Extragalactic Neutral Hydrogen

Abstract

The 21-cm hyperfine transition of neutral hydrogen was first detected in the Milky Way in 1951 by Ewen and Purcell. Two years later (Kerr and Hindman 1953), the HI emission of the Magellanic Clouds was observed from Australia. Largely as a result of the pioneering efforts of M.S. Roberts, by the time of his comprehensive review (Roberts 1975; the reader is referred to this source for the early development of the field), HI in about 140 extragalactic objects had been detected. Since then, mostly with the Green Bank 91-m, the Nançay, the Effelsberg, and more recently the Arecibo telescopes, the line has been observed in emission in thousands of galaxies, to distances 104 times greater than that of the Magellanic Clouds, and in absorption out to redshifts greater than 2. For the first twenty years, single-dish telescopes provided the vast majority of the HI data. By the mid-seventies, the techniques of spectral line aperture synthesis, developed especially at Cambridge, Green Bank, and Owens Valley, bloomed in full maturity at Westerbork and more recently at the VLA. The contributions of single-dish instruments have, however, remained important, especially for survey-oriented projects that require high-sensitivity observations of many noncontiguous regions of sky, and for pure detection experiments.