E. Chatzichristou

University of Patras, Rhion, West Greece, Greece

Are you E. Chatzichristou?

Claim your profile

Publications (22)40.4 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a sample of 17 high-redshift (3.5 z 5.2) quasi-stellar object (QSO) candidates in the 320 arcmin2 area of the Great Observatories Origins Deep Survey, selected in the magnitude range 22.45 < z850 < 25.25 using deep imaging with the Advanced Camera for Surveys on board the Hubble Space Telescope and the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. On the basis of seven spectroscopic and 10 photometric redshifts, we estimate that the final sample will contain between two and four QSOs with 4 < z < 5.2. A dearth of high-redshift moderate-luminosity (M145 -23) QSOs is observed with respect to predictions based on (1) the extrapolation of the z ~ 2.7 luminosity function (LF), according to a pure luminosity evolution calibrated by the results of the Sloan Digital Sky Survey, and (2) a constant universal efficiency in the formation of supermassive black holes (SMBHs) in dark matter halos. Evidence is gathered in favor of a density evolution of the LF at high redshift and of a suppression of the formation or feeding of SMBHs in low-mass halos.
    The Astrophysical Journal 12/2008; 600(2):L119. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We examine the evolution of the sizes and number densities of disk galaxies using the high-resolution images obtained by the Great Observatories Origins Deep Survey (GOODS) with the Advanced Camera for Surveys on the Hubble Space Telescope. The multiwavelength images are used to classify galaxies based on their rest-frame B-band morphologies out to redshift z ~ 1.25. In order to minimize the effect of selection biases, we confine our analysis to galaxies that occupy the region of the magnitude-size plane where the survey is ~90% complete at all redshifts. The observed size distribution is consistent with a lognormal distribution as seen for the disk galaxies in the local universe and does not show any significant evolution over the redshift range 0.25 ≤ z ≤ 1.25. We find that the number densities of disk galaxies remains fairly constant over this redshift range, although a modest evolution by a factor of 4 may be possible within the 2 σ uncertainties.
    The Astrophysical Journal 12/2008; 604(1):L9. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This special issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multiband imaging data obtained with the Hubble Space Telescope and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 arcmin2 in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-Ray Observatory and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory and from the Kitt Peak National Observatory. Deep observations with the Space Infrared Telescope Facility are scheduled. Reduced data from all facilities are being released worldwide within 3-6 months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own Galaxy. This Letter serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.
    The Astrophysical Journal 12/2008; 600(2):L93. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a 3-dimensional study of the local (< 100 ; h-1 kpc) environment of Sy1, Sy2 and Bright IRAS Galaxies. For this purpose we use three galaxy samples (Sy1, Sy2, BIRG) located at high galactic latitudes as well as three control sample of non-active galaxies having the same morphological, redshift and diameter size distributions as the corresponding Seyfert or BIRG sample. Using the CfA2 and SSRS galaxy catalogues as well as our own spectroscopic observations, we find that the fraction of BIRGs with a close neighbour is significantly higher than that of their control sample. We also find that Sy2 galaxies demonstrate the same behaviour with BIRG galaxies but not with Sy1s which do not show any excess of companions with respect to their control sample galaxies. An additional analysis of the relation between FIR colours and activity type of the BIRGs shows a significant difference between the colours of strongly-interacting and non-interacting starbursts and a resemblance between the colours of non-interacting starbursts and Sy2s. Our results support an evolutionary scenario leading from Starbursting to a Sy2 and finally to an un-obscured Sy1 galaxy, where close interactions play the role of the triggering mechanism.
    Memorie della Societa Astronomica Italiana. 01/2008;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Extreme X-ray/Optical ratio sources (EXOs) are a class of source that are robustly detected in our Chandra Deep Field X-ray data, while being completely undetected in our deep multi-band GOODS HST/ACS data, including the long-wavelength z-band data, yet are detected in the deep GOODS near-IR VLT imaging. These sources have values of Fx/Fopt that are factors of 10–100 times higher than those generally found for other AGN. We thus infer two plausible scenarios: (1) if these sources lie at moderately high redshifts (z ~ 3 – 5) then their hosts need to be exceedingly underluminous and/or excessively reddened compared with other AGN hosts; (2) if they lie above z ~ 6 – 7, such that their Lyman-alpha emission is redshifted out of the z-band filter, then their K-band and X-ray properties can be accounted for in terms of relatively normal hosts and moderate-luminosity AGN. In this case, these objects can serve as an invaluable probe of black hole growth and accretion activity in the early universe.
