ALEXANDER D.M., VIGNALI C., BAUER F.E., BRANDT W.N., HORNSCHEMEIER A.E., GARMIRE G.P. and SCHNEIDER D.P.
Abstract (from CDS):
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'.4x8'.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(fx/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.