Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...277..405g&link_type=abstract
Astronomy and Astrophysics, Vol. 277, NO. 2/OCT(I), P. 405, 1993
Astronomy and Astrophysics
Astrophysics
15
Scientific paper
, We observed, using the SEST 15-m telescope, the CO(2-1) line emission toward eight highly luminous infrared galaxies of the southern hemisphere which were previously detected in the CO(1-0) transition. While the shapes of the (2 - 1) and (1 - 0) line profiles (taken with angular resolutions of 24" and 45", respectively) are remarkably similar, the ratios of observed peak main-beam radiation temperatures vary between 1.2 and 2.6. The derived (2-1)/(1-0) ratios of velocity integrated brightness temperature range between 0.33 and 0.77, indicating that the physical conditions of the molecular gas varies from source to source. Assuming that the kinetic temperature of the molecular gas is similar to the temperature derived for the dust (~30-40 K), the low values of the integrated line ratios imply that the CO is subthermally excited and that the bulk of the emission arises from regions of moderate H_2_ density, between 100 and 500 cm^-3^. We find that the use of the Galactic CO-H_2_ conversion factor is appropriate, to within a factor of 1.5, to estimate the molecular mass in this type of galaxies. We also observed four of these galaxies in the ^13^CO(1-0) transition and one in the ^13^CO(2-1) line. The ^12^CO(1-0)/^13^CO(1-0) velocity integrated brightness temperature ratios range from 9 to 27. The largest values, about 5 times greater than the average ratio observed for molecular clouds in the Milky Way disk, are exhibited by the most luminous IR galaxies. They can be explained if molecular clouds in mergers have low and moderate optical depths in ^13^CO and ^12^CO, respectively.
Garay Guido
Mardones Diego
Mirabel Felix I.
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