First Results from the X-Ray and Optical Survey of the Chandra Deep Field South

By R. Giacconi, Piero Rosati, P. Tozzi, M. Nonino, G. Hasinger, C. Norman, J. Bergeron, Stefano Borgani, R. Gilli, R. Gilmozzi and 1 more. 2000

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We present our first results from 120 ks of X-ray observations obtained with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. The field of the two combined exposures is 0.096 deg2 and the detection limit is to a S/N of 2 (corresponding to ~7 net counts). We reach a flux of 2 × 10-16 erg s-1 cm-2 in the 0.5-2 keV soft band and 2 × 10-15 erg s-1 cm-2 in the 2-10 keV hard band. Our combined sample has 144 soft sources and 91 hard sources, for a total of 159 sources. Fifteen sources are detected only in the hard band, and 68 only in the soft band. For the optical identification, we carried out a survey in VRI with the FORS-1 imaging spectrometer on the Antu telescope (UT-1 at VLT) complete to R ≤ 26. This data set was complemented with data from the ESO Imaging Survey (EIS) in the UBJK bands and the ESO Wide Field Imager Survey (WFI) in the B band. The positional accuracy of the X-ray detections is of the order of 1'' in the central 6'. Optical identifications are found for ≃90% of the sources. Optical spectra have been obtained for 12 objects. We obtain the cumulative spectra of the faint and bright X-ray sources in the sample and also the hardness ratios of individual sources. A power-law fit in the range 2-10 keV using the Galactic value of NH ≃ 8 × 1019 cm-2 yields a photon index of Γ = 1.70 ± 0.12 and 1.35 ± 0.20 (errors at 90% confidence level) for the bright and faint samples, respectively, showing a flattening of the spectrum at lower fluxes. Hardness ratio is given as a function of X-ray flux and confirms this result. The spectrum of our sources is approaching the spectrum of the X-ray background (XRB) in the hard band, which has an effective Γ = 1.4. Correlation function analysis for the angular distribution of the sources indicates that they are significantly clustered on scales as large as 100''. The scale dependence of the correlation function is a power law with index γ ~ 2, consistent with that of the galaxy distribution in the local universe. Consequently, the discrete sources detected by deep Chandra-pointed observations can be used as powerful tracers of the large-scale structure at high redshift. We discuss the log N- log S relationship and the discrete source contribution to the integrated X-ray sky flux. In the soft band, the sources detected in the field at fluxes below 10-15 erg s-1 cm-2 contribute (4.0 ± 0.3) × 10-12 erg cm-2 s-1 deg-2 to the total XRB. The flux resolved in the hard band down to the flux limit of 2 × 10-15 erg s-1 cm-2 contributes (1.05 ± 0.2) × 10-11 erg cm-2 s-1 deg-2. Once the contribution from the bright counts resolved by ASCA is included, the total resolved XRB amounts to 1.3 × 10-11 erg cm-2 s-1 deg-2, which is 60%-80% of the total measured background. This result confirms that the XRB is due to the integrated contribution of discrete sources, but shows that there is still a relevant fraction (at least 20%) of the hard XRB to be resolved at fluxes below 10-15 erg s-1 cm-2. We discuss the X-ray flux versus R magnitude relation for the identified sources. We find that ≃10% of the sources in our sample are not immediately identifiable at R > 26. For these sources, SX/Sopt > 15, whereas most of the ROSAT and Chandra sources have SX/Sopt < 10. We have also found a population of objects with unusually low SX/Sopt that are identified as galaxies. The R-K versus R color diagram shows that the Chandra sources continue the trend seen by ROSAT. For our 12 spectroscopically studied objects with redshifts, we observe four QSOs, five Seyfert 2 galaxies, one elliptical, and two interacting galaxies. We compare LX versus z obtained with these measurements and show that Chandra is achieving the predicted sensitivity.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

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Bayesian Estimation of Hardness Ratios: Modeling and Computations (2006, background): “Because these faint sources have only a few counts, hardness ratios are commonly used to study properties of their X-ray emission and absorption (e.g., Alexander et al. 2001, Brandt et al. 2001a, Brandt et al. 2001b, Giacconi et al. 2001, Silverman et al. 2005).”
A Deep Chandra Survey of the Globular Cluster 47 Tucanae: Catalog of Point Sources (2005, neutral): “Assuming a ¼ 1:4 photon index power-law spectrum (Giacconi et al. 2001; Brandt et al. 2001) and NH ¼ 1:3 ; 1020 cm 2, 20 counts is a 0.5–2 keV flux of 1:63 ; 10 16 ergs cm 2 s 1, giving 2860 200 sources deg 2.”
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The Herschel Multi-tiered Extragalactic Survey: HerMES (2012, neutral): “1 13.51 30 416 30 30 42 0.64 0.55 SDP 15 L2 ECDFS Sp. Nom.”
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GALAXY ZOO: THE FUNDAMENTALLY DIFFERENT CO-EVOLUTION OF SUPERMASSIVE BLACK HOLES AND THEIR EARLY- AND LATE-TYPE HOST GALAXIES (2010, methodology): “…account for the the observed peak at z ∼ 0.7 in the number density of moderate-luminosity X-ray selected AGN at z ∼ 0.7, while the mode at work in late-types is responsible for the high redshift peak of luminous AGN (Giacconi et al. 2001; Alexander et al. 2001; Ueda et al. 2003; Gilli et al. 2005).”
A Classic Type 2 QSO (2001, background): “From our multiwaveband studies of the 1 Ms exposure of the Chandra Deep Field–South (CDF-S; Giacconi et al. 2001, 2002; Tozzi et al. 2001; Rosati et al. 2002), we now discuss a classic X-ray–selected type 2 QSO: CXOCDFS J033229.”
ESO imaging survey - Deep public survey: Multi-color optical data for the Chandra Deep Field South (2001, background): “One of the selected regions includes the Chandra Deep Field South (CDF-S; Giacconi et al. 2001) chosen because of its low column-density of hydrogen and absence of bright stars, characteristics that make it wellsuited for deep X-ray observations with the Chandra (AXAF) telescope.”
A long-wavelength view on galaxy evolution from deep surveys by the Infrared Space Observatory (2001, background): “An important diagnostic is being offered by observations of the hard X-ray flux, since starbursts are weaker X-ray emitters than any kinds of AGNs.”
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