NASA/IPAC EXTRAGALACTIC DATABASE
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For refcode 1995AJ....109.1716P:
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1995AJ....109.1716P FIRST OBSERVATIONS OF EXTRAGALACTIC CS J = 1-0 TIMOTHY A. D. PAGLIONE AND JAMES M. JACKSON Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215 Electronic mail: paglione@fish.bu.eilu, jaclison@fish.bu.edu SUMIO ISHIZUKI Astronomical Institute, Tohoku Univeisiry, Aoba, Aramaki, Aoba-ku, Sendai, 980, Japan Electronic mail: ishizuki@astroa.astr.tohoku.ac.jp NGUYEN-Q-RIEU Observatoire de Paris, DEMIRM, 61 Avenue de l'Observatoire, 75014 Paris, France Electronic mail: rieu@mesioa.ohspm.fr Received 1994 December 12; revised 1995 January 17 ABSTRACT We report the first detection of extragalactic CS J = 1 - 0 emission. Because CS requires relatively high densities for thermal excitation (n_H _2__ ~> 10^4^ cm^-3^), CS emission is expected from the denser core regions of molecular clouds. Using the Nobeyama 45 m telescope, we detected unresolved CS J = 1-0 emission from the galaxies NGC 253, M82, NGC 6946, and IC 342, and obtained sensitive upper limits to the emission from Maffei 2 and M51. The CS J= 1-0 line was also detected from NGC 253 with the Nobeyama Millimeter Array, and CS J = 3-2 emission was mapped along the major axis of NGC 253 with the IRAM 30 m. Analysis of a single gas component CS excitation model implies that the cloud properties in these galaxies are very similar to those derived from similar CO studies, even though the critical density of a given line of CS is ~25 times higher. On the other hand, the cloud properties derived from CS studies are different from those derived from HCN, whose lines have critical densities ~13 times those of CS. The CS J = 1-0 emission also correlates better with CO J = 1-0 emission than with that of HCN. From theoretical CO/CS and HCN/CS J = 1-0 line ratio calculations, we find that the observed HCN flux is lower than expected. The simplest explanation for this result is that the dense gas traced by HCN is concentrated in smaller regions than the more diffuse gas traced by CS and CO. Model calculations suggest that the total beam-averaged column density of H_2_ traced by CO is greater than that traced by CS, which in turn is greater than the column density traced by HCN. Therefore, because CO, CS, and HCN apparently trace different physical regions in these galaxies, a single- component, uniform cloud model cannot simultaneously account for emission from species with different critical densities.
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