Characterization of active galactic nuclei through photometric and spectroscopic variability

Nowadays we generally accept the idea that the emission from active galactic nuclei (AGNs) is powered by an accreting supermassive black hole (SMBH) at the center of a galaxy. It is also well known that most -probably all- galaxies go through an active phase at some point in their lives, and that several empirical relations connect black hole and host galaxy properties: this suggests that a tight feedback between the evolution of the black hole and the host galaxy exists; hence, in order to go deeper into galaxy evolution, it is crucial for us to learn more about the formation and evolution of the black holes residing in their centers. Variability is a defining property of AGN emission at all wavebands, and concerns both continuum and broad-line emission. It is generally attributed to instabilities in the AGN accretion disk, together with changes in the accretion rate. Since the extent of variations in different wavelength ranges is not the same, variability measurements can help understand the underlying emission mechanism, constraining the size and structure of the emitting region. The present work investigates AGN variability from two different perspectives. The first part of the project tests the efficiency of optical variability as a tool to select AGNs, since optical continuum variability seems to be a universal feature of broad-line AGNs on timescales from months to years, with variations generally ranging from 1% to 10% of the magnitude, but also much larger (50% of the magnitude) in some cases. Testing techniques for AGN identification based on data from ground-based telescopes is of great relevance in the framework of current and future wide-field surveys (e.g., Dark Energy Survey, Large Synoptic Survey Telescope), since we will need reliable methods to detect and classify the wealth of sources they will provide. The second part of the project investigates the variability of broad absorption lines (BALs) in quasi-stellar object (QSO) spectra, in order to deeply understand the physics and structure of AGNs. BALs originate from outflowing winds along our line of sight; winds are thought to originate from the accretion disk, in the very proximity of the central SMBH; we generally think that they are responsible for a triggering of the accretion mechanism onto the SMBH, as they remove angular momentum from the disk and, since they evacuate gas from the host galaxy, they also play a leading role into galaxy evolution. Several works show that BAL equivalent widths can change on typical timescales from months to years. Such variability is generally attributed to changes in the covering factor (due to rotation and/or changes in the wind structure) and/or in the ionization level. We investigate BAL variability, focusing on BAL disappearance, in a sample of more than 1500 QSOs -the largest sample ever used for such an analysis- to gain insight into the structure and co-evolution of the SMBH and the host galaxy

A structure function analysis of vst-cosmos agn

We present our sixth work in a series dedicated to variability studies of active galactic nuclei (AGN), based on the survey of the COSMOS field by the VLT Survey Telescope (VST). Its 54 r-band visits over 3.3 yr and single-visit depth of 24.6 r-band mag make this dataset a valuable scaled-down version that can help forecast the performance of the Rubin Observatory Legacy Survey of Space and Time (LSST).This work is centered on the analysis of the structure function (SF) of VST-COSMOS AGN, investigating possible differences in its shape and slope related to how the AGN were selected, and explores possible connections between the AGN ensemble variability and the black-hole mass, accretion rate, bolometric luminosity, redshift, and obscuration of the source

C IV Broad Absorption Line Variability in QSO Spectra from SDSS Surveys

Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs) are thought to arise from outflowing winds along our line of sight; winds, in turn, are thought to originate from the accretion disk, in the very surroundings of the central supermassive black hole (SMBH), and they likely affect the accretion process onto the SMBH, as well as galaxy evolution.\\ BALs can exhibit variability on timescales typically ranging from months to years. We analyze such variability and, in particular, BAL disappearance, with the aim of investigating QSO physics and structure.\\ We search for disappearing C {\scriptsize{IV}} BALs in the spectra of 1319 QSOs from different programs from the Sloan Digital Sky Survey (SDSS); the analyzed time span covers $0.28-4.9$ yr (rest frame), and the source redshifts are in the range $1.68-4.27$. This is to date the largest sample ever used for such a study.\\ We find 67 sources ($5.1_{-0.6}^{+0.7}$\% of the sample) with 73 disappearing BALs in total ($3.9_{-0.5}^{+0.5}$\% of the total number of C {\scriptsize{IV}} BALs detected; \textbf{some sources have more than one BAL that disappears}). We compare the sample of disappearing BALs to the whole sample of BALs, and investigate the correlation in the variability of multiple troughs in the same spectrum. We also derive estimates of the average lifetime of a BAL trough and of the BAL phase along our line of sight

Weak Lensing Study in VOICE Survey II: Shear Bias Calibrations

The VST Optical Imaging of the CDFS and ES1 Fields (VOICE) Survey is proposed to obtain deep optical $ugri$ imaging of the CDFS and ES1 fields using the VLT Survey Telescope (VST). At present, the observations for the CDFS field have been completed, and comprise in total about 4.9 deg$^2$ down to $r_\mathrm{AB}$$\sim$26 mag. In the companion paper by Fu et al. (2018), we present the weak lensing shear measurements for $r$-band images with seeing $\le$ 0.9 arcsec. In this paper, we perform image simulations to calibrate possible biases of the measured shear signals. Statistically, the properties of the simulated point spread function (PSF) and galaxies show good agreements with those of observations. The multiplicative bias is calibrated to reach an accuracy of $\sim$3.0%. We study the bias sensitivities to the undetected faint galaxies and to the neighboring galaxies. We find that undetected galaxies contribute to the multiplicative bias at the level of $\sim$0.3%. Further analysis shows that galaxies with lower signal-to-noise ratio (SNR) are impacted more significantly because the undetected galaxies skew the background noise distribution. For the neighboring galaxies, we find that although most have been rejected in the shape measurement procedure, about one third of them still remain in the final shear sample. They show a larger ellipticity dispersion and contribute to $\sim$0.2% of the multiplicative bias. Such a bias can be removed by further eliminating these neighboring galaxies. But the effective number density of the galaxies can be reduced considerably. Therefore efficient methods should be developed for future weak lensing deep surveys.Comment: 11 pages, 13 figures, 2 tables. MNRAS accepte

