arXiv subjects

A. Georgakakis

Publications and source records attributed to A. Georgakakis.

At least 109 records · Page 6Linked to original sources

arXiv2006

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a description of the panchromatic data sets that have been acquired in the Extended Groth Strip region of the sky. Our survey, the All-wavelength Extended Groth strip International Survey (AEGIS), is intended to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray (0.5 - 10 keV), GALEX ultraviolet (1200 - 2500 Angstrom), CFHT/MegaCam Legacy Survey optical (3600 - 9000 Angstroms), CFHT/CFH12K optical (4500 - 9000 Angstroms), Hubble Space Telescope/ACS optical (4400 - 8500 Angstroms), Palomar/WIRC near-infrared (1.2 - 2.2 microns), Spitzer/IRAC mid-infrared (3.6 - 8.0 microns), Spitzer/MIPS far-infrared (24 - 70 microns), and VLA radio continuum (6 - 20 cm). In addition, this region of the sky has been targeted for extensive spectroscopy using the DEIMOS spectrograph on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

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arXiv2006

All-wavelength Extended Groth strip International Survey: the environment of X-ray sources at z~1

We explore the environment of z~1 AGN using a sample of 53 spectroscopically identified X-ray sources in the All-wavelength Extended Groth strip International Survey. We quantify the local density in the vicinity of an X-ray source by measuring the projected surface density of spectroscopically identified optical galaxies within a radius defined by the 3rd nearest neighbour. Our main result is that X-ray selected AGN at z~1 avoid underdense regions at the 99.89% confidence level. Moreover, although we find that the overall population shares the same (rich) environment with optical galaxies of similar U-B and M_B, there is also tentative evidence (96%) that AGN with blue colors (U-B<1) reside in denser environments compared to optical galaxies. We argue that the results above are a consequence of the whereabouts of massive galaxies, capable of hosting supermassive black holes at their centers, with available cold gas reservoirs, the fuel for AGN activity. At z~1 an increasing fraction of such systems are found in dense regions.

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arXiv2006

AEGIS: The color-magnitude relation for X-ray selected AGN

We discuss the relationship between rest-frame color and optical luminosity for X-ray sources in the range 0.6<z<1.4 selected from the Chandra survey of the Extended Groth Strip (EGS). These objects are almost exclusively active galactic nuclei (AGN). While there are a few luminous QSOs, most are relatively weak or obscured AGN whose optical colors should be dominated by host galaxy light. The vast majority of AGN hosts at z~1 are luminous and red, with very few objects fainter than M_{B}=-20.5 or bluer than U-B=0.6. This places the AGN in a distinct region of color-magnitude space, on the ``red sequence'' or at the top of the ``blue cloud'', with many in between these two modes in galaxy color. A key stage in the evolution of massive galaxies is when star formation is quenched, resulting in a migration from the blue cloud to the red sequence. Our results are consistent with scenarios in which AGN either cause or maintain this quenching. The large numbers of red sequence AGN imply that strong, ongoing star formation is not a necessary ingredient for AGN activity, as black hole accretion appears often to persist after star formation has been terminated.

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arXiv2006

AEGIS: The Diversity of Bright Near-IR Selected Distant Red Galaxies

We use deep and wide near infrared (NIR) imaging from the Palomar telescope combined with DEEP2 spectroscopy and Hubble Space Telescope (HST) and Chandra Space Telescope imaging to investigate the nature of galaxies that are red in NIR colors. We locate these `distant red galaxies' (DRGs) through the color cut (J-K)_{vega} > 2.3 over 0.7 deg^{2}, where we find 1010 DRG candidates down to K_s = 20.5. We combine 95 high quality spectroscopic redshifts with photometric redshifts from BRIJK photometry to determine the redshift and stellar mass distributions for these systems, and morphological/structural and X-ray properties for 107 DRGs in the Extended Groth Strip. We find that many bright (J-K)_{vega}>2.3 galaxies with K_s<20.5 are at redshifts z<2, with 64% between 1 2 systems massive with M_*>10^{11} M_solar. HST imaging shows that the structural properties and morphologies of DRGs are also diverse, with the majority elliptical/compact (57%), and the remainder edge-on spirals (7%), and peculiar galaxies (29%). The DRGs at z < 1.4 with high quality spectroscopic redshifts are generally compact, with small half light radii, and span a range in rest-frame optical properties. The spectral energy distributions for these objects differ from higher redshift DRGs: they are bluer by one magnitude in observed (I-J) color. A pure IR color selection of high redshift populations is not sufficient to identify unique populations, and other colors, or spectroscopic redshifts are needed to produce homogeneous samples.

