On the Possibility of Detecting Redshifted 21-CM Absorption Lines in the Spectra of Quasi-Stellar Sources

The expected strengths of redshifted 21-cm absorption lines are estimated by using the properties of the observed optical absorption lines in quasi-stellar sources. It is shown that the absence of 0 i and N i absorption lines in the observed optical absorption spectra of quasi-stellar radio sources indicates that 21-cm absorption lines wider than 100 kllz are likely to be weak unless the heavy-element abundance in the absorbing material is low. Lines narrower than 100 kETz may be strong; their optical counterparts would have escaped detection. A general expression is given for the spin temperature of neutral hydrogen when (following Field) Ly a excitation and de-excitation, 21-cm absorption and emission, and particle collisions are all included. The results are expressed simply in terms of the strength and distance of the radio source and the separation between absorber and emitter. It is shown that the spin temperature of neutral hydrogen is large near a quasi-stellar radio source and is determined either by the ambient 21-cm flux or the ambient Ly a flux for separations between absorber and emitter of less than about 1O~ It-yr. The expected absorption (and emission) strengths of redshifted 21-cm lines are compared with observa- tional capabilities at radio observatories. Some interesting candidates for study are listed