THE IMPACT OF GALAXY INTERACTIONS ON ACTIVE GALACTIC NUCLEUS ACTIVITY IN zCOSMOS

Abstract

Close encounters between galaxies are expected to be a viable mechanism, as predicted by numerical simulations, by which accretion onto supermassive black holes can be initiated. To test this scenario, we construct a sample of 562 galaxies (M(*) > 2.5 x 10(10) M(circle dot)) in kinematic pairs over the redshift range 0.25 <z <1.05 that are more likely to be interacting than a well-matched control sample of 2726 galaxies not identified as being in a pair, both from the zCOSMOS 20k spectroscopic catalog. Galaxies that harbor an active galactic nucleus (AGN) are identified on the basis of their X-ray emission (L(0.5-10) (keV) > 2 x 10(42) erg s(-1)) detected by Chandra. We find a higher fraction of an AGN in galaxies in pairs relative to isolated galaxies of similar stellar mass. Our result is primarily due to an enhancement of AGN activity, by a factor of 1.9 (observed) and 2.6 (intrinsic), for galaxies in pairs of projected separation less than 75 kpc and line-of-sight velocity offset less than 500 km s(-1). This study demonstrates that close kinematic pairs are conducive environments for black hole growth, either indicating a causal physical connection or an inherent relation, such as, to enhanced star formation. In the Appendix, we describe a method for estimating the intrinsic fractions of galaxies (either in pairs or the field) hosting an AGN with confidence intervals, and an excess fraction in pairs. We estimate that 17.8(-7.4)(+8.4)% of all moderate-luminosity AGN activity takes place within galaxies undergoing early stages of interaction that leaves open the question as to what physical processes are responsible for fueling the remaining similar to 80% that may include late-stage mergers.