Red active galactic nuclei in XMM-Newton/Sloan Digital Sky Survey fields

In this paper we combine archival and proprietary XMM-Newton observations (about 5deg²) that overlap with the Sloan Digital Sky Survey to explore the nature of the moderate-z X-ray population. We focus on X-ray sources with optically red colours (g - r > 0.4), which we argue are important for understanding the origin of the X-ray background (XRB). First, these systems constitute a significant fraction, about two-third, of the z <~ 1 X-ray population to the limit f(2-8keV) ~ 2 × 10^-14ergs^-1cm^-2. Secondly, their luminosity function under evolution of the form ~ (1 + z)³ suggests that they could be responsible for about 17 per cent of the diffuse XRB to z = 1. Thirdly, their stacked X-ray spectrum in the range 1-8keV is consistent with a power-law distribution with Γ ~ 1.4 (without fitting intrinsic absorption), that is, similar to the diffuse XRB. We find that the optically red X-ray population comprises a mixed bag of objects, both obscured [hydrogen column density (N_H) > 10²²cm^-2] and unobscured (N_H < 10²²cm^-2), with a wide range of X-ray luminosities up to L_X ~ 10⁴⁴ergs^-1. We argue that dilution of the active galactic nucleus (AGN) light by the host galaxy may play a role in shaping the continuum optical emission of this population. Finally, we explore a possible association of these sources and the moderate-z red (J - Ks> 2mag) AGNs identified in the Two-Micron All-Sky Survey (2MASS). The median N_H of the red X-ray sources studied here is ~10²¹cm^-2, lower than that found for the 2MASS AGNs, suggesting different populations.