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Register a new userThe first high-redshift changing-look quasars
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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$5% in Eddington luminosity.
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If the disappearance of the broad emission lines observed in changing-look quasars originates from the obscuration of the quasar core by dusty clouds moving in the torus, high linear optical polarization would be expected in those objects. We then measured the rest-frame UV-blue linear polarization of a sample of 13 changing-look quasars, 7 of them being in a type 1.9-2 state. For all quasars but one the polarization degree is lower than 1%. This suggests that the disappearance of the broad emission lines cannot be attributed to dust obscuration, and supports the scenario in which changes of look are caused by a change in the rate of accretion onto the supermassive black hole. Such low polarization degrees also indicate that these quasars are seen under inclinations close to the system axis. One type 1.9-2 quasar in our sample shows a high polarization degree of 6.8%. While this polarization could be ascribed to obscuration by a moving dusty cloud, we argue that this is unlikely given the very long time needed for a cloud from the torus to eclipse the broad emission line region of that object. We propose that the high polarization is due to the echo of a past bright phase seen in polar-scattered light. This interpretation raises the possibility that broad emission lines observed in the polarized light of some type 2 active galactic nuclei can be echoes of past type 1 phases and not evidence of hidden broad emission line regions.
Changing-look quasars are a newly-discovered class of luminous active galactic nuclei that undergo rapid ($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.
We present a systematic search for changing-look quasars based on repeat photometry from SDSS and Pan-STARRS1, along with repeat spectra from SDSS and SDSS-III BOSS. Objects with large, |Delta g|>1 mag photometric variations in their light curves are selected as candidates to look for changes in broad emission line (BEL) features. Out of a sample of 1011 objects that satisfy our selection criteria and have more than one epoch of spectroscopy, we find 10 examples of quasars that have variable and/or changing-look BEL features. Four of our objects have emerging BELs; five have disappearing BELs, and one object shows tentative evidence for having both emerging and disappearing BELs. With redshifts in the range 0.20 < z < 0.63, this sample includes the highest-redshift changing-look quasars discovered to date. We highlight the quasar J102152.34+464515.6 at z = 0.204. Here, not only have the Balmer emission lines strongly diminished in prominence, including H$beta$ all but disappearing, but the blue continuum $f_{ u} propto u^{1/3}$ typical of an AGN is also significantly diminished in the second epoch of spectroscopy. Using our selection criteria, we estimate that >15% of strongly variable luminous quasars display changing-look BEL features on rest-frame timescales of 8 to 10 years. Plausible timescales for variable dust extinction are factors of 2-10 too long to explain the dimming and brightening in these sources, and simple dust reddening models cannot reproduce the BEL changes. On the other hand, an advancement such as disk reprocessing is needed if the observed variations are due to accretion rate changes.
Active galactic nuclei (AGN) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as changing-look AGN, or at higher luminosities changing look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a comprehensive search for CLQs across the full SDSS footprint using spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary selection requires large-amplitude (|Delta g|>1 mag, |Delta r|>0.5 mag) variability over any of the available time baselines probed by the SDSS and Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to verify variability behavior in CLQ candidates where available, and confirm CLQs using optical spectroscopy from the William Herschel, MMT, Magellan, and Palomar telescopes. For our adopted S/N threshold on variability of broad Hbeta emission, we find 17 new CLQs, yielding a confirmation rate of >~ 20%. These candidates are at lower Eddington ratio relative to the overall quasar population which supports a disk-wind model for the broad line region. Based on our sample, the CLQ fraction increases from 10% to roughly half as the continuum flux ratio between repeat spectra at 3420 Angstroms increases from 1.5 to 6. We release a catalog of over 200 highly variable candidates to facilitate future CLQ searches.
Context:Quasars radiating at extreme Eddington ratios (xA) are likely a prime mover of galactic evolution and have been hailed as potential distance indicators. Their properties are still scarcely known. Aims:We test the effectiveness of the selection criteria defined on the 4D Eigenvector 1 (4DE1) for identifying xA sources. We provide a quantitative description of their UV spectra in the redshift range 2<z<2.9. Methods:19 extreme quasar candidates were identified using 4DE1 selection criteria applied to SDSS spectra: AlIII1860/SiIII]1892>0.5 and CIII]1909/SiIII]1892<1. The emission line spectra was studied using multicomponent fits of deep spectroscopic observations obtained with the OSIRIS-GTC. Results:Spectra confirm that almost all of these quasars are xA sources with very similar properties. We provide spectrophotometric and line profile measurements for the SiIV1397+OIV]1402, CIV1549+HeII1640, and the 1900A blend composed by AlIII1860, SiIII]1892, FeIII and a weak CIII]1909. The spectra can be characterized as very low ionization (logU~-3), a condition that explains the significant FeIII emission. CIV1549 shows low equivalent width (<30 A for the most sources), and high or extreme blueshift amplitudes (-5000<c(1/2)<-1000 kms-1). Weak-lined quasars appear as extreme xA quasars and not as an independent class. The CIV1549 high amplitude blueshifts coexists in all cases save one with symmetric and narrower AlIII and SiIII] profiles. Estimates of the Eddington ratio using the AlIII FWHM as a virial broadening estimator are consistent with the ones of a previous xA sample. Conclusions:It is now feasible to assemble large samples of xA quasars from the latest data releases of the SDSS. We provide evidence that AlIII1860 could be associated with a low-ionization virialized sub-system, supporting previous suggestions that AlIII is a reliable virial broadening estimator.
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