On Detecting the X-ray Silhouette of a Damped Lyman alpha System

We explore the possibility of resolving an image of a damped Lyman alpha (DLA) system in absorption against an extended, diffuse background X-ray source. Typical columns of neutral hydrogen in DLAs 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%. We discuss the observability of such absorption and find that < 2 arcsecond resolution, and > 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. The detection will be within the reach of a routine observation with a next generation X-ray telescope such as XEUS or Generation X.