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arXiv2006

XMM and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift

We combine bright XMM data with the Chandra Deep Field South observations in order to explore the behavior of the intrinsic AGN absorption, as a function of redshift and luminosity.Our sample consists of 359 sources selected in the hard 2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1 cm^-2 with a high rate of spectroscopic or photometric redshift completeness (100 and 85 per cent respectively for the Chandra and XMM data. We derive the column density values using X-ray spectral fits. We find that the fraction of obscured AGN falls with increasing luminosity in agreement with previous findings. The fraction of obscured AGN shows an apparent increase at high redshifts (z>2). Simulations show that this effect can be most probably attributed to the fact that at high redshifts the column densities are overestimated.

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arXiv2006

The Deep Chandra Survey of the Groth Strip - II. optical identification of the X-ray sources

We discuss the optical and X-ray spectral properties of the sources detected in a single 200ks Chandra pointing in the Groth-Westphal Strip region. Optical identifications and spectroscopic redshifts are primarily from the DEEP2 survey. This is complemented with deeper (r~26mag) multi-waveband data (ugriz) from the Canada France Hawaii Legacy Survey to estimate photometric redshifts and to optically identify sources fainter than the DEEP2 magnitude limit (R(AB)~24.5mag). We focus our study on the 2-10keV selected sample comprising 97 sources to the limit ~8e-16erg/s/cm2, this being the most complete in terms of optical identification rate (86%) and redshift determination fraction (63%; both spectroscopic and photometric). We first construct the redshift distribution of the sample which shows a peak at z~1. This is in broad agreement with models where less luminous AGNs evolve out to z~1 with powerful QSOs peaking at higher redshift, z~2. Evolution similar to that of broad-line QSOs applied to the entire AGN population (both type-I and II) does not fit the data. We also explore the observed N_H distribution of the sample and estimate a fraction of obscured AGN (N_H>1e22) of ~48%. This is found to be consistent with both a luminosity dependent intrinsic N_H distribution, where less luminous systems comprise a higher fraction of type-II AGNs, and models with a fixed ratio 2:1 between type-I and II AGNs. We further compare our results with those obtained in deeper and shallower surveys. We argue that a luminosity dependent parametrisation of the intrinsic N_H distribution is required to account for the fraction of obscured AGN observed in different samples over a wide range of fluxes.

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arXiv2006

X-ray number counts of normal galaxies

We use the number counts of X-ray selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early type galaxies is consistent with no evolution out to z~0.5. For late-type galaxies our analysis suggests that it is the sources with X-ray--to--optical flux ratio logfx/fopt>-2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ~(1+z)^{2.7} out to z~0.4. On the contrary late-type sources with logfx/fopt<-2 are consistent with no evolution. This suggests that the logfx/fopt>-2 population comprises the most powerful and fast evolving starbursts at moderate and high-z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.

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arXiv2006

Searching for X-ray luminous 'normal' galaxies in 2dfGRS

We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database. Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples of galaxies in the local Universe whose X-ray emission is dominated by stellar processes rather than AGN activity ('normal' galaxies) as well as to test the empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs. With XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS galaxies. Using emission-line intensity ratios, we classified 6 2dfGRS spectra as star-forming, H II nuclei, and 2 spectra as possible H II nuclei. The rest of the objects are absorption-line galaxies and AGN, including 3 possible LINERs. No luminous 'normal' galaxies have been found but out of 19 'normal' galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O) > -2. We performed a similar search in two nearby-galaxy samples from the literature. All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2 and L_X < 10^42 erg s^-1. In the Fabbiano et al. (1992) sample, out of a total of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the majority of which are massive ellipticals. Three of these have L_X > 10^42 erg s^-1 .

