The Host Galaxies and Narrow Line Regions of Four Double-Peaked [OIII] AGN

Major gas-rich mergers of galaxies are expected to play an important role in triggering and fuelling luminous AGN. We present deep multi-band (u/r/z) imaging and long slit spectroscopy of four double-peaked [OIII] emitting AGN, a class of objects associated with either kcp-separated binary AGN or final stage major mergers, though AGN with complex narrow-line regions are known contaminants. Such objects are of interest since they represent the onset of AGN activity during the merger process. Three of the objects studied have been confirmed as major mergers using near-infrared imaging, one is a confirmed X-ray binary AGN. All AGN are luminous and have redshifts of 0.1 < z < 0.4. Deep r-band images show that a majority (3/4) of the sources have disturbed host morphologies and tidal features, while the remaining source is morphologically undisturbed down to low surface brightness limits. The lack of morphological disturbances in this galaxy despite the fact that is is a close binary AGN suggests that the merger of a binary black hole can take longer than ~1 Gyr. The narrow line regions (NLRs) have large sizes (10 kpc < r < 100 kpc) and consist of compact clumps with considerable relative velocities (~ 200-650 km/s). We detect broad, predominantly blue, wings with velocities up to ~1500 km/s in [OIII], indicative of powerful outflows. The outflows are compact (<5 kpc) and co-spatial with nuclear regions showing considerable reddening, consistent with enhanced star formation. One source shows an offset between gas and stellar kinematics, consistent with either a bipolar flow or a counter-rotating gas disk. We are not able to unambiguously identify the sources as binary AGN using our data, X-ray or radio data is required for an unambiguous identification. However, the data still yield interesting results for merger triggering of AGN and time-scales of binary black hole mergers.Comment: Accepted for publication in The Astronomical Journal, abstract abridge

Variability and stability in optical blazar jets: the case of OJ287

OJ287 is a BL Lac object at redshift z=0.306 that has shown double-peaked bursts at regular intervals of ~12 yr during the last ~ 40 yr. Due to this behavior, it has been suggested that OJ287 might host a close supermassive binary black hole. We present optical photopolarimetric monitoring data from 2005-2009, during which the latest double-peaked outburst occurred. We find a stable component in the optical jet: the optical polarization core. The optical polarization indicates that the magnetic field is oriented parallel to the jet. Using historical optical polarization data, we trace the evolution of the optical polarization core and find that it has showed a swing in the Stokes plane indicating a reorientation of the jet magnetic field. We also find that changes in the optical jet magnetic field seem tightly related to the double-peaked bursts. We use our findings as a new constraint on possible binary black hole models. Combining all available observations, we find that none of the proposed binary black bole models is able to fully explain the observations. We suggest a new approach to understanding OJ287 that is based on the assumption that changes in the jet magnetic field drive the regular outbursts.Comment: To appear in the proceedings of the meeting "Steady and Transient Jets", held in Bonn, Germany (7-8 April 2010

A new extensive catalog of optically variable AGN in the GOODS Fields and a new statistical approach to variability selection

Variability is a property shared by practically all AGN. This makes variability selection a possible technique for identifying AGN. Given that variability selection makes no prior assumption about spectral properties, it is a powerful technique for detecting both low-luminosity AGN in which the host galaxy emission is dominating and AGN with unusual spectral properties. In this paper, we will discuss and test different statistical methods for the detection of variability in sparsely sampled data that allow full control over the false positive rates. We will apply these methods to the GOODS North and South fields and present a catalog of variable sources in the z band in both GOODS fields. Out of 11931 objects checked, we find 155 variable sources at a significance level of 99.9%, corresponding to about 1.3% of all objects. After rejection of stars and supernovae, 139 variability selected AGN remain. Their magnitudes reach down as faint as 25.5 mag in z. Spectroscopic redshifts are available for 22 of the variability selected AGN, ranging from 0.046 to 3.7. The absolute magnitudes in the rest-frame z-band range from ~ -18 to -24, reaching substantially fainter than the typical luminosities probed by traditional X-ray and spectroscopic AGN selection in these fields. Therefore, this is a powerful technique for future exploration of the evolution of the faint end of the AGN luminosity function up to high redshifts.Comment: Accepted for publication in The Astrophysical Journal (version 2: minor changes to text after receiving comments

Variability in Active Galactic Nuclei: Understanding Emission Mechanisms and Unification Models

Siirretty Doriast

A complete catalogue of merger fractions in AGN hosts: No evidence for an increase in detected merger fraction with AGN luminosity

