Quasar variability: an astrostatistical challenge

Variable behavior on all time scales seems to be ubiquitous phenomenon in almost all classes of active galactic nuclei. This work deals with two topics. One being detection of microvariability (intra-night variability) in the sources which are obscured and were believed to be non variable - type 2 quasars. The detection of a variable behavior in obscured sources is challenging as the detection is hampered by the low contrast between the presumably variable nucleus and the host galaxy. The second part deals with comparing of the variable behavior with larger amplitudes amongst two types of unobscured quasars: core dominated radio loud quasars (CRLQ) and radio quiet quasars (RQQ). Microvariability studies of obscured quasars are the focus of chapters 2 and 3. We have explored the possibility to search for short, small amplitude variations, with statistical tests, namely the F-test and one-way analysis of variance, ANOVA. The data for our sample were obtained during two observing runs, one using the 1.5 m telescope at San Pedro M\'artir observatory in Mexico and another one with Nordic Optical Telescope (NOT) at Observatorio Roque de los Muchachos, La Palma. Both samples were observed in optical wavelengths. The fourth chapter is dedicated to a study independent on the previous two chapters, which studies data set obtained during a year long campaign carried out at San Pedro M\'artir observatory. The data set contains observations of a paired set of unobscured CRLQ and RQQ. The observations were obtained in a weekly cadence, in optical wavelengths, thus focusing on a low frequency observations. We used Bartels test to search for variability in this case. As a result, we report detection of microvariability phenomenon in a subset of targets observed during the microvariability campaigns described in chapters 2 and 3. It is a result previously unaccounted for in obscured type 2 quasars. We explore possible explanations such as structure in the obscuring medium. In the second part of the thesis, we focused no comparing whether it is more likely that CRLQ are variable than RQQ. We have conducted this study on a set of targets which was studied previously. The previous study focused on a similar comparison on high frequencies. We have confirmed the results of the previous study using low frequency observations and report no statistically significant difference between RQQ and CRLQ. We have demonstrated feasibility of different statistical tests in variability studies

Optical microvariability in type 2 quasars

The AGN unified scenario suggests that the type 2 quasars are the obscured counterparts of type 1 quasars where the difference is caused only by the orientation of the object. Optical microvari ability is supposed to arise from the regions which are closer to the supermassive black hole at the center of the AGN. The search for microvariability is an important test for the AGN unified scenario. We present first results from the microvariability campaign carried out at San Pedro Martit observatory. We have targeted three brightest (V mag < 17) type 2 quasars from SDSS. Using analysis of variance (ANOVA), we have detected significant microvariability events in two out of three type 2 quasars we have observe

San Pedro Martir observations of microvariability in obscured quasars

Fast brightness variations are a unique tool to probe the innermost regions of active galactic nuclei (AGN). These variations are called microvariability or intra-night variability, and this phenomenon has been monitored in samples of blazars and unobscured AGNs. Detecting optical microvariations in targets hidden by the obscuring torus is a challenging task because the region responsible for the variations is hidden from our sight. However, there have been reports of fast variations in obscured Seyfert galaxies in X-rays, which rises the question whether microvariations can also be detected in obscured AGNs in the optical regime. Because the expected variations are very small and can easily be lost within the noise, the analysis requires a statistical approach. We report the use of a one-way analysis of variance, ANOVA, with which we searched for microvariability. ANOVA was successfully employed in previous studies of unobscured AGNs. As a result, we found microvariable events during three observing blocks: in two we observed the same object (Mrk 477), and in another, J0759+5050. The results on Mrk 477 confirm previous findings. However, since Mrk 477 is quite a peculiar target with hidden broad-line regions, we cannot rule out the possibility that we have serendipitously chosen a target prone to variations.Comment: Research note, 5 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

Lyman break and ultraviolet-selected galaxies at z ̃ 1-I. Stellar populations from the ALHAMBRA survey

