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 ( AGN ) , star-forming galaxies , and Galactic stars based primarily on X-ray-to-optical flux ratios , optical spectral classifications , X-ray spectra , and intrinsic X-ray luminosities .
Number-count slopes and normalizations below 2 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } were calculated in each band for all source types assuming a single power-law model .
We find that AGN continue to dominate the number counts in the 0.5–2.0 keV and 2–8 keV bands .
At flux limits of \approx 2.5 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } ( 0.5–2.0 keV ) and \approx 1.4 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } ( 2–8 keV ) , the overall AGN source densities are 7166 ^ { +304 } _ { -292 } sources deg ^ { -2 } and 4558 ^ { +216 } _ { -207 } sources deg ^ { -2 } , respectively ; these are factors of \sim 10 –20 higher than found in the deepest optical spectroscopic surveys .
While still a minority , the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 ^ { +187 } _ { -169 } sources deg ^ { -2 } ( 0.5–2.0 keV ) and 711 ^ { +270 } _ { -202 } sources deg ^ { -2 } ( 2-8 keV ) at the CDF flux limits .
The number of star-forming galaxies will likely overtake the number of AGN at \sim 1 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } ( 0.5–2.0 keV ) and dominate the overall number counts thereafter .
Adopting XRB flux densities of ( 7.52 \pm 0.35 ) \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } deg ^ { -2 } ( 0.5–2.0 keV ) and ( 2.24 \pm 0.11 ) \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } deg ^ { -2 } ( 2–8 keV ) , the CDFs resolve a total of 89.5 ^ { +5.9 } _ { -5.7 } % and 86.9 ^ { +6.6 } _ { -6.3 } % of the extragalactic 0.5–2.0 keV and 2–8 keV XRBs , respectively .
AGN as a whole contribute \approx 83 % and \approx 95 % to the these resolved XRB fractions , respectively , while star-forming galaxies comprise only \approx 3 % and \approx 2 % , respectively , and Galactic stars comprise the remainder .
Extrapolation of the number-count slopes can easily account for the entire 0.5–2.0 keV and 2–8 keV XRBs to within statistical errors .
We additionally 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 } erg s ^ { -1 } and N _ { H } < 10 ^ { 22 } cm ^ { -2 } are the dominant contributors to the 0.5–2.0 keV XRB flux density , while sources with L _ { 0.5 - 8 ~ { } keV } = 10 ^ { 42.5 } – 10 ^ { 44.5 } erg 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 AGN 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 2–8 keV detected sources below F _ { 2 - 8 ~ { } keV } \approx 1 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } , where deviations gradually increase to \approx 3.9 \sigma .