News and Views on "Yu et al. 2017"

News and Views on Yu et al. 2017 (RAA 2017 Vol. 17, No. 6, 49)

Carl Heiles

Astronomy Department, University of California, Berkeley, CA 94720-3411, USA

The Lyman-alpha forest is the assembly of Ly\(\alpha\) absorption lines seen over a range of redshift against background galaxies and quasars located at higher redshift. In particular, when the HI column density is high enough, \(\gtrsim 2 \times 10^{20}\) cm\(^{-2}\), the Ly\(\alpha\) absorption lines become saturated (`Damped Lyman-alpha', or DLA). In addition, the associated 21-cm line of the HI becomes detectable. The blind search for these 21-cm absorption lines---the radio version of the DLA Ly\(\alpha\) lines---selectively samples the cold diffuse HI gas in the intervening galaxies, and provides a unique probe with high spatial resolution equal to the size of the background object along the line of sight. This can be used to study the spatial and kinematic distribution of neutral gas in the intervening galaxy. With additional information such as polarization of the absorption line and VLBI maps of the continuum source (e.g. Wilkinson et al. 1979), surprisingly detailed models of physical structure and magnetic field configuration can be derived (e.g. Wolfe et al. 2011).

The 21-cm line absorption of DLAs can provide a wealth of information on the interstellar medium in redshifted galaxies, including gas kinematics, magnetic field, metal abundances, dust content, and excitation conditions. However, detailed study remains difficult. Redshifted 21-cm absorption of DLAs typically selects systems with the highest HI column densities known, and thus provides the best selectors of systems suitable for detailed study of the cold interstellar medium of external galaxies.

Such studies are currently limited by the number of known 21-cm DLA absorbers, which is only \(\sim 20\) (Zwaan et al. 2007). Using FAST for a blind search for 21-cm line absorption in DLAs should rapidly increase this number---at the rate of \(\sim 200\) per month! This will provide a reservoir of objects large enough to contain superb examples for detailed study, and thereby greatly enhance our understanding of the structure, physical conditions, and evolution of cold gas in galaxies.

References

• Wilkinson, P. N.; Readhead, A. C. S.; Anderson, B.; Purcell, G. H. 1979 ApJ, 232, 365 ADS

• Wolfe, Arthur M.; Jorgenson, Regina A.; Robishaw, Timothy; Heiles, Carl; Prochaska, J. Xavier 2011, ApJ, 733, 24 ADS

• Zwaan, M. A.; van der Hulst, J. M.; Briggs, F. H.; Verheijen, M. A. W.; Ryan-Weber, E. V. 2007, Island Universes, Astrophysics and Space Science Proceedings editor R. S. De Jong, ISBN 978-1-4020-5572-0, Springer, p. 501 ADS