The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-energy Cosmic X-Ray Background Population

We report on the first 10 identifications of sources serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) to provide the first sensitive census of the cosmic X-ray background source population at >~ 10 keV. We find that these NuSTAR-detected sources are ≈100 times fainter than those previously detected at >~ 10 keV and have a broad range in redshift and luminosity (z = 0.020-2.923 and L _{10-40 keV} ≈ 4 × 10⁴¹-5 × 10⁴⁵ erg s^-1) the median redshift and luminosity are z ≈ 0.7 and L _{10-40 keV} ≈ 3 × 10⁴⁴ erg s^-1, respectively. We characterize these sources on the basis of broad-band ≈0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared spectral energy distribution analyses. We find that the dominant source population is quasars with L _{10-40 keV} > 10⁴⁴ erg s^-1, of which ≈50% are obscured with N _H >~ 10²² cm^-2. However, none of the 10 NuSTAR sources are Compton thick (N _H >~ 10²⁴ cm^-2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L _{10-40 keV} > 10⁴⁴ erg s^-1) selected at >~ 10 keV of <~ 33% over the redshift range z = 0.5-1.1. We jointly fitted the rest-frame ≈10-40 keV data for all of the non-beamed sources with L _{10-40 keV} > 10⁴³ erg s^-1 to constrain the average strength of reflection; we find R < 1.4 for Γ = 1.8, broadly consistent with that found for local active galactic nuclei (AGNs) observed at >~ 10 keV. We also constrain the host-galaxy masses and find a median stellar mass of ≈10¹¹ M _⊙, a factor ≈5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.