The Chandra deep survey of the Hubble Deep Field North area. IV. An ultradeep image of the DHF-N

We present results from a 479.7 ks Chandra exposure of the Hubble Deep Fiel d North (HDF-N) and its immediate vicinity. In this X-ray image, the deepes t ever reported with a 0.5-2.0 keV flux limit of approximate to4.9 x 10(-17 ) ergs cm(-2) s(-1), four new HDF-N X-ray sources are detected, bringing th e total number of such sources to 12. The new sources include two optically bright (R = 18.3-18.8) low-redshift (z < 0.15) galaxies, a Fanaroff-Riley I radio galaxy, and an edge-on spiral galaxy hosting either a powerful star burst and/or a low-luminosity active galactic nucleus (AGN). Notably, X-ray emission has now been detected from all luminous galaxies (M-V < -18) with z < 0.15 known in the HDF-N. We have also detected the remarkable microjan sky radio source VLA J123642.09+621331.4, which is located just outside the HDF-N and has a likely redshift of z = 4.424. The observed X-ray emission supports the presence of an AGN in this object, and its X-ray-to-optical fl ux ratio (i.e., is consistent with what is <alpha>(ox)) seen for low-redshi ft AGNs. We have detected X-ray variability from two of the previously known HDF-N X -ray sources, and spectral fitting shows clear evidence for X-ray absorptio n in the brightest X-ray source in the HDF-N, a z = 0.960 broad-line AGN wi th associated Mg II absorption. Stacking analyses of optically bright HDF-N galaxies not individually detected in X-rays have provided estimates of th eir average X-ray fluxes, and we find that the X-ray luminosities of "norma l" II spiral galaxies at z approximate to 0.5 are not more than a factor of approximate to2 larger (per unit B-band luminosity) than those of spiral g alaxies in the local universe (z < 0.01). This constrains models for the ev olution of low-mass X-ray binary populations in galaxies in response to the declining cosmic star formation rate. Monte Carlo simulations support the validity of the stacking analyses and show that the Chandra Advanced CCD Im aging Spectrometer (ACIS) performs source detection well even with effectiv e exposure times of <approximate to>8 Ms.