Gas in Galaxies The gaseous content of galaxies is a crucial counterpart to their stars. This is where stars come from, where some of their mater goes when they expire, and has intricate interactions with stars at many evolutionary phases. Furthermore, the spectra of gases afford emission features in the spectrum making them relatively easy to trace. A remarkably useful tracer of overall gas content is the 21-cm H I line . Atomic hydrogen has two states in its ground level, split by spin-orbit coupling. Parallel spins give a slightly higher energy than antiparallel, and decays by emission of a photon at 1420.406 MHz. The upper level is populated by collisions, and has a decay half-life of about 3.5 x 10 14 seconds = 1.1 x 10 7 years; this is detectable only because we are dealing with vast numbers of H atoms. Under some non-restrictive assumptions (spin temperature, optical depth), the total H I mass can be derived from the integrated observed line profile. The intensity unit is frequently taken as brightness
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