12.1. INTRODUCTION

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.

12.1.1. The Role of Neutral Hydrogen in Galaxies

Neutral-hydrogen concentrations, or clouds, provide the starting point for the collapse of matter into stars. Abundant HI in a galaxy indicates potentially active star formation processes, while lack of HI is a guarantee of a barren galaxy and an ineluctably aging stellar population. Thus, from an evolutionary viewpoint, the role of HI is of primary importance. Dynamically, that is, in terms of the fractional contribution to the mass, the perspective is different and, as discussed in detail in Section 12.5, quite dependent on morphological type. The Milky Way provides a reference point. Our Galaxy is an intermediate- to late-type spiral, perhaps an Sbc. The optical surface brightness of the disk decreases exponentially with distance from the center, with one-half of the spatially integrated luminosity falling within an effective radius of 4 or 5 kpc; the luminosity of the entire disk is about 1.1 × 10¹⁰ L, twice that of the nuclear bulge. Although ionized gas is found in the halo, the bulk of the interstellar gas is in the disk; similarly to the light, but with the exception of the very central regions, the total hydrogen gas (HI + H II + H₂) component mimics an exponential disk, with an effective radius close to that of the optical emission. While molecular hydrogen is the dominant constituent of the interstellar medium within the solar circle, atomic hydrogen predominates towards the periphery of the disk. In fact, HI extends farther out than any other galactic disk tracer. By mass, the atomic hydrogen resides mostly in diffuse clouds and in the envelopes surrounding molecular clouds. The thickness of the HI disk is nearly constant within the inner 10 to 12 kpc, the averaged density falling to half its peak value at the plane at about 150 to 200 pc from it. A density cross section perpendicular to the plane does, however, show weak, broad wings, associated with a warmer, less clumped HI component of the interstellar medium, more than twice as thick as the main disk component. The total amount of interstellar hydrogen in the inner 16 kpc of the Galaxy is about 6 × 10⁹ M, with roughly half in molecular and half in atomic form. While the atomic gas extends well outside that radius, the contribution of the gas (as well as that of all luminous matter) to the total mass decreases sharply with distance, and probably the total gaseous mass of the Galaxy does not exceed 10¹⁰ M. As the luminous mass fades rapidly in the outer regions of the Galaxy, the dynamical mass traced by the system of globular clusters and by the dwarf spheroidal satellites of the Milky Way continues to grow almost linearly with distance from the center. Within 16 kpc from the center, this mass is 1.5 × 10¹¹ M, rising to about 10¹² M inside 100 kpc. Atomic hydrogen thus represents less than 10% of the total mass within the easily discernible stellar disk, and about 1% of the total mass out to 100 kpc. Its importance relative to other constituents grows for galaxies of later morphological type than our own; within the region mapped by the HI in some extremely optically-faint blue galaxies, HI can be the principal constituent of the visible mass. By contrast, very early spiral and lenticular galaxies frequently have less than 0.01% of their mass in the form of neutral hydrogen gas, ellipticals sometimes even less.