Using gravitational lensing to study HI clouds at high redshift

We investigate the possibility of detecting H I emission from gravitational ly lensed H I clouds (akin to damped Ly alpha clouds) at high redshift by c arrying out deep radio observations in the fields of known cluster lenses. These observations will be possible with current radio telescopes only if t heir lenses substantially magnify the flux of the H I emission. While at pr esent this holds the only possibility of detecting the H I emission from su ch clouds, it has the disadvantage of being restricted to clouds that lie v ery close to the caustics of the lens. We find that observations at a detec tion threshold of 50 mu Jy at 320 MHz (possible with the Giant Meterwave Ra dio Telescope) have a greater than 20% probability of detecting an H I clou d in the field of a cluster, provided the clouds have H I masses in the ran ge 5 x 10(8) M. less than or similar to 5 M-HI less than or similar to 2.5 x 10(10) M.. The probability of detecting a cloud increases if it has large r H I masses except in cases where the number of H I clouds in the cluster field is very small. The probability of detection at 610 and 233 MHz is com parable to that at 320 MHz, although a definitive statement is difficult ow ing to uncertainties in the H I content at the redshifts corresponding to t hese frequencies. Observations at a detection threshold of 2 mu Jy (possibl e in the future with the Square Kilometer Array) are expected to detect a f ew H I clouds in the field of every cluster, provided the clouds have H I m asses in the range 2 x 10(7) M. less than or similar to M-HI less than or s imilar to 10(9) M.. Even if such observations do not result in the detectio n of H I clouds, they will be able to put useful constraints on the H I con tent of the clouds.