Using Gravitational Lensing to Study HiClouds at High Redshift

We investigate the possibility of detecting HI emission from gravitationallylensed HI clouds (akin to damped Lyman-$\alpha$ clouds) at high redshift bycarrying out deep radio observations in the fields of known cluster lenses.Such observations will be possible with present radio telescopes only if thelens substantially magnifies the flux of the HI emission. While at present thisholds the only possibility of detecting the HI emission from such clouds, ithas the disadvantage of being restricted to clouds that lie very close to thecaustics of the lens. We find that observations at a detection threshold of 50micro Jy at 320 MHz (possible with the GMRT) have a greater than 20%probability of detecting an HI cloud in the field of a cluster, provided theclouds have HI masses in the range 5 X 10^8 M_{\odot} < M_{HI} < 2.5 X 10^{10}M_{\odot}. The probability of detecting a cloud increases if they have largerHI masses, except in the cases where the number of HI clouds in the clusterfield becomes very small. The probability of a detection at 610 MHz and 233 MHzis comparable to that at 320 MHz, though a definitive statement is difficultowing to uncertainties in the HI content at the redshifts corresponding tothese frequencies. Observations at a detection threshold of 2 micro Jy(possible in the future with the SKA) are expected to detect a few HI clouds inthe field of every cluster provided the clouds have HI masses in the range 2 X10^7 M_{\odot} < M_{HI} < 10^9 M_{\odot}. Even if such observations do notresult in the detection of HI clouds, they will be able to put usefulconstraints on the HI content of the clouds.Comment: 21 pages, 7 figures, minor changes in figures, accepted for publication in Ap