Constraining the influence of star formation on the lowest (12)CO line ratios in M33

Abstract

view Abstract Citations (16) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Constraining the Influence of Star Formation on the Lowest 12CO Line Ratios in M33 Thornley, Michele D. ; Wilson, Christine D. Abstract We present new ^12^CO J = 2-1 maps for three regions in the nearby spiral galaxy M33, as well as the first ^12^CO J = 3-2 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 = 1-0 data to measure the CO line ratios in M33. The ratio of the J = 2-1 to J = 1-0 integrated intensities is 0.67 +/- 0.19, in good agreement with measurements of this line ratio in other galaxies, while the J = 3-2 to J = 2-1 integrated intensity ratio is 0.64 +/- 0.28. The J = 2-1 to J = 1-0 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. A comparison of the J = 1-0 peak temperatures estimated for beam sizes ranging from 7" to 54" reveals that the molecular cloud MC 32 has an average areal filling factor substantially less than unity (<= 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'-1' 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 cm^-3^ and the CO column density must be 5 x 10^16^ - 5 x 10^19^ cm^-2^. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173663 Bibcode: 1994ApJ...421..458T Keywords: 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; Astronomy; GALAXIES: INDIVIDUAL MESSIER NUMBER: M33; GALAXIES: ISM; ISM: H II REGIONS; ISM: INDIVIDUAL NGC NUMBER: NGC 595; ISM: MOLECULES full text sources ADS | data products SIMBAD (14) NED (1)