2001ApJ...554..742H

A deep X-ray survey of the Hubble Deep Field-North (HDF-N) and its environs is performed using data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Currently a 221.9 ks exposure is available, the deepest ever presented, and here we give results on X-ray sources located in the 8'.6x8'.7 area covered by the Caltech Faint Field Galaxy Redshift Survey (the ``Caltech area''). This area has (1) deep photometric coverage in several optical and near-infrared bands; (2) extensive coverage at radio, submillimeter, and mid-infrared wavelengths; and (3) some of the deepest and most complete spectroscopic coverage ever obtained. It is also where the X-ray data have the greatest sensitivity; the minimum detectable fluxes in the 0.5-2 keV (soft) and 2-8 keV (hard) bands are ~1.3x10^–16 and ~6.5x10^–16 ergs.cm^–2.s^–1, respectively.

More than ~80% of the extragalactic X-ray background in the hard band is resolved. The 82 Chandra sources detected in the Caltech area are correlated with more than 25 multiwavelength source catalogs, and the results of these correlations as well as spectroscopic follow-up results obtained with the Keck and Hobby-Eberly Telescopes are presented. All but nine of the Chandra sources are detected optically with R≲26.5. Redshifts are available for 39% of the Chandra sources, including 96% of the sources with R<23; the redshift range is 0.1-3.5, with most sources having z<1.5. Eight of the X-ray sources are located in the HDF-N itself, including two not previously reported. A population of X-ray faint, optically bright, nearby galaxies emerges at soft-band fluxes of ≲3x10^–16 ergs.cm^–2.s^–1.

Our multiwavelength correlations have set the tightest constraints to date on the X-ray emission properties of µJy radio sources, mid-infrared sources detected by the Infrared Space Observatory (ISO), and very red (R-K_s>5.0) objects. A total of 16 of the 67 1.4 GHz µJy sources in the Caltech area are detected in the X-ray band, and the detection rates for starburst-type and AGN-candidate µJy sources are comparable. Only two of the 17 red, optically faint (I>25) µJy sources are detected in X-rays. While many of the starburst-type µJy sources appear to contain obscured active galactic nuclei (AGNs), the Chandra data are consistent with the majority of the µJy radio sources being powered by star formation. A total of 11 of the ~100 ISO mid-infrared sources found in and near the HDF-N are detected in X-rays. In the HDF-N itself, where both the infrared coverage and the X-ray coverage are deepest, it is notable that six of the eight Chandra sources are detected by ISO; most of these are known to be AGNs where the X-ray and infrared detections reveal both the direct and indirect accretion power being generated. The high X-ray-to-infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources.

Four of the 33 very red objects that have been identified in the Caltech area are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGNs. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. A stacking analysis of the very red objects not detected individually by Chandra yields a soft-band detection with an average soft-band X-ray flux of ~1.9x10^–17 ergs.cm^–2.s^–1; the observed emission may be associated with the hot interstellar media of moderate-redshift elliptical galaxies.

Constraints on AGN candidates, extended X-ray sources, and Galactic objects in the Caltech area are also presented.