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In some spirals - In some spirals(the best-known example is...

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In some spirals (the best-known example is probably NGC 3198), H I is detected well beyond the optical disk, and similar relative extents are not uncommon for very late Hubble types. This makes H I a powerful dynamical probe of dark-matter halos. It is common to have the edge of the H I disk abruptly cut off at column densities of a few times 10 19 atoms/cm 2 , which may be due to the disk becoming ionized by the general radiation field of AGN and starburst galaxies just below the Lyman limit (see Maloney 1993 ApJ 414, 41). At these low densities, recombination is very slow so that ionization is a good way to hide the gas, since recombination is a collisional process whose rate scales as (density) 2 . As technology allowed measurements in the millimeter regime, where strong transitions of astrophysically important molecules are found, it became clear that molecular gas is as important as atomic (H I) gas in many systems. Molecular hydrogen (H 2 ) is the most abundant molecule, but since this molecule is a symmetric rotor, it has no permanent dipole moment and thus none of the mm-wave rotation and vibrational transitions that make other molecules easy to detect. H 2 in the ground state can be detected only via absorption, in the Lyman and Werner
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