    01/2006: pages 88-93;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: QSOs are intrinsically luminous and therefore can be seen rather easily at large distances; but they are rare, and finding them requires surveys over large areas. As a consequence, at present, the number density of QSOs at high redshift is not well known. Recently, the Sloan Digital Sky Survey (SDSS) has produced a breakthrough, discovering QSOs up to z=6.43 [8] and building a sample of six QSOs with z>5.7. The SDSS, however, has provided information only about very luminous QSOs (M 1450 <~-26.5_{1450} \lesssim -26.5 ), leaving unconstrained the faint end of the high-z QSO Luminosity Function (LF), which is particularly important to understand the interplay between the formation of galaxies and super-massive black holes (SMBH) and to measure the QSO contribution to the UV ionizing background [16]. New deep multi-wavelength surveys like the Great Observatories Origins Deep Survey (GOODS), described by M.Dickinson, M.Giavalisco and H.Ferguson in this conference, provide significant constraints on the space density of less luminous QSOs at high redshift. Here we present a search for high-z QSOs, identified in the two GOODS fields on the basis of deep imaging in the optical (with HST) and X-ray (with Chandra), and discuss the allowed space density of QSOs in the early universe, updating the results presented in [6].
    01/2006: pages 145-150;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate morphological structure parameters and local environments of distant moderate-luminosity active galactic nucleus (AGN) host galaxies in the overlap between the HST/ACS observations of the Great Observatories Origins Deep Survey (GOODS) and the two Chandra Deep Fields. We compute near-neighbor counts and BViz asymmetry (A) and concentration (C) indices for ~35,500 GOODS/ACS galaxies complete to z_850 ~ 26.6, including the resolved hosts of 322 X-ray-selected AGNs. Distributions of (1) z_850 asymmetry for 130 AGN hosts at z_850 < 23 and (2) near-neighbor counts for 173 AGN hosts at z_850 < 24 are both consistent with non-AGN control samples. This implies no close connection between recent galaxy mergers and moderate-luminosity AGN activity out to appreciable look-back times (z < 1.3), approaching the epoch of peak AGN activity in the universe. The distribution of z_850 concentrations for the AGN hosts is offset by +0.5 compared to the non-AGN, a 6.4-sigma discrepancy much larger than can be explained by the possible influence of unresolved emission from the AGN or a circumnuclear starburst. The local universe association between AGN and bulge-dominated galaxies thus persists to substantial look-back time. We discuss implications in the context of the low-redshift supermassive central black hole mass correlation with host galaxy properties, including concentration. Comment: Astrophysical Journal Letters, in press. Figure 1 available at full resolution upon request
    The Astrophysical Journal 07/2005; · 6.73 Impact Factor
  • Source
    E. T. Chatzichristou
    [Show abstract] [Hide abstract]
    ABSTRACT: Several key goals require measuring the number of all AGN in the Universe, and the evolution of the ratio of obscured to unobscured AGN with redshift. Hard X-rays can penetrate most obscuring dust columns to reveal the AGN that remains hidden in all other wavelengths, thus, combined with other wavelengths can provide us with accurate demographics of AGN. I will present and discuss some of the results coming from recent multi-wavelength surveys, placing particular focus on the systematic study of obscured AGN.
    Memorie della Societa Astronomica Italiana Supplementi. 01/2005; 7:21.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We model the X-ray, optical, and mid-infrared flux distributions of AGN in the GOODS fields, starting from hard X-ray luminosity functions and spectral energy distributions appropriate to the unified scheme for AGN. Assuming roughly 3 times as many obscured as unobscured AGN, this model is able to reproduce the observed distributions and statistical properties of the X-ray sources in the GOODS fields and at the same time is consistent with both the spectral shape and normalization of the X-ray background. This model also predicts that only half of the AGN in the GOODS fields have been detected in X-rays with Chandra 1--2 Msec exposures. The remaining obscured/low-luminosity AGN are predicted to appear as luminous infrared sources and as normal early-type galaxies in the optical, which will be identified as AGN using the Spitzer observations of the GOODS fields and with surveys at higher energies (10-100 keV). Based on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc, under NASA contract NAS5-26555. This work was supported by NASA grants HST-GO-09425(.01-A,.13-A,.26-A); NSF CAREER award AST 99-83783; NASA contract number 1224666 issued by JPL/Caltech under NASA contract 1407; ASI grant I/R/088/02; and a Royal Society University Research Fellowship.