The VST Early-type GAlaxy Survey: Exploring the Outskirts and Intra-cluster Regions of Galaxies in the Low-surface-brightness Regime

The VST Early-type GAlaxy Survey (VEGAS) is a deep, multi-band (u, g, r, i) imaging survey, carried out with the 2.6-metre VLT Survey Telescope (VST) at ESO's Paranal Observatory in Chile. VEGAS combines the wide (1-square-degree) OmegaCAM imager and long integration times, together with a specially designed observing strategy. It has proven to be a gold mine for studies of features at very low surface brightness, down to levels of mu_g~27-30 magnitudes arcsec^(-2), over 5-8 magnitudes fainter than the dark sky at Paranal. In this article we highlight the main science results obtained with VEGAS observations of galaxies across different environments, from dense clusters of galaxies to unexplored poor groups and in the field.Comment: Published in The Messenger, vol. 183, p. 25-2

Variability and transient search in the SUDARE-VOICE field: a new method to extract the light curves

The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r ∼ 26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent sub-fields. In this paper, we use the multiepoch r-band imaging data to measure the variability of the detected objects and search for transients. We perform careful astrometric and photometric calibrations and point spread function modelling. A new method, referring to as differential running-average photometry, is proposed to measure the light curves of the detected objects. With the method, the difference of PSFs between different epochs can be reduced, and the background fluctuations are also suppressed. Detailed uncertainty analysis and detrending corrections on the light curves are performed. We visually inspect the light curves to select variable objects, and present some objects with interesting light curves. Further investigation of these objects in combination with multiband data will be presented in our forthcoming paper

C IV Broad Absorption Line Variability in QSO spectra from SDSS I-III Surveys

We present the results of our study of C IV broad absorption line (BAL) variability in the spectra of more than 1500 QSO's from several SDSS I-III surveys. Absorption lines in QSO spectra are due to outflowing winds which originate from the accretion disk, at a distance on the order of 1/100 - 1/10 pc from the central super-massive black hole (SMBH). Winds trigger the accretion mechanism onto the SMBH removing angular momentum from the disk and, since they evacuate gas from the host galaxy, they are believed to play a fundamental role in galaxy evolution. Absorption lines can be classified on the basis of their width and of the observed transitions, and their equivalent width can change on timescales from months to years, due to variations in the covering factor and/or in the ionization level. We analyzed the largest sample ever used for such kind of studies. We find that the fraction of disappearing BALs is three times larger than the one found in previous works. Strong evidence is found for a coordinated variability in spectra with multiple BAL troughs which may be interpreted in terms of disk-wind rotation, and/or variations in the physical status of the shielding gas. We also find that, in spectra with multiple BAL troughs, the disappearing ones are generally those with the highest central velocity

C IV Broad Absorption Line Variability In QSO Spectra

We present the results of our study of C IV broad absorption line (BAL) variability in the spectra of more than 1500 QSOs from several SDSS I-III surveys up to BOSS. Absorption lines in QSO spectra are due to outflowing winds which originate from the accretion disk, at a distance of about 0.01-0.1 pc from the central super-massive black hole (SMBH). Winds trigger the accretion mechanism onto the SMBH removing angular momentum from the disk and, since they evacuate gas from the host galaxy, they are believed to play a fundamental role in galaxy evolution. Absorption lines can be classified on the basis of their width and of the observed transitions, and their equivalent width can change on timescales from months to years, due to variations in the covering factor and/or in the ionization level. We analyzed the largest sample ever used for such kind of studies. We find that the fraction of disappearing BALs is three times larger than the one found in previous works. Strong evidence is found for a coordinated variability in spectra with multiple BAL troughs which may be interpreted in terms of disk-wind rotation, and/or variations in the physical status of the shielding gas. We also find that, in spectra with multiple BAL troughs, the disappearing ones are generally those with the highest central velocity

Serching for AGNs with VST: optical variability in the COSMOS and CDFS regions

Active Galactic Nuclei (AGN) are characterised by strong variability at all wavelengths. We exploited 3 years of VST monitoring observations of the COSMOS and CDFS regions, performed within the SUDARE/VOICE surveys, to assemble a sample of AGN candidates based on variability. Variability selection does not make strong a-priori assumptions about the properties of the sources and can thus integrate and complete samples selected by other techniques. We investigate the effectiveness and reliability of this selection method by comparing it with spectroscopic, X-ray and IR selected samples, showing that variability-selection can yield extremely pure samples, with completeness 〉50% with respect of X-ray and IR selections. We explore the dependence of variability from obscuration, mass and accretion rate, and predict the performance that can be expected by future monitoring surveys such as LSST