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arXiv2006

XMM-Newton and Chandra observations of SHEEP sources

We present Chandra and XMM observations of 12 bright (f(2-10 keV) > 10^-13 cgs) sources from the ASCA SHEEP (Search for the High Energy Extragalactic Population) survey. Most of these have been either not observed or not detected previously with the ROSAT mission and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating with accuracy the optical counterpart of the X-ray sources. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects), and Broad-Line (BL) AGN (five objects) with one source remaining unidentified. Our sources cover the redshift range 0.04 to 1.29 spanning luminosities from 10^42 to 10^45 cgs (2-10 keV). The NL sources have preferentially lower redshift (and luminosity) compared with the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured (N_H>10^22 cgs) and unobscured (N_H<10^22 cgs) sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z=0.5.

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arXiv2006

Mid-infrared properties of X-ray sources in the Extended Groth Strip

Mid-infrared observations of Active Galactic Nuclei (AGN) are important for understanding of the physical conditions around the central accretion engines. Chandra and XMM-Newton X-ray observations of a 300 arcmin^2 region in the Extended Groth Strip are used to select a sample of~150 AGN. The Spitzer instruments IRAC and MIPS detect 68-80% of these sources, which show a wide range of mid-infrared properties. About 40% of the sources have red power-law spectral energy distributions (f_nu ~ nu^alpha, alpha<0) in the 3.6-8 um IRAC bands. In these sources the central engine dominates the emission at both X-ray and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy distributions (alpha>0) with their infrared emission dominated by the host galaxy; the remaining 20% are not well-fit by a power law. Published IRAC color criteria for AGN select most of the red sources, but only some of the blue sources. As with all other known methods, selecting AGN with mid-IR colors will not produce a sample that is simultaneously complete and reliable. The IRAC SED type does not directly correspond to X-ray spectral type (hard/soft). The mid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, and optically-faint sources vary widely and, for the most part, are not distinct from those of the general X-ray/infrared source population. X-ray sources emit 6-11% of the integrated mid--IR light, making them significant contributors to the cosmic infrared background.

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arXiv2005

The Phoenix Deep Survey: The star-formation rates and the stellar masses of EROs

We estimate the star-formation rates and the stellar masses of the Extremely Red objects (EROs) detected in a 180arcmin2 Ks-band survey (Ks~20mag). This sample is complemented by sensitive 1.4GHz radio observations (12micro-Jy; 1sigma rms) and multiwaveband photometric data (UBVRIJ) as part of the Phoenix Deep Survey. For bright K<19.5mag EROs in this sample (I-K>4mag; total of 177) we use photometric methods to discriminate dust-enshrouded active systems from early-type galaxies and to constrain their redshifts. Radio stacking is then employed to estimate mean radio flux densities of 8.6 (3sigma) and 6.4micro-Jy (2.4sigma) for the dusty and early-type subsamples respectively. Assuming that dust enshrouded active EROs are powered by star-formation the above radio flux density at the median redshift of z=1 translates to a radio luminosity of 4.5e22W/Hz and a star-formation rate of SFR=25Mo/yr. Combining this result with photometric redshift estimates we find a lower limit to the star-formation rate density of ~0.02Mo/yr/Mpc^3 for the K<19.5mag dusty EROs in the range z=0.85-1.35. Comparison with the SFR density estimated from previous ERO samples (with similar selection criteria) using optical emission lines, suffering dust attenuation, suggests a mean dust reddening of at least E(B-V)~0.5 for this population. We further use the Ks-band luminosity as proxy to stellar mass and argue that the dust enshrouded EROs in our sample are massive systems, M>5e10Mo. We also find that EROs represent a sizable fraction (~50%) of the number density of galaxies more massive than M=5e10Mo at z~1, with almost equal contributions from dusty and early types. Similarly, we find that EROs contribute about half of the mass density of the Universe at z~1 after taking into account incompleteness because of the limit K=19.5mag.

