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Search Results: 1 - 10 of 208768 matches for " Vicki L. Sarajedini "
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Active Galactic Nuclei surveys with goya on the gtc
Vicki L. Sarajedini
Revista mexicana de astronomía y astrofísica , 2007,
Abstract: Active Galactic Nuclei candidates can be identified in deep, multi-wavelength surveys using a variety of techniques. Here we discuss some of these methods, recent results from the Groth Survey Strip AGN survey, and the application of these techniques to the selec- tion of AGN candidates for the GOYA survey with EMIR on the GTC.
A Multi-wavelength Survey of AGN in Massive Clusters: AGN Distribution and Host Galaxy Properties
Alison J. Klesman,Vicki L. Sarajedini
Physics , 2014, DOI: 10.1093/mnras/stu830
Abstract: We investigate the effect of environment on the presence and fuelling of Active Galactic Nuclei (AGN) by identifying galaxies hosting AGN in massive galaxy clusters and the fields around them. We have identified AGN candidates via optical variability (178), X-ray emission (74), and mid-IR SEDs (64) in multi- wavelength surveys covering regions centered on 12 galaxy clusters at redshifts 0.5 < z < 0.9. In this paper, we present the radial distribution of AGN in clusters to examine how local environment affects the presence of an AGN and its host galaxy. While distributions vary from cluster to cluster, we find that the radial distribution of AGN generally differs from that of normal galaxies. AGN host galaxies also show a different colour distribution than normal galaxies, with many AGN hosts displaying galaxy colours in the "green valley" between the red sequence and blue star-forming normal galaxies. This result is similar to those found in field galaxy studies. The colour distribution of AGN hosts is more pronounced in disturbed clusters where minor mergers, galaxy harassment, and interactions with cluster substructure may continue to prompt star-formation in the hosts. However, we find no relationship between host galaxy colour and cluster radius among AGN hosts. This may indicate that processes related to the accreting supermassive black hole have a greater impact on the star-forming properties of the host galaxy than the intracluster medium and/or local galaxy environment.
The ensemble optical variability of type-1 AGN in the Sloan Digital Sky Survey Data Release 7
Unai Gallastegui-Aizpun,Vicki L. Sarajedini
Physics , 2014, DOI: 10.1093/mnras/stu1648
Abstract: We use a sample of over 5000 active galactic nuclei (AGN) with extended morphologies at z<0.8 from the Sloan Digital Sky Survey (SDSS) to study the ensemble optical variability as a function of rest-frame time lag and AGN luminosity with the aim of investigating these parameter relationships at lower luminosities than previously studied. We compare photometry from imaging data with spectrophotometry obtained weeks to years later in the Sloan g, r, and i bands. We employ quasar and galaxy eigenspectra fitting to separate the AGN and host galaxy components. A strong correlation between the variability amplitude and rest-frame time lag is observed, in agreement with quasar structure functions but extending to AGN several magnitudes fainter than previously studied. The structure function slopes for our fainter AGN sample are slightly shallower than those found in quasars studies. An anticorrelation with luminosity is clearly detected, with lower luminosity AGN displaying greater variability amplitudes. We demonstrate for the first time that this anticorrelation extends to AGN as faint as $M_{AGN_i}\sim-18.5$, with a slight trend towards shallower slopes at luminosities fainter than $M_{AGN_i}\sim-20.2$.
A Multi-wavelength Survey of AGN in Massive Clusters: AGN Detection and Cluster AGN Fraction
Alison J. Klesman,Vicki L. Sarajedini
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.21508.x
Abstract: We aim to study the effect of environment on the presence and fuelling of Active Galactic Nuclei (AGN) in massive galaxy clusters. We explore the use of different AGN detection techniques with the goal of selecting AGN across a broad range of luminosities, AGN/host galaxy flux ratios, and obscuration levels. From a sample of 12 galaxy clusters at redshifts 0.5 < z < 0.9, we identify AGN candidates using optical variability from multi-epoch HST imaging, X-ray point sources in Chandra images, and mid-IR SED power-law fits through the Spitzer IRAC channels. We find 178 optical variables, 74 X-ray point sources, and 64 IR power law sources, resulting in an average of ~25 AGN per cluster. We find no significant difference between the fraction of AGN among galaxies in clusters and the percentage of similarly-detected AGN in field galaxy studies (~2.5%). This result provides evidence that galaxies are still able to fuel accretion onto their supermassive black holes, even in dense environments. We also investigate correlations between the percentage of AGN and cluster physical properties such as mass, X-ray luminosity, size, morphology class and redshift. We find no significant correlations among cluster properties and the percentage of AGN detected.
