The Chandra Deep Field-South: Optical Spectroscopy. I.

By G. Szokoly, J. Bergeron, G. Hasinger, I. Lehmann, Lisa J. Kewley, V. Mainieri, M. Nonino, Piero Rosati, R. Giacconi, R. Gilli and 8 more. 2003

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We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field-South (CDFS). A total of 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R - K colors are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R < 24 observed sample comprises 161 X-ray objects (181 optical counterparts), and 126 of them have unambiguous spectroscopic identification. There are two spikes in the redshift distribution, predominantly populated by type 2 active galactic nuclei (AGNs) but also type 1 AGN and X-ray normal galaxies: the one at z = 0.734 is fairly narrow (in redshift space) and comprises two clusters/groups of galaxies centered on extended X-ray sources, the second one at z = 0.674 is broader and should trace a sheetlike structure. The type 1 and type 2 populations are clearly separated in X-ray/optical diagnostics involving parameters sensitive to absorption/reddening: X-ray hardness ratio (HR), optical/near-IR color, soft X-ray flux, and optical brightness. Nevertheless, these two populations cover similar ranges of hard X-ray luminosity and absolute K magnitude, thus trace similar levels of gravitational accretion. Consequently, we introduce a new classification based solely on X-ray properties, HR, and X-ray luminosity, consistent with the unified AGN model. This X-ray classification uncovers a large fraction of optically obscured, X-ray-luminous AGNs missed by the classical optical classification. We find a similar number of X-ray type 1 and type 2 QSOs [LX(0.5-10 keV) > 1044 ergs s-1] at z > 2 (13 sources with unambiguous spectroscopic identification); most X-ray type 1 QSOs are bright, R ≲ 24, whereas most X-ray type 2 QSOs have R ≳ 24, which may explain the difference with the CDFN results as few spectroscopic redshifts were obtained for R > 24 CDFN X-ray counterparts. There are X-ray type 1 QSOs down to z ~ 0.5, but a strong decrease at z < 2 in the fraction of luminous X-ray type 2 QSOs may indicate a cosmic evolution of the X-ray luminosity function of the type 2 population. An X-ray spectral analysis is required to confirm this possible evolution. The red color of most X-ray type 2 AGNs could be due to dust associated with the X-ray absorbing material and/or a substantial contribution of the host galaxy light. The latter can also be important for some redder X-ray type 1 AGNs. There is a large population of EROs (R - K > 5) as X-ray counterparts, and their fraction strongly increases with decreasing optical flux, up to 25% for the R ≥ 24 sample. They cover the whole range of X-ray hardness ratios, comprise objects of various classes (in particular a high fraction of z ≳ 1 X-ray absorbed AGNs, but also elliptical and starburst galaxies) and more than half of them should be fairly bright X-ray sources [LX(0.5-10 keV) > 1042 ergs s-1]. Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

