Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...421..458t&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 421, no. 2, p. 458-474
Astronomy and Astrophysics
Astronomy
14
Carbon Monoxide, H Ii Regions, Interstellar Matter, Line Spectra, Local Group (Astronomy), Molecular Clouds, Star Formation, Stellar Spectra, Astronomical Interferometry, Astronomical Spectroscopy, Interstellar Gas, Metallicity
Scientific paper
We present new (12)CO J = 2 goes to 1 maps for three regions in the nearby spiral galaxy M33, as well as the first (12)CO J = 3 goes to 2 observations of a Local Group galaxy. Two of the regions observed are centered on molecular clouds in the inner disk, while the third is centered on the giant H II region NGC 595, a region where the star formation rate and efficiency are higher than those of the inner disk by an order of magnitude. These data have been combined with published J = 1 goes to 0 data to measure the CO line ratios in M33. The ratio of the J = 2 goes to 1 to J = 1 goes to 0 integrated intensities is 0.67 +/- 0.19, in good agreement with measurements of this line ratio in other galaxies, while the J = 3 goes to 2 to J = 2 goes to 1 integrated intensity ratio is 0.64 +/- 0.28. The J = 2 goes to 1 to J = 1 goes to 0 line ratio is very uniform throughout the regions studied, and thus the presence of intense high-mass star formation and the lower metallicity in NGC 595 do not seem to have a measurable effect on the CO emission in the lowest two rotational transitions. comparison of the J = 1 goes to 0 peak temperatures estimated for beam sizes ranging from 7 sec to 54 sec reveals that the molecular cloud MC 32 has an average areal filling factor substantially less than unity (less than or equal to 0.3). In addition, CO emission observed with single-dish measurements but not present in interferometric observations ('the missing flux') contributes substantially to the area filling factor of the CO emission within 0.5 min-1 min single-dish beams. The CO line ratios have been combined with a large velocity gradient model to constrain the density, column density, and temperature of the molecular gas. Although our measurements are consistent with recent results from multiple-transition CO studies of Galactic giant molecular clouds, with only two line ratios we cannot place very strong constraints on the physical conditions in the molecular interstellar medium in M33. For an assumed kinetic temperature of 20 K, the molecular hydrogen density must be less than 3000/cc and the CO column density must be 5 x 1016 - 5 x 1019/sq cm.
Thornley Michele D.
Wilson Christine Diana
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