Probing the Evolution of Galaxies Using Redshifted Hα Emission

In this paper we review the present status and implications of Hα surveys at various redshifts. With the advent of sensitive wide-format near-infrared detectors on large telescopes, deep and extensive Hα surveys are now feasible to redshift z ∼ 2.5. The cosmic star formation history can therefore be traced out to this redshift using Hα alone, avoiding complications arising from the comparison of different tracers at different redshifts. The Hα surveys to date confirm the rapid increase in luminosity density from z = 0 out to z = 1, and show that this increase flattens off at higher redshifts, remaining approximately constant out to at least z ∼ 2.2. We also discuss the prospects for determining the masses of high redshift galaxies based on emission lines. A set of high-quality Hα rotation curves of samples of disk galaxies at a number of different redshifts would allow a study of the evolution of the Tully-Fisher relation and address fundamental issues in disk galaxy formation. Such a program remains challenging even with present-day large telescopes.