Variability-selected low luminosity AGNs in the SA57 and in the CDFS

The Astrophysical Journal

Changing look active-galatic-nuclei (CL AGNs) can yield considerable insight into accretion physics as well as the co-evolution of black holes and their host galaxies. A large sample of these CL AGNs is essential to achieve the latter goal. We propose an effective method to select CL candidates from spectroscopic quasar catalogs using the mid-infrared (MIR) variability information derived from ALL-WISE/NEOWISE data releases. Our primary selection criteria include both a large amplitude flux variation and a transition of MIR color from an AGN to a normal galaxy. A pilot spectroscopic followup of 7 candidates among about 300 candidates selected from Sloan Digital Sky Survey low-redshift (z<0.5) AGN sample results in 6 new turn-off CL AGNs. We expect to obtain hundreds of CL AGNs once full spectroscopic follow-up of the sample is carried out.

The Astrophysical Journal, 2011

We identify 85 variable galaxies in the GOODS North and South fields using 5 epochs of HST ACS V-band (F606W) images spanning 6 months. The variables are identified through significant flux changes in the galaxy's nucleus and represent ∼2% of the survey galaxies. With the aim of studying the active galaxy population in the GOODS fields, we compare the variability-selected sample with X-ray and mid-IR AGN candidates. Forty-nine percent of the variables are associated with X-ray sources identified in the 2Ms Chandra surveys. Twenty-four percent of X-ray sources likely to be AGN are optical variables and this percentage increases with decreasing hardness ratio of the X-ray emission. Stacking of the non-X-ray detected variables reveals marginally significant soft X-ray emission. Forty-eight percent of mid-IR power-law sources are optical variables, all but one of which are also X-ray detected. Thus, about half of the optical variables are associated with either X-ray or mid-IR power-law emission. The slope of the power-law fit through the Spitzer IRAC bands indicates that two-thirds of the variables have BLAGN-like SEDs. Among those galaxies spectroscopically identified as AGN, we observe variability in 74% of broad-line AGNs and 15% of NLAGNs. The variables are found in galaxies extending to z∼3.6. We compare the variable galaxy colors and magnitudes to the X-ray and mid-IR sample and find that the non-X-ray detected variable hosts extend to bluer colors and fainter intrinsic magnitudes. The variable AGN candidates have Eddington ratios similar to those of X-ray selected AGN.

The Astrophysical Journal Supplement Series, 2006

We identify active galactic nuclei (AGN) in the Groth-Westphal Survey Strip (GSS) using the independent and complementary selection techniques of optical spectroscopy and photometric variability. We discuss the X-ray properties of these AGN using Chandra/XMM data for this region. From a sample of 576 galaxies with high quality spectra we identify 31 galaxies with AGN signatures. Seven of these have broad emission lines (Type 1 AGNs). We also identify 26 galaxies displaying nuclear variability in HST WFPC2 images of the GSS separated by ∼7 years. The primary overlap of the two selected AGN samples is the set of broad-line AGNs, of which 80% appear as variable. Only a few narrow-line AGNs approach the variability threshold. The broad-line AGNs have an average redshift of <z>≃1.1 while the other spectroscopic AGNs have redshifts closer to the mean of the general galaxy population (<z>≃0.7). Eighty percent of the identified broad-line AGNs are detected in X-rays and these are among the most luminous X-ray sources in the GSS. Only one narrow-line AGN is X-ray detected. Of the variable nuclei galaxies within the X-ray survey, 27% are X-ray detected. We find that 1.9±0.6% of GSS galaxies to V gal =24 are broad-line AGNs, 1.4±0.5% are narrow-line AGNs, and 4.5±1.4% contain variable nuclei. The fraction of spectroscopically identified BLAGNs and NLAGNs at z∼1 reveals a marginally significant increase of 1.3±0.9% when compared to the local population.