    12/2004;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The deep X-ray, optical, and far-infrared fields that constitute the Great Observatories Origins Deep Survey (GOODS) are sensitive to obscured active galactic nuclei (AGNs; NH 1022 cm-2) at the quasar epoch (z ~ 2-3), as well as to unobscured AGNs as distant as z ~ 7. Luminous X-ray emission is a sign of accretion onto a supermassive black hole and thus reveals all but the most heavily obscured AGNs. We combine X-ray luminosity functions with appropriate spectral energy distributions for AGNs to model the X-ray, optical, and far-infrared flux distributions of the X-ray sources in the GOODS fields. A simple model based on the unified paradigm for AGNs, with ~3 times as many obscured AGNs as unobscured, successfully reproduces the z-band flux distributions measured in the deep Hubble Space Telescope ACS observations of the GOODS-North and GOODS-South fields. This model is also consistent with the observed spectroscopic and photometric redshift distributions once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background can be explained by a bias against the most heavily obscured AGNs generated by both X-ray observations and the identification of sources via optical spectroscopy. We predict the AGN number counts for Spitzer MIPS 24 μm and IRAC 3.6-8 μm observations in the GOODS fields, which will verify whether most AGNs in the early universe are obscured in the optical. Such AGNs should be very bright far-infrared sources and include some obscured AGNs missed even by X-ray observations.
    The Astrophysical Journal 11/2004; 616:123-135. · 6.73 Impact Factor
  • N. A. Grogin, C. J. Conselice, E. Chatzichristou
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate morphological structure parameters and local environments of distant AGN host galaxies in the overlap region between the HST/ACS observations of the Great Observatories Origins Deep Survey (GOODS/ACS) and the two Chandra Deep Fields. We compute asymmetry and concentration indices and near-neighbor counts for ~ 34000 GOODS/ACS galaxies, including ~ 350 X-ray-selected AGN hosts. Both the near-neighbor counts and the z850 asymmetries of the 233 resolved, z850 < 23 AGN host galaxies are consistent with the field values, implying no close connection between galaxy mergers and moderate-luminosity AGN activity at appreciable lookback times (z &lsim; 1.3) approaching the epoch of peak AGN activity in the universe. The z850 concentration indices for the AGN hosts are clearly skewed towards larger values; this 7sigma discrepancy is much larger than could be explained by the contribution of the nuclear point source to the hosts' optical emission. From this we infer that the tight correlation observed locally between supermassive central black hole mass and host galaxy properties (including concentration index) persists to the substantial redshifts of our AGN and control samples. The multi-color GOODS/ACS imaging allows us to track the comparative evolution of these morphological and environmental indicators in rest-frame B-band from z ~ 0.4-1.3 using volume-limited samples constructed from AGN redshifts in the literature and ~ 10000 GOODS/ACS field galaxy photometric redshifts. The z850 results described above are separately reproduced within each of the three redshift intervals of the rest-frame B-band analysis.
    10/2004;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We model the X-ray, optical, and far-infrared flux distributions of AGN in the GOODS fields, starting from hard X-ray luminosity functions and spectral energy distributions appropriate to the unified scheme for AGN. The deep optical counts measured from HST ACS images can be well explained by a unified scheme that postulates roughly 3 times as many obscured as unobscured AGN. This scenario is consistent with the observed spectroscopic and photometric redshift distributions of the GOODS AGN once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background (which include a similarly large number of obscured AGN) is explained by bias against the most heavily obscured AGN in both X-ray surveys and optical spectroscopic samples. We present the model predictions for the number counts of AGN in the Spitzer MIPS 24 micron and IRAC 3.6-8 micron bands. The GOODS Spitzer observations will verify whether large numbers of obscured AGN are indeed present in the early Universe; these will be very bright far-infrared sources, including some, missed by X-ray observations, that look like ultraluminous infrared galaxies. Based on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc, under NASA contract NAS5-26555. This work was supported by NASA grants HST-GO-09425(.01-A,.13-A,.26-A); NSF CAREER award AST 99-83783; NASA contract number 1224666 issued by JPL/Caltech under NASA contract 1407; ASI grant I/R/088/02; and a Royal Society University Research Fellowship.
