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Register a new userGemini Imaging of the Host Galaxies of Changing-Look Quasars
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Changing-look quasars are a newly-discovered class of luminous active galactic nuclei that undergo rapid ($lesssim$10 year) transitions between Type 1 and Type 1.9/2, with an associated change in their continuum emission. We characterize the host galaxies of four faded changing-look quasars using broadband optical imaging. We use textit{gri} images obtained with the Gemini Multi Object Spectrograph (GMOS) on Gemini North to characterize the surface brightness profiles of the quasar hosts and search for [O III] $lambda4959,lambda5007$ emission from spatially extended regions, or voorwerpjes, with the goal of using them to examine past luminosity history. Although we do not detect, voorwerpjes surrounding the four quasar host galaxies, we take advantage of the dim nuclear emission to characterize the colors and morphologies of the host galaxies. Three of the four galaxies show morphological evidence of merger activity or tidal features in their residuals. The three galaxies which are not highly distorted are fit with a single Sersic profile to characterize their overall surface brightness profiles. The single-Sersic fits give intermediate Sersic indices between the $n=1$ of disk galaxies and the $n=4$ of ellipticals. On a color-magnitude diagram, our changing-look quasar host galaxies reside in the blue cloud, with other AGN host galaxies and star-forming galaxies. On a color-Sersic index diagram the changing-look quasar hosts reside with other AGN hosts in the green valley. Our analysis suggests that the hosts of changing-look quasars are predominantly disrupted or merging galaxies that resemble AGN hosts, rather than inactive galaxies.
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If the disappearance of the broad emission lines observed in changing-look quasars originates from the obscuration of the quasar core by dusty clouds moving in the torus, high linear optical polarization would be expected in those objects. We then measured the rest-frame UV-blue linear polarization of a sample of 13 changing-look quasars, 7 of them being in a type 1.9-2 state. For all quasars but one the polarization degree is lower than 1%. This suggests that the disappearance of the broad emission lines cannot be attributed to dust obscuration, and supports the scenario in which changes of look are caused by a change in the rate of accretion onto the supermassive black hole. Such low polarization degrees also indicate that these quasars are seen under inclinations close to the system axis. One type 1.9-2 quasar in our sample shows a high polarization degree of 6.8%. While this polarization could be ascribed to obscuration by a moving dusty cloud, we argue that this is unlikely given the very long time needed for a cloud from the torus to eclipse the broad emission line region of that object. We propose that the high polarization is due to the echo of a past bright phase seen in polar-scattered light. This interpretation raises the possibility that broad emission lines observed in the polarized light of some type 2 active galactic nuclei can be echoes of past type 1 phases and not evidence of hidden broad emission line regions.
We report on three redshift $z>2$ quasars with dramatic changes in their C IV emission lines, the first sample of changing-look quasars (CLQs) at high redshift. This is also the first time the changing-look behaviour has been seen in a high-ionisation emission line. SDSS J1205+3422, J1638+2827, and J2228+2201 show interesting behaviour in their observed optical light curves, and subsequent spectroscopy shows significant changes in the C IV broad emission line, with both line collapse and emergence being displayed on rest-frame timescales of $sim$240-1640 days. These are rapid changes, especially when considering virial black hole mass estimates of $M_{rm BH} > 10^{9} M_{odot}$ for all three quasars. Continuum and emission line measurements from the three quasars show changes in the continuum-equivalent width plane with the CLQs seen to be on the edge of the full population distribution, and showing indications of an intrinsic Baldwin effect. We put these observations in context with recent state-change models, and note that even in their observed low-state, the C IV CLQs are generally above $sim$5% in Eddington luminosity.
We study the properties of the host galaxies of Changing-Look Active Galactic Nuclei (CL AGNs) with the aim of understanding the conditions responsible for triggering CL activity. We find that CL AGN hosts primarily reside in the so-called green valley that is located between spiral-like star-forming galaxies and dead ellipticals, implying that CL AGNs are activated during distinct periods of quenching and galaxy transformation processes. CL AGN hosts have low galaxy asymmetry indicators, suggesting that secular evolutionary processes (the influence of bars and spirals, and possibly minor mergers) might be the primary mechanism for transporting gas to the vicinity of the supermassive black hole (SMBH) rather than major mergers. Similar to tidal disruption events (TDEs) and highly variable AGNs, we find that CL AGN hosts are associated with SMBHs residing in high density pseudo-bulges and appear to overlap most significantly with the population of low-ionization nuclear emission-line region (LINER) galaxies. As such, CL AGN are likely fueled by strong episodic bursts of accretion activity, which appear to take place preferentially as the amount of material accessible for star formation and accretion dwindles. We also identify that CL AGN hosts are characterized by either large Sersic indices or high bulge fractions, which suggests a simple metric for identifying candidates for spectroscopic follow-up observations in forthcoming synoptic surveys.
We study the utility of broad-band colours in the SkyMapper Southern Survey for selecting Seyfert galaxies at low luminosity. We find that the $u-v$ index, built from the ultraviolet $u$ and violet $v$ filters, separates normal galaxies, starburst galaxies and type-1 AGN. This $u-v$ index is not sensitive to age or metallicity in a stellar population but is instead a quenching-and-bursting indicator in galaxies and detects power-law continua in type-1 AGN. Using over 25,000 galaxies at $z<0.1$ from 6dFGS, we find a selection cut based on $u-v$ and central $u$ band brightness that identifies type-1 AGN. By eyeballing 6dFGS spectra we classify new Seyfert galaxies of type 1 to 1.8. Our sample includes eight known Changing-Look AGN, two of which show such strong variability that they move across the selection cut during the five years of SkyMapper observations in DR3, along mixing sequences of nuclear and host galaxy light. We identify 46 Changing-Look AGN candidates in our sample, one of which has been reported as a type-IIn supernova. We show that this transient persists for at least five years and marks a flare in a Seyfert-1 period of a new Changing-Look AGN.
Changing-look phenomenon observed now in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. We propose a simple explanation for periodic outbursts in sources operating at a few per cent of the Eddington limit. The mechanism is based on two relatively well understood phenomena: radiation pressure instability and formation of the inner optically thin Advection-Dominated Accretion Flow. The limit cycle behaviour takes place in a relatively narrow transition zone between the standard disk and optically thin flow. Large changes in the cold disk are due to the irradiation by the hot flow with accretion rate strongly varying during the cycle. The model gives quantitative predictions and works well for multiple outbursts of NGC 1566.
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