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Unformatted text preview: 1/15/12 Gala[ieV and Whe UniYeuVe - GaV in Gala[ieV 1/7 ZZZ.aVWu.Xa.edX/keel/gala[ieV/gaV.hWml 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-cP H I OiQe . 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 temperature T B for historical reasons, though more recent VLA observations are expressed in Jy (so that the integrated line intensity is conventionally in Jy km/s). Details are given by, for example, Kerr 1968 (in Nebulae and Interstellar Matter , Chicago, p. 575). The number density of H atoms in the antenna beam is N(H ) = 1.82 x 10 18 B int in atoms/cm 2 , where B int is given in K km/s. This equation applies transparently to a cloud filling the beam (not a typical galaxy). In the more realistic case of an object smaller than the beam, one must multiply by / 4 D 2 , where is the solid angle of the measurement beam and D is the galaxy distance. Following Roberts (SSS vol 9, p. 310) a more useful form is where S is in Jy, V r is in km/s, and D is in Mpc. One frequently uses distance-independent combinations such as M(H I)/L B to sidestep distance uncertainties altogether. H I masses for single galaxies range up to a few 10 10 solar masses. The absolute mass, and mass normalized to optical luminosity, vary systematically along the Hubble sequence as follows: Ellipticals generally have less than 10 7 solar masses, and that is frequently in large configurations possibly acquired from outside. There have been reports of opticall\ thick H I in large amounts in cooling-flow galaxies such as M87, though more recent observations leave this conclusion in serious doubt. S0s may have a large range in H I mass, up to 10 9 solar masses. Sc's typically have several 10 9 solar masses. The very late types Sm,Im often have more mass in H I alone than in the visible stellar population; in this sense they may still be thought of as young galaxies. The trend with Hubble type is illustrated by Fig. 16 from Roberts 1975. Detailed aperture-synthesis maps of H I are becoming available for numerous galaxies, carrying rich information on kinematics as well as the H I distribution...
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- Fall '09