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

We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into active galactic nuclei (AGNs), star-forming galaxies, and Galactic stars primarily on the basis of their X-ray-to-optical flux ratios, optical spectral classifications, X-ray spectra, and intrinsic X-ray luminosities. Number count slopes and normalizations below 2 × 10 ^-15 ergs cm ^-2 s ^-1 were calculated in each band for all source types assuming a single power-law model. We find that AGNs continue to dominate the number counts in the 0.5-2.0 and 2-8 keV bands. At flux limits of ≈2.5 × 10 ^-17 ergs cm ^-2 s ^-1 (0.5-2.0 keV) and ≈2.0 × 10 ^-16 ergs cm ^-2 s ^-1 (2-8 keV), the overall AGN source densities are 7166 _-292 ⁺³⁰⁴and 4558 _-207 ⁺²¹⁶ sources deg ^-2, respectively; these are factors of ∼10-20 higher than those found in the deepest optical spectroscopic surveys. Although still a minority, the number counts of star-forming galaxies climb steeply, such that they eventually achieve source densities of 1727 _-169 ⁺¹⁸⁷ sources deg ^-2 (0.5-2.0 keV) and 711 _-202 ⁺²⁷⁰ sources deg ^-2 (2-8 keV) at the CDF flux limits. The number of star-forming galaxies will likely overtake the number of AGNs at ∼1 × 10 ^-17 ergs cm ^-2 s ^-1 (0.5-2.0 keV) and dominate the overall number counts thereafter. Adopting XRB flux densities of (7.52 ± 0.35) × 10 ^-12 ergs cm ^-2 s ^-1 deg ^-2 for 0.5-2.0 keVand (1.79 ± 0.11) × 10 ^-11 ergs cm ^-2 s ^-1 deg ^-2 for 2-8 keV, the CDFs resolve a total of 89.5 _-5.7 ^+5.9 percent and 92.6 _-6.3 ^+6.6 percent of the extragalactic 0.5-2.0 and 2-8 keV XRBs, respectively. AGNs as a whole contribute ≈83% and ≈95% to these resolved XRB fractions, respectively, whereas star-forming galaxies comprise only ≈3% and ≈2%, respectively, and Galactic stars comprise the remainder. Extrapolation of the number count slopes can easily account for the entire 0.5-2.0 and 2-8 keV XRBs to within statistical errors. We also examine the X-ray number counts as functions of intrinsic X-ray luminosity and absorption, finding that sources with L _{0.5-8 keV} > 10 ^43.5 ergs s ^-1 and N _H < 10 ²² cm ^-2 are the dominant contributors to the 0.5-2.0 keV XRB flux density, whereas sources with L _{0.5-8 keV} = 10 ^42.5-10 ^44.5 ergs s ^-1 and a broad range of absorption column densities primarily contribute to the 2-8 keV XRB flux density. This trend suggests that even less intrinsically luminous, more highly obscured AGNs may dominate the number counts at higher energies, where the XRB intensity peaks. Finally, we revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent with each other except for sources detected at 2-8 keV below F _{2-8 keV} ≈ 1 × 10 ^-15 ergs cm ^-2 s ^-1, for which deviations gradually increase to ≈3. 9 σ.