The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations

Using Chandra observations in the 2.15 deg(2) COSMOS-legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3-7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT similar to 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Gamma = 1.45 +/- 0.02. The 1 keV normalization of the extragalactic component is 10.91 +/- 0.16 keV cm(-2) s(-1) sr(-1) keV(-1). Removing all X-ray-detected sources, the remaining unresolved CXB is best fit by a power law with normalization 4.18 +/- 0.26 keV cm(-2) s(-1) sr(-1) keV(-1) and photon spectral index Gamma = 1.57 +/- 0.10. Removing faint galaxies down to i(AB) similar to 27-28 leaves a hard spectrum with Gamma similar to 1.25 and a 1 keV normalization of similar to 1.37 keV cm(-2) s(-1) sr(-1) keV(-1). This means that similar to 91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute similar to 8%-9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another similar to 1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. According to these limits, we investigate a scenario where early black holes totally account for non-source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the z similar to 6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with N-H > 1.6 x 10(25) cm(-2) and form in extremely low-metallicity environments (Z(circle dot)) similar to 10(-3).