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arXiv2005

Red AGN in XMM-Newton/SDSS fields

In this paper we combine archival and proprietary XMM-Newton observations (about 5deg^2) that overlap with the Sloan Digital Sky Survey to explore the nature of the moderate-z X-ray population. We focus on X-ray sources with optically red colours (g-r>0.4), which we argue are important for understanding the origin of the X-ray background. Firstly, these systems constitute a significant fraction, about 2/3, of the z<1 X-ray population to the limit f(2-8keV)~2e-14cgs. Secondly, their luminosity function under evolution of the form ~(1+z)^3 suggests that they could be responsible for about 17 per cent of the diffuse X-ray background to z=1. Thirdly, their stacked X-ray spectrum in the range 1-8keV is consistent with a power-law distribution with Gamma~1.4 (without fitting intrinsic absorption), i.e. similar to the diffuse X-ray background. We find that the optically red X-ray population comprises a mixed bag of objects, both obscured (N_H>1e22 cm^{-2}) and unobscured (N_H<1e22 cm^{-2}), with a wide range of X-ray luminosities up L_X~1e44cgs. We argue that dilution of the AGN light by the host galaxy may play a role in shaping the continuum optical emission of this population. Finally, we explore a possible association of these sources and the moderate-z red (J-Ks>2mag) AGNs identified in the Two Micron All Sky Survey (2MASS). The median N_H of the red X-ray sources studied here is ~1e21cm^{-2}, lower than that found for the 2MASS AGNs, suggesting different populations.

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arXiv2005

The XMM-Newton/2dF survey-VIII: The extended X-ray sources

We present a sample of eight extended X-ray sources detected in the wide-field (~2.3 deg^2), bright (2-10 ksec) XMM-Newton/2dF survey, reaching a flux limit of ~2 X 10^{-14} erg/s/cm^2. Of these, seven are identified as secure X-ray clusters in the soft 0.3-2 keV band using a standard wavelet algorithm on either the PN or the MOS images. Spectroscopic or photometric redshifts are available for five clusters, spanning a range between 0.12 and 0.68. The X-ray spectral fittings show temperatures between 1 and 4.6 keV, characteristic of poor clusters and groups of galaxies. We derive for the first time the XMM-Newton cluster number count log N-log S distribution albeit with poor statistics. Both the log N-\log S and the Luminosity-Temperature relation are in good agreement with previous ROSAT results.

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arXiv2005

The Phoenix Deep Survey: spectroscopic catalog

The Phoenix Deep Survey is a multi-wavelength survey based on deep 1.4 GHz radio imaging, reaching well into the sub-100 microJy level. One of the aims of this survey is to characterize the sub-mJy radio population, exploring its nature and evolution. In this paper we present the catalog and results of the spectroscopic observations aimed at characterizing the optically ``bright'' (R<~ 21.5 mag) counterparts of faint radio sources. Out of 371 sources with redshift determination, 21% have absorption lines only, 11% show AGN signatures, 32% are star-forming galaxies, 34% show narrow emission lines that do not allow detailed spectral classification (due to poor signal-to-noise ratio and/or lack of diagnostic emission lines) and the remaining 2% are identified with stars. For the star-forming galaxies with a Balmer decrement measurement we find a median extinction of A(Ha)=1.9 mag, higher than that of optically selected samples. This is a result of the radio selection, which is not biased against dusty systems. Using the available spectroscopic information, we estimate the radio luminosity function of star-forming galaxies in two independent redshift bins at z~0.1 and 0.3 respectively. We find direct evidence for strong luminosity evolution of these systems consistent with L(1.4 GHz) ~ (1+z)^(2.7).

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arXiv2005

The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of 'normal' galaxies

In this paper we estimate the local (z<0.22) X-ray luminosity function of `normal' galaxies derived from the XMM-Newton Needles in the Haystack Survey. This is an on-going project that aims to identify X-ray selected `normal' galaxies (i.e. non-AGN dominated) in the local Universe. We are using a total of 70 XMM-Newton fields covering an area of 11 sq. degrees which overlap with the Sloan Digital Sky Survey Data Release-2. `Normal' galaxies are selected on the basis of their resolved optical light profile, their low X-ray--to--optical flux ratio (log(f_x/f_o)<-2) and soft X-ray colours. We find a total of 28 candidate `normal' galaxies to the 0.5-8 keV band flux limit of ~2x10^{-15} cgs. Optical spectra are available for most sources in our sample (82 per cent). These provide additional evidence that our sources are bona-fide 'normal' galaxies with X-ray emission coming from diffuse hot gas emission and/or X-ray binaries rather than a supermassive black hole. Sixteen of our galaxies have narrow emission lines or a late-type Spectral Energy Distribution (SED) while the remaining 12 present only absorption lines or an early-type SED. Combining our XMM-Newton sample with 18 local (z<0.22) galaxies from the Chandra Deep Field North and South surveys, we construct the local X-ray luminosity function of `normal' galaxies. This can be represented with a Schechter form with a break at L* ~ 3x10^{41} cgs and a slope of ~1.78+/-0.12. Using this luminosity function and assuming pure luminosity evolution of the form (1+z)^{3.3} we estimate a contribution to the X-ray background from `normal' galaxies of 10-20 per cent (0.5-8 keV). Finally, we derive, for the first time, the luminosity functions for early and late type systems separately.

