On the Cosmic Evolution of AGN Obscuration and the X-Ray Luminosity Function: XMM-Newton and Chandra Spectral Analysis of the 31.3 deg 2 Stripe 82X

Abstract

We present X-ray spectral analysis of XMM-Newton and Chandra observations in the 31.3 deg 2 Stripe-82X (S82X) field. Of the 6181 unique X-ray sources in this field, we analyze a sample of 2937 candidate active galactic nuclei (AGNs) with solid redshifts and sufficient counts determined by simulations. Our results show an observed population with median values of spectral index Γ = 1.94 − 0.39 + 0.31 , column density log N H / cm − 2 = 20.7 − 0.5 + 1.2 and intrinsic, de-absorbed, 2–10 keV luminosity log L X / erg s − 1 = 44.0 − 1.0 + 0.7 , in the redshift range 0–4. We derive the intrinsic, model-independent, fraction of AGNs that are obscured ( 22 ≤ log N H / cm − 2 < 24 ), finding a significant increase in the obscured AGN fraction with redshift and a decline with increasing luminosity. The average obscured AGN fraction is 57% ± 4% for log L X /erg s −1 > 43. This work constrains the AGN obscuration and spectral shape of the still uncertain high-luminosity and high-redshift regimes (log L X /erg s −1 > 45.5, z > 3), where the obscured AGN fraction rises to 64% ± 12%. We report a luminosity and density evolution of the X-ray luminosity function, with obscured AGNs dominating at all luminosities at z > 2, and unobscured sources prevailing at log L X /erg s −1 > 45 at lower redshifts. Our results agree with the evolutionary models in which the bulk of AGN activity is triggered by gas-rich environments and in a downsizing scenario. Moreover, the black hole accretion density (BHAD) is found to evolve similarly to the star formation rate density, confirming the coevolution between AGN and host galaxy, but suggesting different timescales in their growing history. The derived BHAD evolution shows that Compton-thick AGNs contribute to the accretion history of AGNs as much as all other AGN populations combined.