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 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

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

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

Accretion disc time lag distributions: applying CREAM to simulated AGN light curves

DAS acknowledges the support of the Science and Technologies Funding Council studentship. KH acknowledges support from the UK Science and Technology Facilities Council (STFC) consolidated grant to St Andrews (ST/M001296/1).Active galactic nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet-optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (Continuum REprocessed AGN Markov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination.We test CREAM's ability to recover both inclination and product of black hole mass and accretion rate MM˙ , and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 s exposures of a 17th magnitude AGN with a 2-m telescope in Sloan g and i bands with Signal-to-Noise Ratio (SNR) of 500-900 depending on the filter and lunar phase.We also test CREAM on poorer quality g and i light curves with SNR = 100. We find in the high-SNR case that CREAM can recover the accretion disc inclination to within an uncertainty of 5° and an MM˙to within 0.04 dex.Publisher PDFPeer reviewe

ECOSMOG: An Efficient Code for Simulating Modified Gravity

We introduce a new code, ECOSMOG, to run N-body simulations for a wide class of modified gravity and dynamical dark energy theories. These theories generally have one or more new dynamical degrees of freedom, the dynamics of which are governed by their (usually rather nonlinear) equations of motion. Solving these non-linear equations has been a great challenge in cosmology. Our code is based on the RAMSES code, which solves the Poisson equation on adaptively refined meshes to gain high resolutions in the high-density regions. We have added a solver for the extra degree(s) of freedom and performed numerous tests for the f(R) gravity model as an example to show its reliability. We find that much higher efficiency could be achieved compared with other existing mesh/grid-based codes thanks to two new features of the present code: (1) the efficient parallelisation and (2) the usage of the multigrid relaxation to solve the extra equation(s) on both the regular domain grid and refinements, giving much faster convergence even under much more stringent convergence criteria. This code is designed for performing high-accuracy, high-resolution and large-volume cosmological simulations for modified gravity and general dark energy theories, which can be utilised to test gravity and the dark energy hypothesis using the upcoming and future deep and high-resolution galaxy surveys.Comment: 17 pages, 12 figures, 1 tabl