The 4Ms Chandra Deep Field-South number counts apportioned by source class: Pervasive active galactic nuclei and the ascent of normal galaxies

B. D. Lehmer, Y. Q. Xue, W. N. Brandt, D. M. Alexander, F. E. Bauer, M. Brusa, A. Comastri, R. Gilli, A. E. Hornschemeier, B. Luo, M. Paolillo, A. Ptak, O. Shemmer, D. P. Schneider, P. Tozzi, C. Vignali

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174 Scopus citations


We present 0.5-2keV, 2-8keV, 4-8keV, and 0.5-8keV (hereafter soft, hard, ultra-hard, and full bands, respectively) cumulative and differential number-count (log N-log S) measurements for the recently completed 4Ms Chandra Deep Field-South (CDF-S) survey, the deepest X-ray survey to date. We implement a new Bayesian approach, which allows reliable calculation of number counts down to flux limits that are factors of 1.9-4.3times fainter than the previously deepest number-count investigations. In the soft band (SB), the most sensitive bandpass in our analysis, the 4Ms CDF-S reaches a maximum source density of 27,800deg-2. By virtue of the exquisite X-ray and multiwavelength data available in the CDF-S, we are able to measure the number counts from a variety of source populations (active galactic nuclei (AGNs), normal galaxies, and Galactic stars) and subpopulations (as a function of redshift, AGN absorption, luminosity, and galaxy morphology) and test models that describe their evolution. We find that AGNs still dominate the X-ray number counts down to the faintest flux levels for all bands and reach a limiting SB source density of 14,900deg-2, the highest reliable AGN source density measured at any wavelength. We find that the normal-galaxy counts rise rapidly near the flux limits and, at the limiting SB flux, reach source densities of 12,700deg -2 and make up 46% ± 5% of the total number counts. The rapid rise of the galaxy counts toward faint fluxes, as well as significant normal-galaxy contributions to the overall number counts, indicates that normal galaxies will overtake AGNs just below the 4Ms SB flux limit and will provide a numerically significant new X-ray source population in future surveys that reach below the 4Ms sensitivity limit. We show that a future 10Ms CDF-S would allow for a significant increase in X-ray-detected sources, with many of the new sources being cosmologically distant (z ≳ 0.6) normal galaxies.

Original languageEnglish (US)
Article number46
JournalAstrophysical Journal
Issue number1
StatePublished - Jun 10 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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