SIMBAD references

We present an optical and near-infrared catalog for the X-ray sources in the ~2 Ms Chandra observation of the Hubble Deep Field North region. We have high-quality multicolor imaging data for all 503 X-ray point sources in the X-ray-selected catalog and reliable spectroscopic redshifts for 284. We spectroscopically identify six high-redshift (z>1) type II quasars (L_2–8keV>10⁴⁴ ergs.s^–1) in our sample. Our spectroscopic completeness for the R≤24 sources is 87%. The spectroscopic redshift distribution shows two broad redshift spikes that have clearly grown over those originally seen in the ~1 Ms exposure. The spectroscopically identified extragalactic sources already comprise 75% of the measured 2-8 keV light. Redshift slices versus 2-8 keV flux show that an impressive 54% of the measured 2-8 keV light arises from sources at z<1 and 68% from sources at z<2. Thus, major accretion onto supermassive black holes has occurred since the universe was half its present age. We use seven broadband colors and a Bayesian photometric redshift estimation code to obtain photometric redshifts for the X-ray sources. We find that the photometric redshifts are within 25% of the spectroscopic redshifts for 94% of the non-broad-line sources with both photometric and spectroscopic measurements. The photometrically identified sources show a smooth continuation of the spectroscopically identified sources to redder R-HK' color with increasing redshift, consistent with the galaxy tracks of evolved bulge-dominated galaxies. Fourteen have colors R-HK'>5.7 that would classify them as extremely red objects (EROs). The photometric redshifts of these EROs are all between z∼1.5 and z∼2.5. We use our wide wavelength coverage to determine rest-frame colors for the X-ray sources with spectroscopic or photometric redshifts. We find that many of the X-ray sources have the rest-frame colors of evolved red galaxies and that there is very little evolution in these colors with redshift. We also determine absolute magnitudes and find that many of the non-broad-line sources are more luminous than M^*_I, even at high redshifts. We therefore infer that deep X-ray observations may provide an effective way of locating M^* galaxies with colors similar to present-day early-type galaxies to high redshifts.