Active Galactic Nucleus Pairs From the Sloan Digital Sky Survey. Ii. Evidence for Tidally Enhanced Star Formation and Black Hole Accretion

Abstract

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 h −1 70 kpc and velocity offsets < 600 km s −1 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] λ5007 luminosities (corrected for contribution from star formation) and Eddington ratios, than do control samples of single AGNs matched in redshift and host-galaxy stellar mass. Their host galaxies have stronger post-starburst activity and younger mean stellar ages, as indicated by stronger Hδ absorption and smaller 4000 Å 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 h −1 70 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 host-galaxy star formation in AGN pairs, even though the majority of low redshift AGNs is not coincident with on-going interactions.

Galaxy pairs in the Sloan Digital Sky Survey – VIII. The observational properties of post-merger galaxies

“…The statistics are noisier than for the other metrics investigated in this paper due to the small number of 'pure' Seyferts in our sample. Nonetheless, there is a clear enhancement in the [OIII] luminosity by a factor of ∼ 2 -3 for close pairs separated by less than 50 h −1 70 kpc, consistent with, for example, the recent work by Liu et al (2012). The post-mergers show the largest accretion rate enhancement, albeit with a large error bar, as predicted by simulations of major mergers (e.g.…”

“…Kauffmann et al 2003b;Heckman et al 2004;Chen et al 2009;Liu et al 2012). However, since star formation can also contribute to the [OIII] line flux, some studies have attempted to remove this possible contamination (e.g.…”

“…By combining the imaging and spectroscopic samples, it has been possible to tackle many aspects of galaxy mergers with unparalleled statistics. Samples of either close spectroscopic pairs, or morphologically classified mergers have been used to investigate the impact of galaxy interactions on colours (Ellison et al 2010;Patton et al 2011;Alonso et al 2012;Lambas et al 2012), star formation rates (Nikolic, Cullen & Alexander 2004;Alonso et al 2006;Li et al 2008a;Scudder et al 2012b;Patton et al 2013), metallicity (Michel-Dansac et al 2008;Ellison et al 2008;Reichard et (Hwang et al 2010(Hwang et al , 2011Ellison et al 2013) and nuclear activity (Alonso et al 2007;Woods & Geller 2007;Li et al 2008b;Ellison et al 2008Ellison et al , 2011Rogers et al 2009;Darg et al 2010;Liu, Shen & Strauss 2012;Sabater et al 2013).…”

The AMIGA sample of isolated galaxies

“…Park & Choi 2009;Liu et al 2012, and references therein). Galaxy interactions are thought to induce nuclear activity by removing angular momentum from the gas and, in this way, feeding the central black hole (Shlosman et al 1990; Barnes & Hernquist 1991;Haan et al 2009;Liu et al 2011).…”

“…However, different studies have yielded contradictory results: some studies find a higher local density of companions near galaxies hosting an AGN or a higher prevalence of AGN in interacting galaxies (Petrosian 1982;Dahari 1985;MacKenty 1990;Rafanelli et al 1995Rafanelli et al , 1997Alonso et al 2007;Ellison et al 2011;Liu et al 2012), while others find no excess, or only a marginal excess (Bushouse 1986;Laurikainen & Salo 1995;Schmitt 2001;Miller et al 2003;Ellison et al 2008;Li et al 2008). Schawinski et al (2009Schawinski et al ( , 2010 found a clear relation are only available in electronic form at http://amiga.iaa.es/ and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/545/A15 Appendix A is available in electronic form at http://www.aanda.org between some AGN and the signatures of a previous merger.…”