On Detecting the X–ray Silhouette of a Damped Lyα System

We explore the possibility of resolving an image of a damped Lyα (DLA) system in absorption against an extended, diffuse background X–ray source. Typical columns of neutral hydrogen in DLA systems are high enough to block out up to ∼ 30% of the soft X–ray flux at an observed photon energy of 0.5 keV, and we find that ∼ 1% of the area of extended X–ray sources at z & 1 have their 0.5 keV flux reduced by at least 20% because of intervening DLA systems. We discuss the observability of such absorption and find that & 300 photons per angular resolution element are required in the 0.3–8 keV band for its detection, and in order to distinguish it from intrinsic surface brightness fluctuations. For the surface brightness of the currently known high–redshift extended X–ray sources, this requires an integration time of a few Msec on Chandra if the maps are smoothed spatially to ≈ 2 resolution. The exact required integration time depends on the DLA system’s column density, metallicity and most strongly, its redshift. Current X–ray telescopes are likely to detect DLA systems with NHI < 1022 cm−2 only out to z ≈ 2.3. The availability of DLA systems with a suitably high column–density for a silhouette detection is currently poorly known. We suggest that at low redshifts, archival data of bright X–ray point sources may be useful in constraining the high–NHI end of the column density distribution. We briefly discuss an alternative strategy of searching for extended X–ray sources behind known DLA systems. Although with current X–ray telescopes the detections are challenging, they will be within the reach of a routine observation with a next generation X–ray telescope such as the X-Ray Evolving Universe Spectrometer (XEUS) or Generation–X, and will deliver novel constraints on the nature of proto–galaxies. Subject headings: cosmology: theory – galaxies: formation – galaxies: high-redshift – quasars: absorption lines