New Results from the X-Ray and Optical Survey of the Chandra Deep Field-South: The 300 Kilosecond Exposure. II.

By P. Tozzi, Piero Rosati, M. Nonino, Jacqueline Bergeron, Stefano Borgani, R. Gilli, R. Gilmozzi, G. Hasinger, Norman Grogin, Lisa J. Kewley and 9 more. 2001

169 citations recorded by the source corpus.

We present results from 300 ks of X-ray observations of the Chandra Deep Field-South. The field of the four combined exposures is now 0.1035 deg2, and we reach a flux limit of 10-16 ergs s-1 cm-2 in the 0.5-2 keV soft band and 10-15 ergs s-1 cm-2 in the 2-10 keV hard band, i.e., a factor of 2 fainter than the previous 120 ks exposure. The total catalog is composed of 197 sources including 22 sources detected only in the hard band, 51 only in the soft band, and 124 detected in both bands. We now have the optical spectra for 86 optical counterparts. The log N-log S relationship of the whole sample confirms the flattening with respect to the ASCA hard counts and the ROSAT soft counts. The average logarithmic slopes of the number counts are α = 0.66 ± 0.06 and α = 0.92 ± 0.12 in the soft and hard bands, respectively. Double power-law fits to the differential counts show evidence of further flattening at the very faint end to slopes of 0.5 ± 0.1 and 0.6 ± 0.2 in the soft and hard bands, respectively. We compute the total contribution to the X-ray background (XRB) in the 2-10 keV band, which now amounts to (1.45 ± 0.15) × 10-11 ergs cm-2 s-1 deg-2 (after the inclusion of the ASCA sources to account for the bright end) to a lower flux limit of 10-15 ergs s-1 cm-2. This corresponds to 60%-90% of the unresolved hard XRB, given the uncertainties on its actual value. We confirm previous findings on the average spectrum of the sources, which is well described by a power law with Γ = 1.44 ± 0.03, and the progressive hardening of the sources at lower fluxes. In particular, we find that the average spectral slope of the sources is flatter than the average for fluxes lower than 9 × 10-15 ergs s-1 cm-2 in the hard band. The hardening of the spectra is consistent with an increasing fraction of absorbed objects (NH > 1022 cm-2) at low fluxes. From 86 redshifts available at present, we find that hard sources have on average lower redshifts (z ≤ 1) than soft sources. Their typical luminosities and optical spectra show that most of these sources are obscured active galactic nuclei (AGNs), as expected by AGN population synthesis models of the XRB. We are still in the process of finding hard sources that constitute the remaining fraction of the total XRB. Most of the sources detected only in the soft band appear to be optically normal galaxies with luminosities LX ≃ 1040-1042 ergs s-1. This population appears to be a mix of normal galaxies, possibly with enhanced star formation, and galaxies with low-level nuclear activity.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/b23f6fb5c5ec286fbdbcb767ab815a38fe410ebb