    05/2004;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a possible new class of X-ray sources that have robust detections in ultra-deep Chandra data, yet have no detections at all in our deep multi-band GOODS Hubble Space Telescope (HST) ACS images, which represent the highest quality optical imaging obtained to date on these fields. These extreme X-ray / Optical ratio sources ("EXO"s) have values of Fx/Fopt at least an order of magnitude above those generally found for other AGN, even those that are harbored by reddened hosts. We thus infer two possible scenarios: (1) if these sources lie at redshifts z < 6, then their hosts need to be exceedingly underluminous, or more reddened, compared with other known sources; (2) if these sources lie above z ~ 6-7, such that even their Lyman-alpha emission is redshifted out of the bandpass of our ACS z(850) filter, then their optical and X-ray fluxes can be accounted for in terms of relatively normal L* hosts and moderate-luminosity AGN. Comment: AASTEX-Latex, 12 pages, 3 figures, including 1 jpg figure. Accepted by the Astrophysical Journal Letters
    The Astrophysical Journal 06/2003; · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The ratio of obscured to unobscured AGN, and the evolution of that ratio with redshift, is important for understanding (1) the AGN phenomenon and (2) the production of the extragalactic X-ray and infrared ``backgrounds.'' Current models for these backgrounds predict a different evolution for obscured AGN compared to unobscured (type 1) AGN. Deep X-ray/infrared surveys like GOODS (the Great Observatories Origins Deep Survey, a deep multi-wavelength survey using SIRTF, HST/ACS, Chandra, XMM, and an array of ground-based telescopes) test these models at virtually all redshifts. We present the expected number of obscured and unobscured sources in several wavebands, ranging from the SIRTF 24-micron MIPS band to optical HST/ACS bands to Chandra X-ray bands, based on various plausible assumptions about their luminosity functions and evolution. These semi-theoretical predictions are compared with the observed number counts obtained from the GOODS data.
    05/2003;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structural parameters of galaxies reflect the effects of physical processes related to their formation and evolution. The high spatial resolution (0.1 " ) HST ACS images obtained in the B, V, i, and z bands by the Great Observatories Origins Deep Survey (GOODS) over a large survey area (320 sq. arcmin), provides a unique opportunity to quantify the morphological evolution of galaxies out to redshifts z = 1. We present the preliminary results from an analysis based on two-dimensional modeling of the surface brightness distributions of galaxies in the GOODS-South field. The decomposition of multiple galaxy components (such as the bulge, inner and outer disks, bars, and central compact sources) is performed using multiple Sérsic profiles. The observed scale lengths, bulge magnitudes, bulge-to-disk ratios, Sérsic indices, and the correlations among these structural parameters are used to examine the evolution of the bulge and disk properties of the galaxies with redshift.
    11/2002; 201:611.
  • E. T. Chatzichristou, C. M. Urry, R. Nichol
    [Show abstract] [Hide abstract]
    ABSTRACT: The Great Observatories Origins Deep Survey (GOODS), a SIRTF Legacy and HST Treasury program, will obtain the deepest multiwavelength observations, across the broadest wavelength range, of any astronomical field. It includes infrared (3.6-24 mu m) coverage with SIRTF IRAC and MIPS, high-resolution optical coverage with ACS, and ultra-deep X-ray observations with Chandra and XMM-Newton. The scientific goals are to trace the mass assembly history of galaxies and to probe the nature of their energetic output (from stars and AGN) over a broad span of cosmic history. To study the AGN in particular, we must distinguish them from the tens of thousands of normal galaxies. Here we present our approach to identifying AGN candidates through color selection, with particular emphasis on high-redshift and/or very dusty objects as candidates for follow-up spectroscopic studies. We have compiled a library of Spectral Energy Distributions (SED) of extragalactic objects, which we combine with appropriate evolution and cosmology to populate multi-dimensional color-color space. We then incorporate novel algorithms to characterize survey objects relative to template AGN, to obtain an efficient method for finding AGN in the GOODS fields.