Astronomy & Astrophysics, 2015

Context. One of the most peculiar characteristics of Active Galactic Nuclei (AGN) is their variability over all wavelengths. This property has been used in the past to select AGN samples and is foreseen to be one of the detection techniques applied in future multi-epoch surveys, complementing photometric and spectroscopic methods. Aims. In this paper, we aim to construct and characterise an AGN sample using a multi-epoch dataset in the r band from the SUDARE-VOICE survey. Methods. Our work makes use of the VST monitoring program of an area surrounding the Chandra Deep Field South to select variable sources. We use data spanning a six month period over an area of 2 square degrees, to identify AGN based on their photometric variability.

Annual Review of Astronomy and Astrophysics, 1997

Continuum and emission-line variability of active galactic nuclei provides a powerful probe of microarcsecond scale structures in the central regions of these sources. In this contribution, we review basic concepts and methodologies used in analyzing AGN variability. We develop from first principles the basics of reverberation mapping, and pay special attention to emission-line transfer functions. We discuss application of cross-correlation analysis to AGN light curves. Finally, we provide a short review of recent important results in the field.

Astrophysical Journal, 2008

We present the mid-infrared colors of X-ray-detected AGN and explore mid-infrared selection criteria. Using a statistical matching technique, the likelihood ratio, over 900 IRAC counterparts were identified with a new MUSYC X-ray source catalog that includes ∼ 1000 published X-ray sources in the Chandra Deep Field-South and Extended Chandra Deep Field-South. Most X-ray-selected AGN have IRAC spectral shapes consistent with power-law slopes, f ν ∝ ν α , and display a wide range of colors, −2 ≤ α ≤ 2. Although X-ray sources typically fit to redder (more negative α) power-laws than non-X-ray detected galaxies, more than 50% do have flat or blue (galaxy-like) spectral shapes in the observed 3-8 µm band. Only a quarter of the X-ray selected AGN detected at 24 µm are well fit by featureless red power laws in the observed 3.6-24 µm, likely the subset of our sample whose infrared spectra are dominated by emission from the central AGN region. Most IRAC color-selection criteria fail to identify the majority of X-ray-selected AGN, finding only the more luminous AGN, the majority of which have broad emission lines. In deep surveys, these color-selection criteria select 10-20% of the entire galaxy population and miss many moderate luminosity AGN.

Research in Astronomy and Astrophysics, 2009

Active galactic nuclei (AGN) in low surface brightness galaxies (LSBGs) have received little attention in previous studies. In this paper, we present detailed spectral analysis of 194 LSBGs from the Impey et al. (1996) APM LSBG sample which have been observed spectroscopically by the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). Our elaborate spectral analysis enables us to carry out, for the first time, reliable spectral classification of nuclear activities in LSBGs based on the standard emission line diagnostic diagrams in a rigorous way. Star-forming galaxies are common, as found in about 52% LSBGs. We find, contrary to some of the previous claims, that the fraction of galaxies containing an AGN is significantly lower than that found in nearby normal galaxies of high surface brightness. This is qualitatively in line with the finding of Impey et al. (2001). This result holds true even within each morphological type from Sa to Sc. LSBGs having larger central stellar velocity dispersions, or larger physical sizes, tend to have a higher chance to harbor an AGN. For three AGNs with broad emission lines, the black hole masses estimated from the emission lines are broadly consistent with the well known M-σ * relation established for normal galaxies and AGNs.