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arXiv2005

The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): Sample selection and the cluster log N - log S

In this paper we serendipitously identify X-ray cluster candidates using XMM-Newton archival observations complemented by 5-band optical photometric follow-up observations (r~23 mag) as part of the X-ray Identification (XID) programme. Our sample covers an area of ~2.1 sq. deg (15 XMM-Newton fields) and comprises a total of 21 (19 serendipitous + 2 target) extended X-ray sources to the limit f(0.5-2keV) ~ 6x10^{-15} cgs with a high probability (> 99.9%) of being extended on the XMM-Newton images. Of the 21 cluster candidates 7 are spectroscopically confirmed in the literature. Exploiting the optical data available for these fields we discover that 68% of the X-ray cluster candidates are associated with optical galaxy overdensities. We also attempt to constrain the redshifts of our cluster candidates using photometric methods. We thus construct the photometric redshift distribution of galaxies in the vicinity of each X-ray selected cluster candidate and search for statistically significant redshift peaks against that of the background distribution of field galaxies. Comparison of the photometric with spectroscopic redshift estimates for the confirmed clusters suggest that our simple method is robust out to z~0.5. For clusters at higher z, deeper optical data are required to estimate reliable photometric redshifts. Finally, using the sample of the 19 serendipitous X-ray selected cluster candidates we estimate their surface density down to f(0.5-2keV) ~ 6x10^{-15} cgs and find it to be in fair agreement with previous and recent studies.

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arXiv2005

Dust in a merging galaxy sequence: the SCUBA view

We investigate the cold and warm dust properties during galaxy interactions using a merging galaxy sample ordered into a chronological sequence from pre- to post-mergers. Our sample comprises a total of 29 merging systems selected to have far-infrared and sub-millimeter observations. We use the 100-to-850 micron flux density ratio, f100/f850, as a proxy to the mass fraction of the warm and the cold dust in these systems. We find evidence for an increase in f100/f850 along the merging sequence from early to advanced mergers and interpret this trend as an increase of the warm relative to the cold dust mass. We argue that the two key parameters affecting the f100/f850 flux ratio is the star-formation rate and the dust content of individual systems relative to the stars. Using a sophisticated model for the absorption and re-emission of the stellar UV radiation by dust we show that these parameters can indeed explain both the increase and the observed scatter in the f100/f850 along the merging galaxy sequence.

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arXiv2004

On the use of photometric redshifts for X-ray selected AGNs

(Abridged) In this paper we present photometric redshift estimates for a sample of X-ray selected sources detected in the wide field (~2 deg^2), bright [f_{X} (0.5-8 keV)~10^{-14} cgs] XMM-Newton/2dF survey. Unlike deeper X-ray samples comprising a large fraction of sources with colours dominated by the host galaxy, our bright survey primarily probes the QSO X-ray population. Therefore photometric redshift methods employing both galaxy and QSO templates need to be used. We employ the photometric redshift technique of Hatziminaoglou, Mathez & Pello (2000) using 5-band photometry from the SDSS. We separate our X-ray sources according to their optical profile to point-like and extended. We apply QSO and galaxy templates to the point-like and extended sources respectively. X-ray sources associated with Galactic stars are identified and discarded from our point-like sample on the basis of their low X-ray--to--optical flux ratio and their broad band colours that are best fit by stellar templates. Comparison of our results with spectroscopic redshifts available, allows calibration of our method and estimation of the photometric redshift accuracy. For ~70 per cent of the point-like sources photometric redshifts are correct within dz <= 0.3 (or ~75 per cent have dz/(1+z) <= 0.2), and the rms scatter is estimated to be sigma_z = 0.30. For the optically extended objects the photometric redshifts work only in the case of red (g - r > 0.5 mag) sources yielding dz <= 0.15 and dz/(1+z) <= 0.2 for 73 and 93 per cent respectively. However, we find that the above photometric redshift technique does not work in the case of extended sources with blue colours (g - r < 0.5): such sources cannot be fit successfully by QSO or galaxy templates, or any linear combination of the two.

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