The Large-Scale Distribution of Neutral Hydrogen in the Galaxy

Abstract

Although insight into the structure of our Galaxy can be sought in a number of different ways, the study of the neutral hydrogen component is particularly suitable. Neutral hydrogen is the main observed constituent of the interstellar medium. The physical characteristics of the hydrogen are closely related to the characteristics of other galactic constituents, both stellar and interstellar. The interstellar medium is transparent enough to hydrogen radio emission that, with the exception of a few directions along the galactic equator, investigation of the entire Galaxy is possible. This transparency allows investigation of regions of the Galaxy which are too distant to be studied optically. Interstellar neutral hydrogen is so abundant and is distributed in such a general fashion throughout the Galaxy that the 21-cm hyperfine transition line has been detected in emission for every direction in the sky at which a suitably equipped radio-telescope has been pointed. No time variation of a neutral hydrogen line has been found. What we detect is a line profile giving intensity, usually expressed as a brightness temperature, as a function of frequency. The frequency measures a Doppler shift from the natural frequency of 1420.406 MHz. In practice the measured frequency shifts are converted to radial velocities (1 km sec⁻¹ = 4.74 kHz), and in Milky Way studies, they are corrected to the local standard of rest. The local standard of rest is conventionallydefined by the standard solar motion of 20 km sec⁻¹ toward a, δ = 18^h, + 30° (1900).