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For refcode 1991A&A...247..320S:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1991A&A...247..320S Star formation, molecular clouds and ^13^CO emission from nearby galaxies L.J. Sage and D.W. Isbell Max Planck Institut fur Radioastronomie, Auf dem Huge] 69, W-5300 Bonn, Federal Republic of Germany Astrophysics Research Center, Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Technical Communication Program, Humanities Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Received June 7, 1990; accepted March 27, 1991 Abstract. This paper presents observations of ^13^CO 1-0 emission from 16 nearby spiral galaxies. Through comparisons with ^12^CO and model calculations we have determined that ^12^CO 1-0 is a reliable tracer of molecular mass with, at most, a factor of two uncertainty when a standard Galactic CO to H_2_ conversion ratio is applied. The ratio R of ^12^CO to ^13^CO emission is independent of metallicity within the range examined 9.1 <= 12+log([O]/[H])< 9.6. We found that R is insensitive to variations in global parameters such as inclination angle and Hubble type, although there may be a dependence on average dust temperature for the galaxies with T_dust_~> 40 K. We expected R to be correlated with the ratio of CO to CS emission (which traces very dense gas), but this was not seen. We detected ^13^CO emission from the centers of all 16 galaxies. Central beam ratios range from 5 to 16.6, with most in the 7 to 11 range. We also determined R along the major axes of four galaxies and at four positions in the disk of M 101, with detections at all but one location. The observed line ratios lie between 2.5 and >~ 17, but most are again in the 7 to 11 range. The ^12^CO and ^13^CO line profiles agree very well. Key words: interstellar medium: molecules - galaxies: general - millimeter lines - radio lines: molecular - stars: formation of
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