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Date and Time of the Query: 2019-06-19 T22:13:09 PDT
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For refcode 1997ApJ...484..656P:
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Copyright by American Astronomical Society. Reproduced by permission
1997ApJ...484..656P The Average Properties of the Dense Molecular Gas in Galaxies TIMOTHY A. D. PAGLIONE AND JAMES M. JACKSON Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 AND SUMIO ISHIZUKI Nobeyama Radio Observatory, National Astronomical Observatory, Minamimaki, Minamisaku, Nagano 384-13, Japan Received 1996 August 16; accepted 1996 March 6 ABSTRACT We have observed the HCN J = 3 - 2 and J = 1 -0 emission from several nearby starburst and normal galaxies. These lines have large critical densities (n_H_2__ > 10^6^ cm^-3^) and excitation energies (E_u_ > 25 K). Thus, they probe the warm and dense molecular gas where massive stars typically form. The average cloud densities of these galaxies, as estimated from the ratio of the HCN J = 3 - 2 and 1 - 0 integrated intensities, is correlated with their star formation efficiency. Therefore, the average densities of the molecular clouds in starburst nuclei are higher than those of more quiescent galaxies. Further, the starburst galaxies NGC 253 and M82 have a much higher fraction of molecular mass at high density (n_H_2__ > 10^4^ cm^-3^) than the normal galaxies IC 342 and the Milky Way. These results imply that the clouds in starburst nuclei form stars more efficiently than those in normal galaxies. Subject headings: galaxies: ISM-galaxies: starburst-galaxies: stellar content-radio lines: galaxies
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