We present an observational constraint for the typical active galactic nucleus (AGN) phase lifetime. The argument is based on the time lag between an AGN central engine switching on and becoming visible in X-rays, and the time the AGN then requires to photoionize a large fraction of the host galaxy. Based on the typical light travel time across massive galaxies, and the observed fraction of X-ray selected AGN without AGN-photoionized narrow lines, we estimate that the AGN phase typically lasts ∼ 10 5 years. This lifetime is short compared to the total growth time of 10 7 − 10 9 years estimated from e.g. the Soltan argument and implies that black holes grow via many such short bursts and that AGN therefore "flicker" on and off. We discuss some consequences of this flickering behavior for AGN feedback and the analogy of X-ray binaries and AGN lifecycles.

We present the first catalog and data release of the Swift-BAT AGN Spectroscopic Survey. We analyze optical spectra of the majority of the detected AGNs (77%, 642/836)based on their 14-195 keV emission in the 70-month Swift-BATall-sky catalog. This includes redshift determination, absorption and emission-line measurements, and black hole mass and accretion rate estimates for the majority of obscured and unobscured AGNs (74%, 473/642), with 340 measured for the first time. With ∼90% of sources at < z 0.2, the survey represents a significant advance in the census of hard X-ray-selected AGNs in the local universe. In this first catalog paper, we describe the spectroscopic observations and data sets, and our initial spectral analysis. The FWHMs of the emission lines show broad agreement with the X-ray obscuration (∼94%), such that Sy 1-1.8 have < N 10 H 21.9 cm −2 , and Seyfert 2 have > N 10 H 21.9 cm −2 . Seyfert 1.9, however, show a range of column densities. Compared to narrow-line AGNs in the SDSS, the X-ray-selected AGNs have a larger fraction of dusty host galaxies ( a b > H H 5), suggesting that these types of AGN are missed in optical surveys. Using the [O III] λ5007/Hβ and [N II] λ6583/Hα emission-line diagnostic, about half of the sources are classified as Seyferts; ∼15% reside in dusty galaxies that lack an Hβ detection, but for which the upper limits on line emission imply either a Seyfert or LINER,~15% are in galaxies with weak or no emission lines despite high-quality spectra, and a few percent each are LINERS, composite galaxies, H II regions, or in known beamed AGNs.

We investigated AGN activity in low-mass galaxies, an important regime that can shed light onto BH formation and evolution, and their interaction with their host galaxies. We identified 336 AGN candidates from a parent sample of ∼ 48, 000 nearby low-mass galaxies (M 10 9.5 M , z < 0.1) in the SDSS. We selected the AGN using the classical BPT diagram, a similar optical emission line diagnostic based on the He ii λ4686 line, and mid-IR color cuts. Different criteria select host galaxies with different physical properties such as stellar mass and optical color and only 3 out of 336 sources fulfill all three criteria. This could be in part due to selection biases. The resulting AGN fraction of ∼ 0.7% is at least one order of magnitude below the one estimated for more massive galaxies. At optical wavelengths, the He ii -based AGN selection appears to be more sensitive to AGN hosted in star-forming galaxies than the classical BPT diagram, at least in the low-mass regime. The archival X-ray and radio data available for some of the optically selected AGN candidates seem to confirm their AGN nature, but follow-up observations are needed to confirm the AGN nature of the rest of the sample, especially in the case of mid-IR selection. Our sample will be important for future follow-up studies aiming to understand the relation between BHs and host galaxies in the low-mass regime.

We consider the energy budgets and radiative history of eight fading AGN, identified from an energy shortfall between the requirements to ionize very extended (radius > 10 kpc) ionized clouds and the luminosity of the nucleus as we view it directly. All show evidence of significant fading on ≈ 50, 000-year timescales. We explore the use of minimum ionizing luminosity Q ion derived from photoionization balance in the brightest pixels in Hα at each projected radius. Tests using presumably constant Palomar-Green (PG) QSOs, and one of our targets with detailed photoionization modeling, suggest that we can derive useful histories of individual AGN, with the caveat that the minimum ionizing luminosity is always an underestimate and subject to uncertainties about fine structure in the ionized material. These consistency tests suggest that the degree of underestimation from the upper envelope of reconstructed Q ion values is roughly constant for a given object and therefore does not prevent such derivation. The AGN in our sample show a range of behaviors, with rapid drops and standstills; the common feature is a rapid drop in the last ≈ 2 × 10 4 years before the direct view of the nucleus. The e-folding timescales for ionizing luminosity are mostly in the thousands of years, with a few episodes as short as 400 years. In the limit of largely obscured AGN, we find additional evidence for fading from the shortfall between even the lower limits from recombination balance and the maximum luminosities derived from from infrared fluxes. We compare these long-term light curves, and the occurrence of these fading objects among all optically identified AGN, to simulations of AGN accretion; the strongest variations on these timespans are seen in models with strong and local (parsec-scale) feedback. We present Gemini integral-field optical spectroscopy, which shows a very limited role for outflows in these ionized structures. While rings and loops of emission, morphologically suggestive of outflow, are common, their kinematic structure shows some to be in regular rotation. UGC 7342 exhibits local signatures of outflows < 300 km s −1 , largely associated with very diffuse emission, and possibly entraining gas in one of the clouds seen in HST images. Only in the Teacup AGN do we see outflow signatures of order 1000 km s −1 . In contrast to the extended emission regions around many radio-loud AGN, the clouds around these fading AGN consist largely of tidal debris being externally illuminated but not displaced by AGN outflows.

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 ...