We take advantage of the exceptional photometric coverage provided by the combination of GALEX data in the ultraviolet (UV) and the ALHAMBRA survey in the optical and near-infrared to analyse the physical properties of a sample of 1225 GALEX-selected Lyman break galaxies (LBGs) at 0.8 ≲ z ≲ 1.2 that are located in the COSMOS field. This is the largest sample of LBGs studied in this redshift range to date. According to a spectral energy distribution (SED) fitting with synthetic stellar population templates, we find that LBGs at z ̃ 1 are mostly young galaxies with a median age of 341 Myr and have intermediate dust attenuation, (Es(B - V)) ̃ 0.20. Owing to the selection criterion, LBGs at z ̃ 1 are UV-bright galaxies and have a high dust-corrected total star formation rate (SFR), with a median value of 16.9M⊙ yr-1. Their median stellar mass is log (M*/M⊙) = 9.74. We find that the dustcorrected total SFR of LBGs increases with stellar mass and that the specific SFR is lower for more massive galaxies (downsizing scenario). Only 2 per cent of the galaxies selected through the Lyman break criterion have an active galactic nucleus nature. LBGs at z ̃ 1 are located mostly over the blue cloud of the colour-magnitude diagram of galaxies at their redshift, with only the oldest and/or the dustiest deviating towards the green valley and red sequence. Morphologically, 69 per cent of LBGs are disc-like galaxies, with the fractions of interacting, compact, or irregular systems being much lower, below 12 per cent. LBGs have a median effective radius of 2.5 kpc, and larger galaxies have a higher total SFR and stellar mass. Compared with their high-redshift analogues, we find evidence that LBGs at lower redshifts are larger, redder in the UV continuum, and have a major presence of older stellar populations in their SEDs. However, we do not find significant differences in the distributions of stellar mass or dust attenuation.NASA NAS5-26555NASA Office of Space Science NNX09AF08GEuropean Southern Observatory LP175.A-0839.Junta de Andalucia TIC-114, P08-TIC03531Ministerio de Economía y Competitividad AYA2011-29517-C03- 01, AYA2010-1516

San Pedro Mártir observations of microvariability in obscured quasars

Fast brightness variations are a unique tool to probe the innermost regions of active galactic nuclei (AGN). These variations are called microvariability or intra-night variability, and this phenomenon has been monitored in samples of blazars and unobscured AGNs. Detecting optical microvariations in targets hidden by the obscuring torus is a challenging task because the region responsible for the variations is hidden from our sight. However, there have been reports of fast variations in obscured Seyfert galaxies in X-rays, which rises the question whether microvariations can also be detected in obscured AGNs in the optical regime. Because the expected variations are very small and can easily be lost within the noise, the analysis requires a statistical approach. We report the use of a one-way analysis of variance, ANOVA, with which we searched for microvariability. ANOVA was successfully employed in previous studies of unobscured AGNs. As a result, we found microvariable events during three observing blocks: in two we observed the same object (Mrk 477), and in another, J0759+5050. The results on Mrk 477 confirm previous findings. However, since Mrk 477 is quite a peculiar target with hidden broad-line regions, we cannot rule out the possibility that we have serendipitously chosen a target prone to variations.This research has been supported by the Spanish Ministerio de Economía y Competitividad (MINECO) under the grant AYA2014-58861-C3-1. I.O. acknowledges support from the European Research Council (ERC) in the form of Advanced Grant, COSMICISM. J.A.D. is grateful for the support from the grant UNAM-DGAPA-PAPIIT IN110013 Program and the Canary islands CIE: Tricontinental Atlantic Campus. A.E. acknowledges support by the grant AYA2012-30789. J. I. González-Serrano is grateful for the support from AYA2011-29517-C03-02.Peer Reviewe

Optical microvariability of bright type 2 quasars

We present results from a project focused on searching optical microvariabilty (also known as “intra-night” variability) in type 2 - obscured - quasars. Optical microvariability can be described as very small changes in the flux, typically in the order of hundredths of magnitude, which can be observed on timescales of hours. Such studies have been so far conducted for samples of blazars and type 1, unobscured, AGNs, where the optical microvariability was detected with success. We have focused on obscured targets which would pose a challenge to the AGN standard model. In the present work, however, we have observed a sample of three bright (g mag< 17) type 2 quasar, based on the catalog of type 2 quasars from SDSS of Reyes et al. (2008). The observations were carried out with the 1.5 meter telescope at San Pedro Martir observatory in Mexico. The sample was observed during an observation period of four days in Johnsons V filter, resulting in at least two continuous intervals of observations per target during the observational run. We have obtained differential light curves for our sources as well as for the comparison stars. They were analyzed using one-way analysis of variance statistical test (ANOVA), which has been repeatedly used in the past for studies of unobscured targets. Based on the results from the statistical analysis, we show that at least two out of three observed targets appear to be variable on time scales of hours. So far, this is the first study which confirmed existence of optical microvariability in type 2 quasars.This research has been supported by the Spanish Ministerio de Economía y Competitividad (MINECO) under the grant AYA2011-29517-C03-01. J. A. de Diego is grateful for support from grants SAB2010-0011 awarded by the Spanish MIED, and PAPIIT IN110013 awarded by the UNAM. J. I. González-Serrano is grateful for the support from AYA2011-29517-C03-02. The authors also would like to thank our colleague Yair Krongold for useful suggestions on this paper