Despite the importance of Active Galactic Nuclei (AGN) in galaxy evolution, the mechanisms that fuel AGN activity remain poorly understood. Theoretical models suggest that major mergers of galaxies contribute strongly to AGN fuelling, particularly at high AGN luminosities. The connection between mergers and AGN activity has therefore been widely studied, although with contradictory results. Some studies find a strong connection between mergers and AGN, while others find merger fractions in AGN hosts to match those in the inactive galaxy population. To address these apparent contradictions, I present a complete and systematic analysis of detected merger fractions in AGN hosts from the literature. I assess if discrepancies between studies are indicative of systematic uncertainties and biases and analyse the detected merger fraction as a function of luminosity, redshift, and AGN selection method. X-ray selected AGN samples show comparable detected merger fractions across studies and major mergers do not dominate triggering in this AGN population. On the other hand, signatures of significant merger contribution to the AGN population are observed in a small fraction of primarily radio selected and reddened AGN samples. It is unclear if this is due to observational biases or physical differences in the host galaxies. There is no correlation between the detected merger fraction and AGN luminosity. This lack of correlation between detected merger fraction and AGN luminosity, which has previously been reported in the literature, cannot be explained by systematic uncertainties and observational biases.Comment: accepted for publication in The Open Journal of Astrophysic

A systematic search for changing-look quasars in SDSS-II using difference spectra

Context. ‘Changing-look quasars’ (CLQs) are active galactic nuclei (AGN) showing extreme variability that results in a transition from type 1 to type 2 AGN. The short timescales of these transitions present a challenge to the unified model of AGN and the physical processes causing these transitions remain poorly understood. CLQs also provide interesting samples for the study of AGN host galaxies since the central emission disappears almost entirely. Aims. Previous searches for CLQs have utilised photometric variability or SDSS classification changes to systematically identify CLQs; this approach may miss lower luminosity CLQs. In this paper, we aim to use spectroscopic data to asses if analysis difference spectra can be used to detect further CLQs that have been missed by photometric searches. Methods. We searched SDSS-II DR 7 repeat spectra for sources that exhibit either a disappearance or appearance of both broad line emission and accretion disc continuum emission by directly analysing the difference spectrum between two epochs of observation. Results. From a sample of 24 782 objects with difference spectra, our search yielded six CLQs within the redshift range 0.1 ⩽ z ≤ 0.3, including four newly identified sources. Spectral analysis indicates that changes in the accretion rate can explain the changing-look behaviour. While a change in dust extinction fits the changes in the spectral shape, the timescales of the changes observed are too short for obscuration from torus clouds. Conclusions. Using difference spectra was shown to be an effective and sensitive way to detect CLQs. We recover CLQs an order of magnitude lower in luminosities than those found by photometric searches and achieve higher completeness than spectroscopic searches relying on pipeline classification

Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical imaging

We present the results of deep I-band imaging of two BL Lacerate objects, RGB 0136+391 and PKS 0735+178, during an epoch when the optical nucleus was in a faint state in both targets. In PKS 0735+178 we find a significant excess over a point source, which, if fitted by the de Vaucouleurs model, corresponds to a galaxy with I = 18.64 +- 0.11 and r_eff = 1.8 +- 0.4 arcsec. Interpreting this galaxy as the host galaxy of PKS 0735+178 we derive z = 0.45 +- 0.06 using the host galaxy as a "standard candle". We also discuss the immediate optical environment of PKS 0735+178 and the identity of the MgII absorber at z = 0.424. Despite of the optimally chosen epoch and deep imaging we find the surface brightness profile of RGB 0136+391 to be consistent with a point source. By determining a lower limit for the host galaxy brightness by simulations, we derive z > 0.40 for this target.Comment: 6 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

Morphologies of z ∼ 0.7 AGN host galaxies in CANDELS: no trend of merger incidence with AGN luminosity

The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z < 0.8 and have luminosities 41 ≲ log (L_X [erg s^(−1)]) ≲ 44.5. ‘Fake AGN’ are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ∼950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for <6 per cent of all AGN in our sample as well as <40 per cent of the highest luminosity AGN (log  (L_X [erg s^(−1)]) ∼ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible

The SEDs, Host Galaxies and Environments of Variability Selected AGN in GOODS-S

Variability selection has been proposed as a powerful tool for identifying both low-luminosity AGN and those with unusual SEDs. However, a systematic study of sources selected in such a way has been lacking. In this paper, we present the multi-wavelength properties of the variability selected AGN in GOODS South. We demonstrate that variability selection indeed reliably identifies AGN, predominantly of low luminosity. We find contamination from stars as well as a very small sample of sources that show no sign of AGN activity, their number is consistent with the expected false positive rate. We also study the host galaxies and environments of the AGN in the sample. Disturbed host morphologies are relatively common. The host galaxies span a wide range in the level of ongoing star-formation. However, massive star-bursts are only present in the hosts of the most luminous AGN in the sample. There is no clear environmental preference for the AGN sample in general but we find that the most luminous AGN on average avoid dense regions while some low-luminosity AGN hosted by late-type galaxies are found near the centres of groups. AGN in our sample have closer nearest neighbours than the general galaxy population. We find no indications that major mergers are a dominant triggering process for the moderate to low luminosity AGN in this sample. The environments and host galaxy properties instead suggest secular processes, in particular tidal processes at first passage and minor mergers, as likely triggers for the objects studied. This study demonstrates the strength of variability selection for AGN and gives first hints at possibly triggering mechanisms for high-redshift low luminosity AGN.Comment: Accepted for publication in MNRAS (missing references added in version 2