The Chandra Deep Field North Survey. X. X-Ray Emission from Very Red Objects

Abstract

The multiwavelength properties of very red objects [VROs; (I-K)>=4] are largely unknown since many of these sources are optically faint (I>=24) and undetected at most wavelengths. Here we provide constraints on the X-ray (0.5-8.0 keV) properties of VROs using the 1 Ms Chandra exposure of an 8.4‧×8.4‧ region within the Hawaii flanking-field area containing the Hubble Deep Field North (HDF-N). We find that VROs detected in the hard band (2.0-8.0 keV) have flat X-ray spectral slopes (Γ~0.9) and X-ray properties consistent with those expected from luminous obscured AGNs. The fraction of such sources in the K<=20.1 VRO population is 14⁺¹¹_-7%. Conversely, the average X-ray spectral slope of VROs detected in the soft band (0.5-2.0 keV) but not in the hard band is comparatively steep (Γ>1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (>=99% confidence level) in the soft band and in the full band (0.5-8.0 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from nonactive VROs with low X-ray-to-optical flux ratios [log(f_x/f_I)~-2] this is consistent with that expected if the majority of these VROs are ~M^*_I elliptical galaxies. A number of VROs are also detected with mid-infrared (15 μm) and radio emission, and we provide constraints on the nature of this emission.