SPECTROSCOPY OF BROAD-LINE BLAZARS FROM 1LAC (2012, unknown): “This correction term was constructed following Szokoly et al. (2004).”
Cosmic X-ray surveys of distant active galaxies (2015, background): “…complete spectroscopic identification down to I = 22–24 are now available for statistical studies (e.g., Barger et al 2003b; Fiore et al 2003; Szokoly et al 2004; Eckart et al 2006; Della Ceca et al 2008; Trouille et al 2008; Trump et al 2009; Silverman et al 2010; Kochanek et al 2012;…”
AN ALMA SURVEY OF SUBMILLIMETER GALAXIES IN THE EXTENDED CHANDRA DEEP FIELD SOUTH: THE REDSHIFT DISTRIBUTION AND EVOLUTION OF SUBMILLIMETER GALAXIES (2013, background): “…(Cristiani et al. 2000; Croom et al. 2001; Cimatti et al. 2002; Teplitz et al. 2003; Bunker et al. 2003; Le Fèvre et al. 2004; Zheng et al. 2004; Szokoly et al. 2004; Strolger et al. 2004; Stanway et al. 2004; van der Wel et al. 2005; Mignoli et al. 2005; Daddi et al. 2005; Doherty et al.…”
The evolution of the hard X-ray luminosity function of AGN (2009, methodology): “Large efforts have been made to obtain follow-up observations of X-ray sources in various X-ray surveys to determine their redshifts, and thus allow the X-ray luminosity function (XLF) to be determined (e.g. Barger et al. 2003; Szokoly et al. 2004; Trouille et al. 2008).”
THE CHANDRA DEEP FIELD-SOUTH SURVEY: 4 Ms SOURCE CATALOGS (2008, methodology): “Spectroscopic redshifts were collected from Le Fèvre et al. (2004), Szokoly et al. (2004), Zheng et al. (2004), Mignoli et al. (2005), Ravikumar et al. (2007), Vanzella et al. (2008), Popesso et al. (2009), Treister et al. (2009)44, Balestra 42 The AB magnitudes for the radio counterparts were…”
THE EVOLUTION OF THE GALAXY REST-FRAME ULTRAVIOLET LUMINOSITY FUNCTION OVER THE FIRST TWO BILLION YEARS (2014, background): “…The spectroscopic redshifts come from a compilation made by N. Hathi (2015, private communication), which include data from the following studies: Szokoly et al. (2004), Grazian et al. (2006), Barger et al. (2008), Hathi et al. (2008), Vanzella et al. (2008, 2009), Rhoads et al. (2009), Wuyts et…”
The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era (2006, methodology): “…programs have shown that the vast majority of the XRB sources are identified as Active Galactic Nuclei (AGN) with different degrees of obscuration (e.g. Szokoly et al. 2004, Barger et al. 2005) confirming early predictions by AGN synthesis models (Setti & Woltjer 1989, Comastri et al. 1995).”
GALAXY STELLAR MASS FUNCTIONS FROM ZFOURGE/CANDELS: AN EXCESS OF LOW-MASS GALAXIES SINCE z = 2 AND THE RAPID BUILDUP OF QUIESCENT GALAXIES (2013, methodology): “Spectroscopic redshifts from CDFS come from Vanzella et al. (2008), Le Fèvre et al. (2005), Szokoly et al. (2004), Doherty et al. (2005), Popesso et al. (2009), and Balestra et al. (2010).”
SPACE DENSITY OF OPTICALLY SELECTED TYPE 2 QUASARS (2008, background): “…samples, at redshifts z ∼ 3, suggest that the fraction of obscured quasars is large at low luminosities, and then decreases at higher luminosities (Ueda et al. 2003; Szokoly et al. 2004; Barger et al. 2005; Markwardt et al. 2005; Treister et al. 2006; Beckmann et al. 2006; Sazonov et al. 2007).”
DEEP EXTRAGALACTIC X-RAY SURVEYS (2005, background): “A number of X-ray obscured quasars have been found in deep surveys (e.g., Norman et al. 2002; Stern et al. 2002b; Barger et al. 2003a; Szokoly et al. 2004).”
Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey (2006, background): “There is evidence that the fraction of obscured AGNs decreases with higher intrinsic luminosities (Ueda et al. 2003; Szokoly et al. 2004; Barger et al. 2005; Treister & Urry 2005) and possibly increases with redshift (La Franca et al. 2005).”
Multiwavelength Study of Massive Galaxies at z~2. I. Star Formation and Galaxy Growth (2007, methodology): “…of publicly available redshifts (VVDS, Le Fèvre et al. 2004; GOODS/ FORS2, Vanzella et al. 2005, 2006; K20 survey, Mignoli et al. 2005; CDFS survey, Szokoly et al. 2004), we have most notably used results of redshift surveys developed as part of GOODS or in collaboration with GOODS that…”
THE 2 Ms CHANDRA DEEP FIELD-NORTH SURVEY AND THE 250 Ks EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: IMPROVED POINT-SOURCE CATALOGS (2016, unknown): “Columns 50–52 give the zspec, zspec quality flag, and zspec reference. zspec’s are collected from (1) Szokoly et al. (2004), (2) Zheng et al. (2004), (3) Ravikumar et al. (2007), (4) Treister et al. (2009), (5) Balestra et al. (2010), (6) Silverman et al. (2010), (7) Bonzini et al. (2012), (8)…”
A CRITICAL ASSESSMENT OF PHOTOMETRIC REDSHIFT METHODS: A CANDELS INVESTIGATION (2013, unknown): “…(2000), Croom et al. (2001), Bunker et al. (2003), Dickinson et al. (2004), LeFèvre et al. (2004), Stanway et al. (2004), Strolger et al. (2004), Szokoly et al. (2004), van der Wel et al. (2004), Doherty et al. (2005), Mignoli et al. (2005), Roche et al. (2006), Ravikumar et al. (2007),…”
Spitzer Observations of Massive, Red Galaxies at High Redshift (2005, background): “Spectroscopic redshifts are available for 12 of the DRGs (Szokoly et al. 2004; Mignoli et al. 2005; Vanzella et al. 2005; D. Stern 2005, private communication).”
The MUSE Hubble Ultra Deep Field Survey: I. Survey description, data reduction and source detection (2017, background): “…confidence ground-based spectroscopic redshifts from various instruments and surveys (see their Table 3): VIMOSVVDS (Le Fèvre et al. 2004), FORS1&2 (Szokoly et al. 2004; Mignoli et al. 2005) VIMOS-GOODS (Vanzella et al. 2005– 2009; Popesso et al. 2009; Balestra et al. 2010) and VIMOSGMASS (Kurk et…”
CALIFA : a diameter-selected sample for an integral field spectroscopy galaxy survey (2014, background): “When the X-ray luminosity of a source exceeds log(L2−10 keV/[erg s−1]) > 42 (e.g. Szokoly et al. 2004) in the soft or log (L15−195 keV/[erg s−1]) > 42.2 in the hard band, it is most likely harbouring an AGN.”