Optical microvariability of bright type 2 quasars

We present results from a project focused on searching optical microvariabilty (also known as “intra-night” variability) in type 2 - obscured - quasars. Optical microvariability can be described as very small changes in the flux, typically in the order of hundredths of magnitude, which can be observed on timescales of hours. Such studies have been so far conducted for samples of blazars and type 1, unobscured, AGNs, where the optical microvariability was detected with success. We have focused on obscured targets which would pose a challenge to the AGN standard model. In the present work, however, we have observed a sample of three bright (g mag< 17) type 2 quasar, based on the catalog of type 2 quasars from SDSS of Reyes et al. (2008). The observations were carried out with the 1.5 meter telescope at San Pedro Martir observatory in Mexico. The sample was observed during an observation period of four days in Johnsons V filter, resulting in at least two continuous intervals of observations per target during the observational run. We have obtained differential light curves for our sources as well as for the comparison stars. They were analyzed using one-way analysis of variance statistical test (ANOVA), which has been repeatedly used in the past for studies of unobscured targets. Based on the results from the statistical analysis, we show that at least two out of three observed targets appear to be variable on time scales of hours. So far, this is the first study which confirmed existence of optical microvariability in type 2 quasars.This research has been supported by the Spanish Ministerio de Economía y Competitividad (MINECO) under the grant AYA2011-29517-C03-01. J. A. de Diego is grateful for support from grants SAB2010-0011 awarded by the Spanish MIED, and PAPIIT IN110013 awarded by the UNAM. J. I. González-Serrano is grateful for the support from AYA2011-29517-C03-02. The authors also would like to thank our colleague Yair Krongold for useful suggestions on this paper

Lyman break and ultraviolet-selected galaxies at z ∼ 1 - I. Stellar populations from the ALHAMBRA survey

We take advantage of the exceptional photometric coverage provided by the combination of GALEX data in the ultraviolet (UV) and the ALHAMBRA survey in the optical and near-infrared to analyse the physical properties of a sample of 1225 GALEX-selected Lyman break galaxies (LBGs) at 0.8 ≲ z ≲ 1.2 that are located in the COSMOS field. This is the largest sample of LBGs studied in this redshift range to date. According to a spectral energy distribution (SED) fitting with synthetic stellar population templates, we find that LBGs at z ̃ 1 are mostly young galaxies with a median age of 341 Myr and have intermediate dust attenuation, (Es(B - V)) ̃ 0.20. Owing to the selection criterion, LBGs at z ̃ 1 are UV-bright galaxies and have a high dust-corrected total star formation rate (SFR), with a median value of 16.9M⊙ yr-1. Their median stellar mass is log (M*/M⊙) = 9.74. We find that the dustcorrected total SFR of LBGs increases with stellar mass and that the specific SFR is lower for more massive galaxies (downsizing scenario). Only 2 per cent of the galaxies selected through the Lyman break criterion have an active galactic nucleus nature. LBGs at z ̃ 1 are located mostly over the blue cloud of the colour-magnitude diagram of galaxies at their redshift, with only the oldest and/or the dustiest deviating towards the green valley and red sequence. Morphologically, 69 per cent of LBGs are disc-like galaxies, with the fractions of interacting, compact, or irregular systems being much lower, below 12 per cent. LBGs have a median effective radius of 2.5 kpc, and larger galaxies have a higher total SFR and stellar mass. Compared with their high-redshift analogues, we find evidence that LBGs at lower redshifts are larger, redder in the UV continuum, and have a major presence of older stellar populations in their SEDs. However, we do not find significant differences in the distributions of stellar mass or dust attenuation. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.This research has been supported by the Spanish Ministerio de Economía y Competitividad (MINECO) under grant AYA2011-29517-C03-01. Funding for the SDSS and SDSS-II was provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. Financial support from the Spanish grant AYA2010-15169 and from the Junta de Andalucía through TIC-114 and the Excellence Project P08-TIC-03531 is acknowledged.Peer Reviewe