The [ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] Deep Survey of the Hubble Deep Field North Area. IV. An Ultradeep Image of the HDF-N

Open Access
Publisher Website
Google Scholar
Abstract
We present results from a 479.7 ks Chandra exposure of the Hubble Deep Field North (HDF-N) and its immediate vicinity. In this X-ray image, the deepest ever reported with a 0.5–2.0 keV flux limit of ≈4.9 × 10-17 ergs cm-2 s-1, four new HDF-N X-ray sources are detected, bringing the 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 starburst and/or a low-luminosity active galactic nucleus (AGN). Notably, X-ray emission has now been detected from all luminous galaxies (MV < -18) with z < 0.15 known in the HDF-N. We have also detected the remarkable microjansky 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 flux ratio (i.e., αox) is consistent with what is seen for low-redshift 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 absorption in the brightest X-ray source in the HDF-N, a z = 0.960 broad-line AGN with associated Mg II absorption. Stacking analyses of optically bright HDF-N galaxies not individually detected in X-rays have provided estimates of their average X-ray fluxes, and we find that the X-ray luminosities of "normal" spiral galaxies at z ≈ 0.5 are not more than a factor of ≈2 larger (per unit B-band luminosity) than those of spiral galaxies in the local universe (z < 0.01). This constrains models for the evolution 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 Imaging Spectrometer (ACIS) performs source detection well even with effective exposure times of ≈8 Ms.