Contrasts between disk

Contrasts between disk - Contrasts between disk- and...

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Contrasts between disk- and nuclear-dominated CO distributions are seen in comparing NGC 4414 and NGC 6574 from the survey by Sakamoto et al. (1999 ApJS 124, 403, again courtesy of the AAS): Even at the global level, there remain major unknowns in interpreting CO data. At lower abundances, characteristic of very late-type and irregular galaxies, the molecular clouds may be undetectable - there's not enough C and O present to give a signal, though the H 2 should still form clouds (which can sometimes actually be seen by their effects on the neighboring H II regions; McCall, Hill, and English 1990 AJ 100, 193). In very dense regions, the molecular balance can shift in favor of more astronomically exotic species (from CS and HCN on up), so that we need to look at different transitions and consider a wide mix of cloud properties. The whole relationship between atomic and molecular gas has been re-evaluated by several studies of nearby galaxies (such as Allen et al. 1997 ApJ 487, 171) which suggest that molecular gas is the more basic form, with H I now being generated by photodissociation of H 2 rather than H I being dominant and H 2 being created transiently from association on grain surfaces. The grains are
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important, because the association rate for most astrophysical molecules in a pure gas phase is vanishingly small, but this is catalyzed by grains; atoms can be adsorbed onto grain surfaces, where the grain can act as a sink for momentum during molecule formation. In fact, there is
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Contrasts between disk - Contrasts between disk- and...

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