    05/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present realistic simulations of galaxies in the GOODS fields, especially AGN host galaxies, to determine the accuracy with which host galaxy properties can be measured through multi-color HST ACS images. In particular, we study the effect of dilution when galaxies are observed at increasing distance. Spatial resolution might bias the detection of emission from the central source (AGN) or its surrounding star formation regions, and the accuracy with which we can measure the host morphological and structural parameters. In extreme cases, the AGN contribution may completely dominate the total emission coming from a galaxy or on the other hand, a faint/partly obscured AGN can be completely dominated by the host disk emission. We simulate host galaxies of various spectral types and luminosities, to investigate the effects of limited spatial resolution on the observed AGN/starburst/host galaxy dominance and the accuracy with which we can recover the AGN host characteristics, with various fitting routines of these synthetic images.
    01/2002;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: There is growing evidence that the formation of galaxies and the growth of central black holes must be closely related. This ``grand unification'' of AGN and galaxies can be tested through studies of the AGN host galaxies. Using the deep multi-color HST ACS imaging data, we describe the morphologies of AGN host galaxies in the GOODS (Great Observatories Origins Deep Survey) southern field, which coincides with the Chandra Deep Field - South. We compare these to the morphologies of normal galaxies at the same (photometric) redshifts, also taken from the GOODS ACS data. Ground-based imaging data were taken at ESO 2.2m, NTT, VLT, and NOAO 4m telescopes and made publicly available. This work is supported by NASA.
    01/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present morphological CAS (concentration, asymmetry, and clumpiness) parameters out to z 1 for galaxies in an area covering and surrounding the CDF-S which we are imaging in 4 bands (B,V,i,z) with the ACS Wide Field Camera. These parameters will allow us to determine, based on a nearby galaxy calibration, the similarities between each galaxy and the various major types. We will focus in particular on the morphological mix in the redshift-limited sample, with a special emphasis on the evolution in the fraction of interactions and mergers with redshift. To do this, we will first calculate rest-frame B-band morphologies for the galaxies brighter than a given magnitude limit. From this, and using a local calibration, the morphological mix (fraction of mergers, late types, and early types) can be computed as a function of redshift. It should also be possible to track changes in the parameter values themselves as a function of redshift, and this can be used to assess their implications for the morphological mix. Other relations between variables such as spectral type (from our photometric redshift catalogs) and the degree of asymmetry can also be used to probe questions such as the relationship between starburst galaxies and mergers, for example.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present infrared through X-ray spectral energy distributions (SEDs) of the 198 hard X-ray selected, z-band identified, active galactic nuclei (AGNs) from the GOODS multiwavelength survey. These SEDs include photometry from Spitzer/IRAC observations. Fits to the optical/IR SEDs with local empirical SED templates of both normal and active galaxies identify 36 Type 1 AGN, 89 Type 2 AGN, 64 early-type galaxies and 9 late-type spiral galaxies. Because the majority of the ``early-type galaxies'' are strong X-ray sources with high X-ray hardness ratios and strong infrared emission, they likely harbor fully obscured nuclear activity. Assuming a 15% contamination factor of starbursts in the Type 2 classification, the ratio of obscured to unobscured AGN in our sample is ˜ 3.7:1 for the full redshift range of 0.3 < z < 3.7, and ˜ 4.1:1 for 0.5 < z 2, but these values are suspect due to low number statistics and reliance on photometric redshifts due to the R < 24 spectroscopic cutoff. We compare NH column densities as obtained from model SED fits with predicted distributions (Treister et al. 2004), as well as comparing optical morphology classifications to the observed SEDs. Color-color diagrams of IRAC bands and bolometric luminosity estimates demonstrate the utility of using Spitzer to identify high redshift, moderate luminosity, highly-obscured AGN. Due to the breadth of wavelength coverage, the depth of the images, and precision of source positions, the present sample constitiues one of the largest, least-biased AGN samples to date, representative of the true AGN population.

Publication Stats

319 Citations
40.40 Total Impact Points

Institutions

  • 2008
    • University of Patras
      Rhion, West Greece, Greece
    • National Observatory of Athens
      Athínai, Attica, Greece
  • 2003–2008
    • Yale University
      • Department of Astronomy
      New Haven, CT, United States