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Register a new userThe Chandra Deep Field South/GOODS survey. Optically faint X-ray sources
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Vincenzo Mainieri Dr.
Publication date
2005
fields
Physics
and research's language is
English
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We provide important new constraints on the nature and redshift distribution of optically faint (R>25) X-ray sources in the Chandra Deep Field South Survey. We show that we can derive accurate photometric redshifts for the spectroscopically unidentified sources thus maximizing the redshift completeness for the whole X-ray sample. Our new redshift distribution for the X-ray source population is in better agreement with that predicted by X-ray background synthesis models; however, we still find an overdensity of low redshift (z<1) sources. The optically faint sources are mainly X-ray absorbed AGN, as determined from direct X-ray spectral analysis and other diagnostics. Many of these optically faint sources have high (>10) X-ray-to-optical flux ratios. We also find that ~71% of them are well fitted with the SED of an early-type galaxy with <z_phot>~1.9 and the remaining 29% with irregular or starburst galaxies mainly at z_phot>3. We estimate that 23% of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a ~15% fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ~38% contribution.
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We have analyzed optically bright, X-ray faint [OBXF; i.e., log(fX/fR) < -2] sources identified in an 178.9 square arcminute area within the Chandra Deep Field-North (CDF-N) 2 Ms survey. We find 43 OBXF sources in this area, comprising ~15% of the X-ray sources above a 0.5--2 keV flux of 2.3e-17 erg cm^-2 s^-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including 5 previously unpublished redshifts. Deep optical imaging data (either HST or ground-based) are presented for all the OBXF sources. The OBXF population consists mainly of normal and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z=0.297 and a full redshift range z=0.06-0.845). This is notable since these distances equate to look-back times of up to ~8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to Chandra. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of normal galaxies indicating that a significant fraction are likely dominated by low-luminosity AGN (LLAGN) or vigorous star formation. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a Gamma=2.0 power law. The 0.5--2 keV log N-log S for the OBXF galaxies is much steeper (alpha=-1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1~Ms and 2~Ms survey, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ~1-2% of the soft extragalactic X-ray background.
We present the results of a program to acquire high-quality optical spectra of X-ray sources detected in the E-CDF-S and its central area. New spectroscopic redshifts are measured for 283 counterparts to Chandra sources with deep exposures (t~2-9 hr per pointing) using multi-slit facilities on both the VLT and Keck thus bringing the total number of spectroscopically-identified X-ray sources to over 500 in this survey field. We provide a comprehensive catalog of X-ray sources detected in the E-CDF-S including the optical and near-infrared counterparts, and redshifts (both spectroscopic and photometric) that incorporate published spectroscopic catalogs thus resulting in a final sample with a high fraction (80%) of X-ray sources having secure identifications. We demonstrate the remarkable coverage of the Lx-z plane now accessible from our data while emphasizing the detection of AGNs that contribute to the faint end of the luminosity function at 1.5<z<3. Our redshift catalog includes 17 type 2 QSOs that significantly increases such samples (2x). With our deepest VIMOS observation, we identify elusive optically-faint galaxies (R~25) at z~2-3 based upon the detection of interstellar absorption lines; we highlight one such case, an absorption-line galaxy at z=3.208 having no obvious signs of an AGN in its optical spectrum. In addition, we determine distances to eight galaxy groups with extended X-ray emission. Finally, we measure the physical extent of known large-scale structures (z~0.7) evident in the CDF-S. While a thick sheet (radial size of 67.7 Mpc) at z~0.67 extends over the full field, the z~0.73 structure is thin (18.8 Mpc) and filamentary as traced by both AGNs and galaxy groups. In the appendix, we provide spectroscopic redshifts for 49 counterparts to fainter X-ray sources detected only in the 1 and 2 Ms catalogs, and 48 VLA radio sources not detected in X-rays.
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the AGN host galaxies have on average redder rest-frame optical colors than non-active galaxies, and that they live mostly in the green valley. The dependence of the fraction of AGN that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN space density is compared with the expectations from existing luminosity functions. These AGN show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGN, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.
The Extended Chandra Deep Field-South (ECDFS) survey consists of 4 Chandra ACIS-I pointings and covers $approx$ 1100 square arcminutes ($approx$ 0.3 deg$^2$) centered on the original CDF-S field to a depth of approximately 228 ks. This is the largest Chandra survey ever conducted at such depth, and only one XMM-Newton survey reaches a lower flux limit in the hard 2.0--8.0 keV band. We detect 651 unique sources -- 587 using a conservative source detection threshold and 64 using a lower source detection threshold. These are presented as two separate catalogs. Of the 651 total sources, 561 are detected in the full 0.5--8.0 keV band, 529 in the soft 0.5--2.0 keV band, and 335 in the hard 2.0--8.0 keV band. For point sources near the aim point, the limiting fluxes are approximately $1.7 times 10^{-16}$ $rm{erg cm^{-2} s^{-1}}$ and $3.9 times 10^{-16}$ $rm{erg cm^{-2} s^{-1}}$ in the 0.5--2.0 keV and 2.0--8.0 keV bands, respectively. Using simulations, we determine the catalog completeness as a function of flux and assess uncertainties in the derived fluxes due to incomplete spectral information. We present the differential and cumulative flux distributions, which are in good agreement with the number counts from previous deep X-ray surveys and with the predictions from an AGN population synthesis model that can explain the X-ray background. In general, fainter sources have harder X-ray spectra, consistent with the hypothesis that these sources are mainly obscured AGN.
We discuss the X-ray properties of the radio sources detected in a deep 1.4 and 5 GHz VLA Radio survey of the Extended Chandra Deep Field South (E-CDFS). Among the 266 radio sources detected, we find 89 sources (1/3 of the total) with X-ray counterparts in the catalog of the 1Ms exposure of the central 0.08 deg^2 (Giacconi et al. 2002; Alexander et al. 2003) or in the catalog of the 250 ks exposure of the 0.3 deg^2 E-CDFS field (Lehmer et al. 2005). For 76 (85%) of these sources we have spectroscopic or photometric redshifts, and therefore we are able to derive their intrinsic properties from X-ray spectral analysis, namely intrinsic absorption and total X-ray luminosities. We find that the population of submillijansky radio sources with X-ray counterparts is composed of a mix of roughly 1/3 star forming galaxies and 2/3 AGN. The distribution of intrinsic absorption among X-ray detected radio sources is different from that of the X-ray selected sample. Namely, the fraction of low absorption sources is at least two times larger than that of X-ray selected sources in the CDFS. This is mostly due to the larger fraction of star forming galaxies present among the X-ray detected radio sources. If we investigate the distribution of intrinsic absorption among sources with L_X>10^42 erg s^-1 in the hard 2-10 keV band (therefore in the AGN luminosity regime), we find agreement between the X-ray population with and without radio emission. In general, radio detected X-ray AGN are not more heavily obscured than the non radio detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGN.
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