A population of luminous accreting black holes with hidden mergers

Abstract

Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black-holes 1 . However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars 2, 3 and other studies showing no such association 4, 5 . Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated 6-8 (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated 9 (less than 3 kiloparsecs). Resolving this final stage requires a combination of highspatial-resolution infrared imaging and high-sensitivity hard-Xray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations 10 . Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive black-hole mergers. We find that luminous obscured black holes (bolometric luminosity higher than 2 × 10 44 ergs per second) show a significant (P < 0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage. As the parent population of supermassive black hole mergers, the study of nuclear mergers can provide crucial benchmarks for models of black-hole inspiral and gravitational-wave signals.The Burst Alert Telescope (BAT) on the Neil Gehrels Swif t Observatory has surveyed the entire sky at unprecedented depths in the ultra-hard X-ray band (14-195 keV) and primarily detects accretion on to supermassive black holes (SMBHs) at the centres of nearby galaxies.Detection in the ultra-hard X-ray band is possible even when obscuring gas and dust in the host galaxy significantly attenuates the ultraviolet, optical, and/or softer X-ray emission around the growing black holes. At the distance to the nearest luminous AGN (about 220 Mpc or z≈0.05) ground-based optical imaging typically achieves a resolution of order 1 or 1 kpc in the host galaxy. This spatial resolution is not sufficient to resolve the final merger stage in the host galaxies down to the 100s of pc scales. However these can be resolved with near-infrared adaptive optics, which provide an improvement by a factor of 10 in spatial resolution (about 0.1 ). These scales are still above the black hole sphere of influence, which is of the order 10-100 pc for black holes with masses of 10 7 − 10 9 ...

Galaxy mergers in eagle do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN

“…Encouragingly, studies have shown that post-merger features could have observability time-scales of ≈ 0.2-0.4 Gyr (e.g., Lotz et al 2010;Ji et al 2014), which would mean that the peak excess of BH activity that arises ≈ 300 Myr after the coalescence of the two galaxies found in Figure 8 at z < 1 could be captured, and therefore would be correctly attributed to a post-merger system. Indeed, some observational results have reported that the largest excess of AGN activity has been found in post-merger systems (e.g., Ellison et al 2013;Koss et al 2018), in agreement with the findings of this study.…”

“…When trying to investigate correlations over a wide dynamic range of AGN luminosities, the small samples sizes of many of these studies can be particularly restrictive. More fundamentally, dust obscured AGN in merging systems may be missed entirely in surveys that only focus on shorter wavelengths (e.g., Goulding & Alexander 2009;Weston et al 2017;Koss et al 2018), indicating that surveys in the infrared and rest frame hard x-rays may be the most effective measure of AGN selection (e.g., Brandt & Alexander 2015). Perhaps most crucially, the process of identifying merging systems through morphological disturbances or asymmetry is especially challenging, and often done by eye (however this process is becoming increasingly automated with improving success, e.g., Pawlik et al 2016;Goulding et al 2018;Bottrell et al 2019).…”

Resolving the Interstellar Medium in Ultraluminous Infrared QSO Hosts with ALMA

“…The diverse host galaxy properties revealed in our observations show evidence that the merging between luminous QSOs and companion galaxies is still ongoing in some of IR QSO systems. This is unexpected since the objects in IR QSO phase are typically characterized by the final coalescence of the galaxies, according to the theoretical models of merger-driven evolutionary sequence (e.g., Sanders et al 1988;Barnes & Hernquist 1992;Hopkins et al 2008;Narayanan et al 2010), but consistent with recent discovery of a population of late-stage nuclear mergers with separation < 3 kpc in obscured luminous black holes by high-resolution near-IR observations (Koss et al 2018). We caution, however, given that our classification of IR QSOs is based on a small sample with only nine CO-mapped sources, it is thus a first-order kinematic classification and may be too simplistic.…”

NGC 6240: A triple nucleus system in the advanced or final state of merging

“…Of these systems, the most interesting ones are those in an advanced state of merging (i.e., those with the smallest distances between their SMBHs). The closest known optical or infrared distances of two nuclei correspond to a projected separation of ∼ one kiloparsec, except for only one example, MCG+02-21-013, with a projected separation of 300 pc (see Table 1 in Koss et al 2018). Arp 220, which is the nearest ultraluminous infrared galaxy (ULIRG), has two near-IR nuclei at a separation of 330 pc (Genzel et al 2001, and references therein).…”