The Astrophysical Journal, 2008

We present an analysis of 109 moderate-luminosity (41.9 ≤ log L 0.5−8.0 keV ≤ 43.7) AGN in the Extended Chandra Deep Field-South survey, which is drawn from 5,549 galaxies from the COMBO-17 and GEMS surveys having 0.4 ≤ z ≤ 1.1. These obscured or optically-weak AGN facilitate the study of their host galaxies since the AGN provide an insubstantial amount of contamination to the galaxy light. We find that the color distribution of AGN host galaxies is highly dependent upon (1) the strong color-evolution of luminous (M V < −20.7) galaxies, and (2) the influence of ∼ 10 Mpc scale structures. When excluding galaxies within the redshift range 0.63 ≤ z ≤ 0.76, a regime dominated by sources in large-scale structures at z = 0.67 and z = 0.73, we observe a bimodality in the host galaxy colors. Galaxies hosting AGN at z 0.8 preferentially have bluer (rest-frame U − V < 0.7) colors than their z 0.6 counterparts (many of which fall along the red sequence). The fraction of galaxies hosting AGN peaks in the "green valley" (0.5 < U − V < 1.0); this is primarily due to enhanced AGN activity in the redshift interval 0.63 ≤ z ≤ 0.76. The AGN fraction in this redshift and color interval is 12.8% (compared to its 'field' value of 7.8%) and reaches a maximum of 14.8% at U − V ∼ 0.8. We further find that blue, bulge-dominated (Sérsic index n > 2.5) galaxies have the highest fraction of AGN (21%) in our sample. We explore the scenario that the evolution of AGN hosts is driven by galaxy mergers and illustrate that an accurate assessment requires a larger area survey since only three hosts may be undergoing a merger with timescales 1 Gyr following a starburst phase.

The Astrophysical Journal, 2014

In this work, we investigate the dependence of the covering factor (CF) of active galactic nuclei (AGNs) (i) on the mid-infrared (MIR) luminosity and (ii) on the redshift. We constructed 12-and 22-µm luminosity functions (LFs) at 0.006 ≤ z ≤ 0.3 using the Wide-field Infrared Survey Explorer (WISE) data. Combining the WISE catalog with the Sloan Digital Sky Survey (SDSS) spectroscopic data, we selected 223,982 galaxies at 12 µm and 25,721 galaxies at 22 µm for spectroscopic classification. We then identified 16,355 AGNs at 12 µm and 4,683

The Astrophysical Journal, 2012

Spitzer/IRAC selection is a powerful tool for identifying luminous active galactic nuclei (AGNs). For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGNs and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high-redshift star-forming galaxies selected via the BzK, distant red galaxy, Lyman-break galaxy, and submillimeter galaxy criteria. At QSO luminosities of log L 2-10 keV (erg s −1 ) 44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N H (cm −2 ) = 23.5 ± 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGNs, it is incomplete to low-luminosity and host-dominated AGNs.

The Astrophysical Journal, 2013

We discuss some practical aspects of measuring the variability amplitude of faint and distant active galactic nuclei (AGN), characterized by sparsely sampled lightcurves and low statistic. In such cases the excess variance, commonly used to estimate the intrinsic lightcurve variance, is affected by strong biases and uncertainties since it represents a maximum likelihood variability estimator only for identical/normal distributed measurements errors and uniform sampling. We performed realistic Monte Carlo simulations of AGN lightcurves, reproducing both the sampling pattern and measurement errors typical of multi-epoch deep surveys, such as the XMM-Newton observations of the Chandra Deep Field South (CDFS), or assuming different sampling patterns that may characterize long surveys with sub-optimal observing conditions. We used the results to estimate our ability to measure the intrinsic source variability as well as to constrain the observing strategy of future X-ray missions studying distant and/or faint AGN populations.

The Astrophysical Journal, 2009

We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared (IR) wavebands. We study a sample of 585 AGNs at 0.25 < z < 0.8 using redshifts from the AGN and Galaxy Evolution Survey (AGES). We select AGNs with observations in the radio at 1.4 GHz from the Westerbork Synthesis Radio Telescope, X-rays from the Chandra XBoötes Survey, and mid-IR from the Spitzer IRAC Shallow Survey. The radio, X-ray, and IR AGN samples show modest overlap, indicating that to the flux limits of the survey, they represent largely distinct classes of AGNs. We derive host galaxy colors and luminosities, as well as Eddington ratios, for obscured or optically faint AGNs. We also measure the two-point cross-correlation between AGNs and galaxies on scales of 0.3-10 h −1 Mpc, and derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly found in luminous red sequence galaxies, are strongly clustered (with M halo ∼ 3 × 10 13 h −1 M ⊙ ), and have very low Eddington ratios (λ 10 −3 ); (2) X-rayselected AGNs are preferentially found in galaxies that lie in the "green valley" of color-magnitude space and are clustered similar to typical AGES galaxies (M halo ∼ 10 13 h −1 M ⊙ ), with 10 −3 λ 1; (3) IR AGNs reside in slightly bluer, slightly less luminous galaxies than X-ray AGNs, are weakly clustered (M halo 10 12 h −1 M ⊙ ), and have λ > 10 −2 . We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ∼10 12 and 10 13 M ⊙ . After this event, star formation ceases and AGN accretion shifts from radiatively efficient (optical-and IR-bright) to radiatively inefficient (optically faint, radio-bright) modes.

