X-ray, Optical, and Infrared Imaging and Spectral Properties of the 1 Ms   Chandra Deep Field North Sources
  We present the optical, near-infrared, submillimeter, and radio follow-up catalog of the X-ray selected sources from the 1 Ms Chandra observation of the Hubble Deep Field North region. We have B, V, R, I, and z' magnitudes for the 370 X-ray point sources, HK' magnitudes for 276, and spectroscopic redshifts for 182. We present high-quality spectra for 175 of these. The redshift distribution shows indications of structures at z=0.843 and z=1.0175 (also detected in optical surveys) which could account for a part of the field-to-field variation seen in the X-ray number counts. The flux contributions separated into unit bins of redshift show that the z<1 spectroscopically identified sources already contribute about one-third of the total flux in both the hard and soft bands. We find from ratios of the X-ray counts that the X-ray spectra are well-described by absorption of an intrinsic Gamma=1.8 power-law, with log NH values ranging from 21 to 23.7. We estimate that the Chandra sources that produce 87% of the HEAO-A X-ray background (XRB) at 3 keV produce 57% at 20 keV, provided that at high energies the spectral shape of the sources continues to be well-described by a Gamma=1.8 power-law. However, when the Chandra contributions are renormalized to the BeppoSAX XRB at 3 keV, the shape matches fairly well the observed XRB at both energies. Thus, whether a substantial population of as-yet undetected Compton-thick sources is required to completely resolve the XRB above 10 keV depends critically on how the currently discrepant XRB measurements in the 1-10 keV energy range tie together with the higher energy XRB. (Abridged) 
A.J. Barger, L.L. Cowie, W.N. Brandt, P. Capak, G.P. Garmire, A.E.   Hornschemier, A.T. Steffen, E.H. Wehner