xin Liu, Yue Shen, Michael A. Strauss
astro-ph.CO