Investigations of Redshifted Clouds with the 21 cm Line

Abstract

21 cm studies, with the 305 m telescope at Arecibo, of QSO absorption-line clouds are described. A search for 21 cm absorption in 16 redshift systems exhibiting Mg II and Fe II absorption yield a single detection; at za = 0.394 985 in PKS 1229-02. The null observations are interpreted in terms of turbulent gas that is optically thick in the resonance transitions, but thin to 21 cm radiation. Occasionally a very opaque cloud with low velocity dispersion is present which gives rise to 21 cm absorption. The latter has little effect on saturated equivalent widths. Hence one cannot predict 21 cm absorption from properties of Mg II and Fe II absorption.Lα absorption, however, is sensitive to the presence of very opaque clouds, and a 21 cm absorption line was discovered at zA = 1.776 42 on the basis of strong Lα absorption in MC3 1331 + 170. Comparison between the Lα and 21 cm absorption spectra restricts the hydrogen spin temperature to Ts < 103 K. When combined with optical limits on fine-structure excitation this restricts the distance separating the absorber from the QSO to d > 1 kpc. However, Lα photons produced by the QSO can control Ts in the absorber, in which case d is not so restricted. Calculations of Lα transport through the absorber predict a characteristic emission feature that is observable if d ∼ 1 kpc. Sensitive observations do indicate a feature, but one which is easier to interpret in terms of an intervening galaxy at zA.A search for 21 cm emission from extended H I envelopes in a complete sample of 28 spiral galaxies is described. So far, 21 cm emission is found in only 9% of the sample galaxies, and of these in only 1 isolated galaxy, NGC 628. The upper limits restrict the equivalent width of Mg II absorption to be smaller than that normally detected in QSOs. Thus Mg II absorption lines cannot arise in galaxies like those in our sample unless Mg+/H I ≳ 0.3X(Mg).. The gas detected in NGC 628 is 100 kpc from the nucleus, and might be an intergalactic cloud.The coincidence of 21 cm and optical absorption redshifts imply space-time invariance of certain physical constants throughout most of the observable universe. Because one of these depends on all the details of strong interaction physics, it is suggested that all physical laws are globally invariant.