Submillimeter HCN and HCO(+) emission from galaxies

Details Submillimeter HCN and HCO(+) emission from galaxies Submillimeter HCN and HCO(+) emission from galaxies

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...438..695j&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 438, no. 2, p. 695-701

Physics

40

Interstellar Matter, Molecular Clouds, Molecular Gases, Star Formation, Starburst Galaxies, Carbon Monoxide, Gas Density, Molecular Excitation, Stellar Physics

Scientific paper

We report the first detection of extragalactic HCN and HCQ(+) J = 4 goes to 3 emission at lambda = 850 micrometers. Because their critical densities (n approximately 107 and excitation energies (Eu approximately 45 K) are large, these lines trace dense, warm gas. We have detected HCN J = 4 goes to 3 toward NGC 253, NGC 4945, and IC 342 and report a sensitive upper limit toward M82. We have also detected HCO(+) J = 4 goes to 3 in both NGC 253 and M82. The HCN J = 4 goes to 3/ J = 1 goes to 0 intensity ratio, which is larger for higher gas densities, is at least 12 times larger in NGC 253 than that in M82. Single-component gas excitation model calculations for NGC 253 show that, averaged over the central 300 pc, the molecular gas is warm (T approximately less than 50 K) and dense (n approximately 5 x 105 cm-3). For M82, the lower HCN J = 4 goes to 3/J = 1 goes to 0 line ratio requires an average gas density at least 10 times smaller. In both M82 and NGC 253, the J = 4 goes to 3/J = 1 goes to 0 line ratios peak at the nucleus. Consequently, molecular gas is densest in the central 300 pc. The submillimeter HCN and HCO(+) lines reveal large differences in the dense gas properties, despite similar CO and far-infrared luminosities.

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