Astrophys. J., Suppl. Ser., 155, 271-349 (2004/December-0)
The Chandra Deep Field-South: Optical spectroscopy. I..
SZOKOLY G.P., BERGERON J., HASINGER G., LEHMANN I., KEWLEY L., MAINIERI V., NONINO M., ROSATI P., GIACCONI R., GILLI R., GILMOZZI R., NORMAN C., ROMANIELLO M., SCHREIER E., TOZZI P., WANG J.X., ZHENG W. and ZIRM A.
Abstract (from CDS):
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(0.5-10keV)>1044 ergs/s] 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(0.5-10keV)>1042 ergs/s]. Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.
Cosmology: Observations - Galaxies: Active - Galaxies: Quasars: General - Surveys - Techniques: Spectroscopic - X-Rays: Galaxies: Clusters
VizieR on-line data:
<Available at CDS (J/ApJS/155/271): table4.dat table5.dat table8.dat table9.dat>
Tables 4, 8: [SBH2004] JHHMMSS.ss+DDMMSS.s N=288+139. Tables 4, 8: [SBH2004] XID NNNa N=103+50. Table 9: [SBH2004] NNNNN N=124 among (Nos 12-60041). Table 5: [GZW2002] XID NNN (Nos 901-903) added.
Status at CDS:
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