Astrophys. J., 551, 624-634 (2001/April-3)
First results from the X-ray and optical survey of the Chandra deep field south.
GIACCONI R., ROSATI P., TOZZI P., NONINO M., HASINGER G., NORMAN C., BERGERON J., BORGANI S., GILLI R., GILMOZZI R. and ZHENG W.
Abstract (from CDS):
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 2x10–16 erg.s–1.cm–2 in the 0.5-2 keV soft band and 2x10–15 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≃8x1019 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 logN-logS 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)x10–12 erg.cm–2.s–1.deg–2 to the total XRB. The flux resolved in the hard band down to the flux limit of 2x10–15 erg.s–1.cm–2 contributes (1.05±0.2)x10–11 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 1.3x10–11 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.
Cosmology: Observations - Cosmology: Diffuse Radiation - Galaxies: Active - Surveys - X-Rays: Galaxies
Table 3: same objects as in 2001MmSAI..72..831G Table 1: objects not identified.
View the references in ADS
To bookmark this query, right click on this link: simbad:2001ApJ...551..624G and select 'bookmark this link' or equivalent in the popup menu