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

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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 84 × 84 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%. 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 (fx/fI) ≈ -2]; this is consistent with that expected if the majority of these VROs are ≈M 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.
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