Selection Effects at 21cm

Abstract

Surveys in the 21cm line of neutral hydrogen are testing the completeness of the catalogs of nearby galaxies. The remarkable observational fact is that the potential wells that confine gas to sufficient density that it can remain neutral in the face of ionizing radiation also provide sites for star formation, so that there are no known cases of neutral intergalactic clouds without associated star light.

8 pages, 2 figures, LaTeX. To Appear in the proceeding of the IAU colloquium 171, "The Low Surface Brightness Universe", held in July 1998 in Cardiff, U.K; Typo in Eq. 2 corrected