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 8.6 ^ { \prime } \times 8.7 ^ { \prime } 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 \approx 1.3 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } and \approx 6.5 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } , respectively .
More than \approx 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 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 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 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 3 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } .

Our multiwavelength correlations have set the tightest constraints to date on the X-ray emission properties of \mu Jy radio sources , mid-infrared sources detected by ISO , and very red ( { \cal R } - K _ { s } > 5.0 ) objects .
Sixteen of the 67 1.4 GHz \mu Jy sources in the Caltech area are detected in the X-ray band , and the detection rates for starburst-type and AGN-candidate \mu Jy sources are comparable .
Only two of the 17 red , optically faint ( I > 25 ) \mu Jy sources are detected in X-rays .
While many of the starburst-type \mu Jy sources appear to contain obscured AGN , the Chandra data are consistent with the majority of the \mu Jy radio sources being powered by star formation .
Eleven of the \approx 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 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 AGN 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 by Hogg et al .
( 2000a ) are detected in X-rays ; these four are among our hardest Chandra sources , and we argue that they contain moderately luminous obscured AGN .
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 \approx 1.9 \times 10 ^ { -17 } erg 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 .