A V-Band Survey for Variable Galactic Nuclei in the Hubble Deep Field
Vicki L. Sarajedini,Ronald L. Gilliland,Christina Kasm
Physics , 2003, DOI: 10.1086/379168
Abstract: We present the results of a 2-epoch variability survey in the Hubble Deep Field with the goal of investigating the population of AGN to z=1. The primary data sets analyzed for galactic variability are the original HDF observations obtained in 1995 and a second V-band image obtained 5 years later in 2000. We find evidence for nuclear variability in 16 of 217 galaxies brighter than V_nuc=27.5. Correcting for incompleteness and spurious detections, variable nuclei make up ~8% of the surveyed galaxies. Seven of our variable sources are coincident with X-ray sources detected in the 2Ms Chandra survey. We find that 44% of the variable nuclei are associated with mid-IR detections at 15 microns and 31% are detected at 1.4GHz. Optical spectra are available for 13 of the 16 variables. One is a broad-line AGN and 2 others show weak evidence of type 2 AGNs. With the assumption that these variables are all active nuclei, we estimate the AGN LF at 0.4
Requirements for Investigating the Connection Between Lyman Alpha Absorption Clouds and the Large-Scale Distribution of Galaxies
Vicki L. Sarajedini,Richard F. Green,Buell T. Jannuzi
Physics , 1995, DOI: 10.1086/176750
Abstract: We model the requirements on observational data that would allow an accurate determination of the degree of association between Lyman alpha absorbers and peaks in the redshift distribution of galaxies (large-scale structures like clusters of galaxies). We compare simulated distributions of low-redshift Lyman alpha absorption systems, constrained to be consistent with the distribution observed with HST, with the large-scale distribution of galaxies determined from pencil-beam redshift surveys. We estimate the amount of observational data required from catalogues of Lyman alpha absorbers and galaxies to allow a statistically significant test of the association of absorbers with large-scale structures of galaxies. We find that for each line-of-sight observed for Lyman alpha absorption lines (assuming that the entire redshift range out to z=0.4 is observable), redshifts must be obtained for at least 18 galaxies brighter than Mb=-18 and having redshifts between 0.2 and 0.4. Based on the redshift surveys used in this study, a search radius of 10' from the quasar line-of-sight is required. This will ensure that all peaks in the galaxy redshift distribution are represented by at least one galaxy in the observed sample. If Lyman alpha absorbers are intrinsically uncorrelated with galaxies, we find that 8 lines-of-sight must be observed to show that the distributions are different at the 95% confidence level. However, if a fraction of the Lyman alpha absorbers are distributed with the peaks in the galaxy distribution, 38 lines-of-sight must be mapped for the distribution of both Lyman alpha absorbers and galaxies in order to determine the fraction of absorbers distributed with the peaks of the galaxy distribution to an accuracy of 10%.
Spectroscopic and Variability Surveys for AGN in the Groth Survey Strip
Vicki Sarajedini
Physics , 2001,
Abstract: Preliminary results are presented for a spectroscopic survey of the Groth Survey Strip (GSS), a 40 by 3.5 arcminute region of the sky imaged with HST, for which several hundred galaxy spectra have been obtained as part of the DEEP project (http://deep.ucolick.org). At least 6 broad-line AGNs (primarily Seyfert 1s) have been detected as well as several narrow-line Seyfert 2 candidates. The Seyfert galaxies detected in our survey have integrated absolute magnitudes extending to M(B)~-17.5, probing fainter magnitudes and higher redshifts than existing optical spectroscopic surveys. We also discuss a variability study of the GSS using the original HST images from 1994 and new images obtained in 2001. The high resolution obtained with HST allows us to isolate and measure variable galactic nuclei too faint to be detected from the ground, reaching nuclear magnitudes of M(B)~-16 in galaxies to z=0.8. The combination of these techniques provides a powerful probe of the population of low-luminosity AGNs at moderate redshifts.
