Galactic neutral hydrogen

Abstract

The neutral atomic hydrogen in the Milky Way constitutes two different thermal phases of the interstellar gas: the warm neutral medium and the cool neutral medium. The best way to trace these phases on the scale of the entire Galactic disk is by using the 21-cm line at radio frequencies. This chapter explains with examples how observations of the 21-cm line are interpreted, with emission and absorption spectra leading to estimates of the column density and the excitation- or spin-temperature of the gas. The structure of the disk has both a thin component, with half-width 100 pc that includes both warm and cool gas, plus a thick disk, 400 pc half-width, that is mostly all warm. In the inner Galaxy the distribution of brightness in longitude-velocity coordinates is largely determined by the velocity gradient along the line of sight, with self-absorption by cool clouds competing with irregularities in the density and velocity field to modulate the intensity of the 21-cm emission. In the outer Galaxy the disk becomes more and more dominated, with an increase in the scale height and a strong warp that becomes very one-sided at Galactic radius greater tha kpc. The thermal balance in the two atomic phases is set by heating and cooling equilibrium, but the system does not have time to reach hydrostatic balance before supernova remnants disrupt the gas and broaden the pressure distribution function. Pressure variations can drive the gas either way between warm and cool phases, but the net flux must be from warm to cool, and on to molecular clouds, in order to explain the continuous cycle of star formation and chemical enrichment of the Galaxy. The small-scale structure of the H I shows a power-law spectrum of velocity and density fluctuations similar to that of a turbulent fluid. In some cases, the structure is enhanced in the cool neutral medium on scales of a few hundred astronomical units. The magnetic field may be important dynamically, particularly on the smaller scales. These and other issues about the disk will be better answered when the next generation of telescopes becomes available for Galactic survey projects.