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

Citations in the corpus

The Fall of Active Galactic Nuclei and the Rise of Star-forming Galaxies: A Close Look at the Chandra Deep Field X-Ray Number Counts (2004, background): “…et al. 1998; Akiyama et al. 2003), whereas below this level several other populations emerge, such as obscured AGNs (e.g., Alexander et al. 2001; Tozzi et al. 2001; Barger et al. 2002; Mainieri et al. 2002) and starburst and quiescent galaxies (e.g., Giacconi et al. 2001; Alexander et al.…”
The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era (2006, unknown): “…stressing that the above described steepening of the intrinsic source spectra towards faint fluxes is due tothe assumption of an intrinsic dispersion of the average power law slopes and is by no means in contrast with the flattening of the average slope observed in deep fields (Tozzi et al. 2001b).”
The Extended Chandra Deep Field-South Survey: Chandra Point-Source Catalogs (2005, result): “Presently, the two deepest X-ray surveys are the 2 Ms Chandra Deep Field–North (CDF-N; Brandt et al. 2001, hereafter B01; Alexander et al. 2003, hereafter A03) and the 1 Ms Chandra Deep Field–South (CDF-S; Giacconi et al. 2002, hereafter G02).”
The Resolved Fraction of the Cosmic X-Ray Background (2003, background): “…low as 2 10 16 ergs s 1 cm 2, resolving about 80%–90% of the hard CXB (Mushotzky et al. 2000; Hornschemeier et al. 2000, 2001; Brandt et al. 2001; Hasinger et al. 2001; Tozzi et al. 2001; Campana et al. 2001; Rosati et al. 2002; Moretti et al. 2002; Miyaji & Griffiths 2002; Giacconi et al. 2002).”
“Hidden” Seyfert 2 Galaxies and the X-Ray Background (2002, background): “Recently published spectroscopic studies of faint Chandra sources are based mainly on data acquired with 8–10 m telescopes (Barger et al. 2002; Hornschemeier et al. 2001; Tozzi et al. 2001).”
The Chandra Deep Field-South: Optical Spectroscopy. I. (2003, background): “…fields are still in progress, however, a mixture of obscured and unobscured AGNs with an increasing fraction of obscuration at lower flux levels seems to be the dominant population in these samples (Fiore et al. 2000; Barger et al. 2001a; Tozzi et al. 2001; Rosati et al. 2002; Stern et al. 2002).”
DEEP EXTRAGALACTIC X-RAY SURVEYS (2005, background): “…for the shallower Lockman Hole (Hasinger et al. 2001) and Lynx (Stern et al. 2002a) fields agree with those for the Chandra Deep Fields to within statistical errors, whereas those for the SSA13 (Mushotzky et al. 2000) field appear to be ≈ 40% higher in the 2–8 keV band (see Tozzi et al. 2001).”
A Classic Type 2 QSO (2001, unknown): “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.”
The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy (2002, background): “%) of the X–ray background emission below 8 keV have been detected byChandradeep surveys down to extremely faint fluxes S2−8keV < 10−15 erg cm−2 s−1 (Mushotzky et al. 2000; Giacconi et al. 2001; Hornschmeier et al. 2001; Brandt et al. 2001; Tozzi et al. 2001).”
Chandra Deep Field South: The 1 Ms Catalog (2002, methodology): “The definition of S/N was that No. 2, 2002 CHANDRA DEEP FIELD SOUTH CATALOG 371 adopted by Tozzi et al. (2001).”
The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei (2004, background): “…selected broad-line AGNs comprise only about one-third (e.g., Barger et al. 2003b) of the X-ray background (Giacconi et al. 1962), and many X-ray sources show no signs of AGN activity at all in their optical spectra (e.g., Barger et al. 2001b; Tozzi et al. 2001; Hornschemeier et al. 2001).”
An Overview of the Performance and Scientific Results from the Chandra X‐Ray Observatory (2001, background): “The spectrum of all of the sources has a slope very near to 1.4 (Tozzi et al. 2001; Garmire et al. 2001), consistent with the wide field-of-view measurements made with detectors that could not resolve the fainter sources.”
The Great Observatories Origins Deep Survey: Initial Results from Optical and Near-Infrared Imaging (2003, unknown): “The data and their reduction are described in Tozzi et al. (2001).”
Study of the X-Ray Background Spectrum and its Large-Scale Fluctuation with ASCA (2002, background): “Moreover, recent Chandra and XMM-Newton results indicate a turn-over of around S ∼ 2 × 10−14 ergcm−2 s−1 (Campana et al. 2001; Hasinger et al. 2001; Baldi et al. 2002).”
The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs (2003, result): “This hardening of the X-ray spectral slope has been reported in other studies (e.g., della Ceca et al. 1999; Ueda et al. 1999; Mushotzky et al. 2000; Tozzi et al. 2001; B01) and appears to be due to an increase in the number of absorbed AGNs detected.”