KISS AGNs in the Soft X-ray Band: Correlation with the ROSAT All-Sky Survey
Samantha L. Stevenson,John J. Salzer,Vicki L. Sarajedini,Edward C. Moran
Physics , 2002, DOI: 10.1086/344680
Abstract: We present a study of the X-ray properties of a volume-limited sample of optically selected emission-line galaxies. The sample is derived from a correlation between the KPNO International Spectroscopic Survey (KISS), an H-alpha-selected objective-prism survey of AGNs and starbursting galaxies, and the ROSAT All-Sky Survey (RASS). After elimination of all spurious matches, we identify 18 ROSAT-detected X-ray sources within the KISS sample in the 0.1-2.4 keV band. Due to soft X-ray selection effects, the majority of the ROSAT sources are Seyfert 1 galaxies. The majority (54%) of the ROSAT-KISS Seyferts are classified as narrow-line Seyfert 1 objects, a relatively high percentage compared to previous objective-prism-selected Seyfert galaxy samples. We estimate the X-ray luminosities of the ROSAT-detected KISS objects and derive volume emissivities of 6.63 x 10^38 ergs/s/Mpc^3 and 1.45 x 10^38 ergs/s/Mpc^3 for the 30 deg and 43 deg survey strips, respectively. For those KISS AGNs not detected by RASS, we use the median L_X/L_H-alpha ratio derived from a previous study to estimate L_X. The total 0.5-2 keV volume emissivity we predict for the overall KISS AGN sample is sufficient to account for 22.1 +/- 8.9% of the soft X-ray background (XRB), averaged over both survey strips. The KISS AGN sample is made up predominantly of intermediate-luminosity Seyfert 2's and LINERs, which tend to be weak soft X-ray sources. They may, however, represent a much more significant contribution to the hard XRB.
Compact Nuclei in Moderately Redshifted Galaxies
Vicki L. Sarajedini,Richard F. Green,Richard E. Griffiths,Kavan U. Ratnatunga
Physics , 1996, DOI: 10.1086/310333
Abstract: The Hubble Space Telescope WFPC2 is being used to obtain high-resolution images in the V and I bands for several thousand distant galaxies as part of the Medium Deep Survey (MDS). An important scientific aim of the MDS is to identify possible AGN candidates from these images in order to measure the faint end of the AGN luminosity function as well as to study the host galaxies of AGNs and nuclear starburst systems. We are able to identify candidate objects based on morphology. Candidates are selected by fitting bulge+disk models and bulge+disk+point source nuclei models to HST imaged galaxies and determining the best model fit to the galaxy light profile. We present results from a sample of MDS galaxies with I less than 21.5 mag that have been searched for AGN/starburst nuclei in this manner. We identify 84 candidates with unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy redshifts, all normal bulges are resolved. Most of the candidates are found in galaxies displaying exponential disks with some containing an additional bulge component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color distribution of the nuclei is consistent with a dominant population of Seyfert-type nuclei combined with an additional population of starbursts. Our results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host galaxies.
Luminosity Functions of Elliptical Galaxies at z < 1.2
Myungshin Im,Richard E. Griffiths,Kavan U. Ratnatunga,Vicki L. Sarajedini
Physics , 1996, DOI: 10.1086/310018
Abstract: The luminosity functions of E/S0 galaxies are constructed in 3 different redshift bins (0.2 < z < 0.55, 0.55 < z < 0.8, 0.8 < z < 1.2), using the data from the Hubble Space Telescope Medium Deep Survey (HST MDS) and other HST surveys. These independent luminosity functions show the brightening in the luminosity of E/S0s by about 0.5~1.0 magnitude at z~1, and no sign of significant number evolution. This is the first direct measurement of the luminosity evolution of E/S0 galaxies, and our results support the hypothesis of a high redshift of formation (z > 1) for elliptical galaxies, together with weak evolution of the major merger rate at z < 1.
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