CHANDRAX-RAY ANDHUBBLE SPACE TELESCOPEIMAGING OF OPTICALLY SELECTED KILOPARSEC-SCALE BINARY ACTIVE GALACTIC NUCLEI. I. NATURE OF THE NUCLEAR IONIZING SOURCES

Abstract

Kiloparsec-scale binary active galactic nuclei (AGNs) signal active supermassive black hole (SMBH) pairs in merging galaxies. Despite their significance, unambiguously confirmed cases remain scarce and most have been discovered serendipitously. In a previous systematic search, we optically identified four kpc-scale binary AGNs from candidates selected with double-peaked narrow emission lines at z = 0.1-0.2. Here we present Chandra and Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging of these four systems. We critically examine and confirm the binary-AGN scenario for two of the four targets, by combining high angular resolution X-ray imaging spectroscopy with Chandra ACIS-S, better nuclear position constraints from WFC3 F105W imaging, and direct starburst estimates from WFC3 F336W imaging; for the other two targets, the existing data are still consistent with the binary-AGN scenario, but we cannot rule out the possibility of only one AGN ionizing gas in both merging galaxies. We find tentative evidence for a systematically smaller X-rayto-[O III] luminosity ratio and/or higher Compton-thick fraction in optically selected kpc-scale binary AGNs than in single AGNs, possibly caused by a higher nuclear gas column due to mergers and/or a viewing angle bias related to the double-peak narrow line selection. While our result lends some further support to the general approach of optically identifying kpc-scale binary AGNs, it also highlights the challenge and ambiguity of Xray confirmation.

Galactic-scale Feedback Observed in the 3C 298 Quasar Host Galaxy

“…The offset between the dynamical center of the second disk and the secondary outflow is on the order of 3 kpc. We note that this is a typical separation that is found between the stellar centroid and AGN location in local late-stage mergers (Liu et al 2013b;Comerford et al 2015;Barrows et al 2016). We infer that the 3C 298 system likely has two massive galaxies with distinct rotating disks that are currently experiencing a close passage, where their galactic nuclei and SMBHs have yet to merge.…”

Spatially Offset Active Galactic Nuclei. II. Triggering in Galaxy Mergers

“…As with the dual AGNs from Liu et al (2013), both of the X-ray AGNs in SDSSJ1126 +2944satisfy the spatially offset criteria from Paper I(3σ and 20 kpc). The brighter of the two X-ray AGNs (SDSSJ1126+2944NW) passes the L 2-10 keV,unabs.…”

“…One particular example is the source SDSSJ110851.04+065901.5(SDSSJ1108+0659), which was selected as a spatially offset AGN in Paper Ibut was also previously discussed in Liu et al (2013), who interpreted the system to be consistent with a dual AGN. Although SDSSJ1108+0659is excluded from the sample because it was targeted by a Chandraprogram as a dual-AGN candidate, secondary AGNs that are currently undetected may exist in some of our other offset AGN systems.…”

“…Since we require Chandradetections to spatially isolate the AGN relative positions, we have cross-matched the SDSS spectroscopic AGNs with unique detections from the ChandraSource Catalog (CSC; Evans et al 2010) within 1 5 to create the final parent sample of the optically selected AGNs. From this parent sample, we use the previously identified dual-AGN systems found in the two studies that have selected dual AGNs starting from the SDSS spectroscopic AGN sample and for which Chandra imaging reveals spatially distinct X-ray AGNs (Liu et al 2013;Comerford et al 2015). The two samples are described below.…”

Finding binary active galactic nuclei candidates by the centroid shift in imaging surveys

“…Thus, binary active galactic nuclei (AGNs; binary AGNs are generally defined here as any double sources discovered by imaging or spectroscopy, and we do not strictly distinguish binary AGNs from dual AGNs or AGN pairs) are expected to be common because galaxy mergers can trigger the activity of SMBHs (Begelman et al 1980;Hernquist 1989;Kauffmann & Haehnelt 2000;Hopkins et al 2008). Although hundreds of binary AGNs are found with separations of tens of kiloparsec (kpc), only about ten binary AGNs are known with separations ∼10 kpc (Junkkarinen et al 2001;Komossa et al 2003;Hennawi et al 2006;Fu et al 2011;Koss et al 2011;Mazzarella et al 2012;Liu et al 2013;Müller-Sánchez et al 2015). The occurrence rate of kpc-scale binary AGNs (∼1%) is lower than the expected value by a factor of ten assuming that each major merger can induce a binary AGN (Yu et al 2011).…”