No abstract

We present a Chandra archival study of optically selected active galactic nucleus (AGN) pairs at a median redshift . Out of 1286 AGN pairs (with projected separations r p  < 100 kpc and velocity offsets Δv < 600 km s−1) optically identified from the Sloan Digital Sky Survey Seventh Data Release, we find 67 systems with archival Chandra observations, which represents the largest sample of optically selected AGN pairs studied in the X-ray. Among the 67 AGN pairs, 21 systems have both nuclei detected in the X-ray, 36 have one nucleus detected in the X-ray, and 10 have no X-ray detection. The X-ray detection rate, 78/134 = 58% (±7% 1σ Poisson errors), is significantly higher than that (23/134 = 17% ± 4%) of a comparison sample of star-forming galaxy pairs, lending support to the optical AGN classification. In the conservative case where X-ray contamination from star formation is removed, the X-ray detection rate becomes 27% ± 4%, consistent with predictions from the latest galaxy merger simulations. The 2–10 keV X-ray luminosity L 2–10 keV increases with decreasing projected separation in AGN pairs for r p ≳ 15 kpc, suggesting an enhancement of black hole accretion even in early-stage mergers. On the other hand, L 2–10 keV appears to decrease with decreasing projected separation at r p ≲15 kpc, which is contradictory to predictions from merger simulations. The apparent decrease in L 2–10 keV of AGN pairs at r p ≲ 15 kpc may be caused by (i) enhanced absorbing columns from merger-induced gas inflows, (ii) feedback effects from early-stage mergers, and/or (iii) small number statistics. Future X-ray studies with larger samples are needed to put our results on firmer statistical ground.

We report the discovery of SDSS J0849+1114 as the first known triple Type 2 Seyfert nucleus. It represents three active black holes that are identified from new spatially resolved optical slit spectroscopy using the Dual Imaging Spectrograph on the 3.5 m telescope at the Apache Point Observatory. We also present new complementary observations including the Hubble Space Telescope Wide Field Camera 3 U- and Y-band imaging, Chandra Advanced CCD Imaging Spectrometer S-array X-ray 0.5–8 keV imaging spectroscopy, and NSF Karl G. Jansky Very Large Array radio 9.0 GHz imaging in its most extended A configuration. These comprehensive multiwavelength observations, when combined together, strongly suggest that all three nuclei are active galactic nuclei. While they are now still at kiloparsec-scale separations, where the host-galaxy gravitational potential dominates, the black holes may evolve into a bound triple system in ≲2 Gyr. These triple merger systems may explain the overly massive stellar cores that have been observed in some elliptical galaxies such as M87, which are expected to be unique gravitational wave sources. Similar systems may be more common in the early universe, when galaxy mergers are thought to have been more frequent.