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Session 85 - Quasar Absorption Line Systems.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[85.05] A Search for Redshifted HI 21cm Absorption by Damped Ly-alpha Absorption Systems

Using data from the recently upgraded WSRT we search for HI 21cm absorption associated with three high redshift damped Ly\alpha systems. We detect no absorption at higher than a 3\sigma level in any object. We calculate a lower limit on the temperatures of the absorbing gas by comparing column densities in HI derived from HI 21 cm absorption with those derived from damped Ly\alpha absorption, and combine our results with previous HI 21 cm observations of damped Ly\alpha systems. We find a typical value for the harmonic mean spin temperature of gas in such systems of: \sim 10^3 K. We compare results from damped Ly\alpha systems to results for Galactic gas and gas in M31, using a standard - \tau relationship (Braun and Walterbos, 1992). We see that at a given optical depth, the values of for the damped Ly\alpha systems are systematically higher than those for the Galaxy by about a factor of 5. This suggests, in a two phase medium, either a larger amount of warm gas than is found in the Galaxy, a higher temperature for the cold phase than is found in the Galaxy, or a combination of the two. The data at low opacity, or high radius (R > 15kpc) for M31 converge with the damped Ly\alpha data, suggesting that conditions in the damped Ly\alpha sytems may be similar to those found in the outer reaches of galaxies. We also search for HI 21cm emission and find none at a 5\sigma or higher level in the fields of any of the sources. We derive an upper limit on the mass of Hydrogen present in the system which effectively rules out the possibility that any of these systems are proto-clusters according to the model defined by Oort (1984). Finally we find a molecular hydrogen fraction in the system towards 0528-2505, of F = 2N(H_2)/N(HI) > 2%.

References:

Braun and Walterbos, 1992 Ap.J., 157, 1055

Oort, 1984, A.A., 139, 211

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