Monthly Notices of the Royal Astronomical Society, 1999

We report the detection of optical and X-ray variability in the least luminous known Seyfert galaxy, NGC 4395. Between July 1996 and January 1997 the featureless continuum changed by a factor of 2, which is typical of more luminous AGN. The largest variation was seen at shorter wavelengths, so that the spectrum becomes 'harder' during higher activity states. During the same period the broad emission-line flux changed by ∼ 20 − 30 per cent. In a one week optical broad band monitoring program, a 20 per cent change was seen between successive nights. The difference in flux observed between the spectroscopy of July 1996 and the broad-band observations implies variation by a factor of 3 at 4400Å in just one month. In the same period, the spectral shape changed from a power law with spectral index α ∼ 0 (characteristic of quasars) to a spectral index α ∼ 2 (as observed in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray source coincident with the galactic nucleus. A change in X-ray flux by a factor ∼ 2 in 15 days has been observed. When compared with more luminous AGN, NGC 4395 appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data suggests that the spectrum could be absorbed. We also report the discovery of weak CaIIK absorption, suggesting the presence of a young stellar cluster providing of the order of 10% of the blue light. The stellar cluster may be directly observed as a diffuse component in HST optical imaging. Using HST UV archive data, together with the optical and X-ray observations, we examine the spectral energy distribution for NGC 4395 and discuss the physical conditions implied by the nuclear activity under the standard AGN model. When in the low state, the extrapolated UV continuum is insufficient to explain the observed broad emission-lines. This could be explained by intrinsic variability or absorption or may imply an extra heating source for the BLR. The observations can be explained by either an accreting massive black hole emitting at about 10 −3 L Edd or by a single old compact SNR with an age of 50 to 500 years generated by a small nuclear starburst.

2003

We conclude that the Type-1 AGN population found in the near-IR is not dramatically different from that found in optical samples. There is no evidence for a large population of AGNs that could not be found at optical wavelengths, though we can only place very weak constraints on any population of dusty high-redshift QSOs.

The Astrophysical Journal Supplement Series

A new, complete sample of 14,584 broad-line active galactic nuclei (AGNs) at z < 0.35 is presented, which are uncovered homogeneously from the complete database of galaxies and quasars observed spectroscopically in the Sloan Digital Sky Survey Seventh Data Release. The stellar continuum is properly removed for each spectrum with significant host absorption line features, and careful analyses of the emission line spectra, particularly in the Hα and Hβ wavebands, are carried out. The broad Balmer emission line, particularly Hα, is used to indicate the presence of an AGN. The broad Hα lines have luminosities in a range of 10 38.5-10 44.3 erg s −1 , and line widths (FWHMs) of 500-34,000 km s −1. The virial black hole masses, estimated from the broad-line measurements, span a range of 10 5.1-10 10.3 M , and the Eddington ratios vary from −3.3 to 1.3 in logarithmic scale. Other quantities such as multiwavelength photometric properties and flags denoting peculiar line profiles are also included in this catalog. We describe the construction of this catalog and briefly discuss its properties. The catalog is publicly available online. This homogeneously selected AGN catalog, along with the accurately measured spectral parameters, provides the most updated, largest AGN sample data, which will enable further comprehensive investigations of the properties of the AGN population in the low-redshift universe.