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Date and Time of the Query: 2019-06-20 T11:10:07 PDT
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For refcode 1992A&A...264..433B:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1992A&A...264..433B A CO (1-0) and CO (2-1) survey of nearby spiral galaxies II. Physical conditions in the nuclear gas J. Braine and F. Combes Observatoire de Paris, Section de Meudon, F-92 190 Meudon, France Received June 18, 1991; accepted June 26, 1992 Abstract. We present the analysis of the CO(1-0) and CO(2-1) survey of nearby galaxies carried out at the IRAM 30m telescope (Braine et al. 1992). The average CO(2-1) to CO(1-0) line ratio is 0.89+/-0.06 in galactic nuclei (inner kpc), and varies only between 0.82 for isolated to 1.11 for perturbed galaxies, with a large overlap. Since the excitation (as measured by the line ratio) of the gas is not highly different in isolated and perturbed galaxies, we assume that the standard N_H_2_/W_CO_ conversion ratio is valid for all galaxies (except for NGC 3310, where the line ratio is 2.6). Our data therefore indicate that disturbed galaxies have more H_2_ gas in the central ~kpc than galaxies without signs of perturbation. The observed line ratio of 0.9 is compatible with optically thick gas being the dominant component of the nuclear interstellar medium. Only in one galaxy, NGC 3310, do the physical conditions in the molecular gas appear to be different. As the sample contains all galaxies with B_T_ < 12 and with distance and positional criteria only, this result illustrates the rarity of such objects. We show that even high line ratios (>1) do not necessarily imply different N_H_2_/W_CO_ conversion ratios. The differences in the CO emission due to galactic interactions are more pronounced than those due to morphological type, presence of bars, or nuclear activity. Key words: galaxies: nuclei - spiral galaxies - interstellar medium: molecules - radio lines: molecular
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