Physics
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt.........1x&link_type=abstract
Ph.D. Thesis Massachusetts Univ., Amherst.
Physics
Carbon Monoxide, Cosmic Dust, Cosmic Gases, Gas Density, Molecular Clouds, Molecular Gases, Temperature Distribution, Atmospheric Models, Grain Size, Interstellar Matter, Local Thermodynamic Equilibrium, Luminosity, Mass Ratios, Radiative Transfer
Scientific paper
We investigate three aspects in the determinations of molecular gas masses from 12CO, 13CO, and dust emission. Using the 12CO, 13CO J = 2 - 1 and 1 - O as well as CS J = 2 - 1 data obtained with the 14 m FCRAO telescope, in conjunction with radiative transfer calculations, we estimate the average temperature and density of the molecular gas at the centers of two actively star-forming galaxies, NGC 2146 and IC 342. The physical conditions of the gas in the nuclei of these two galaxies differ from those in the disk of our own Galaxy. However, the CO luminosity to molecular gas mass ratios are found to be similar in these two galaxies. The ratios are within a factor of 2 of the value in the Milky Way if the Galactic CO abundance is assumed. We present a simple model to estimate the dust temperature distributions in galaxies based on the FIR/sub mm observations. The uncertainties in the dust mass owing to inexact modeling of the temperature distribution, uncertainty in the emissivity law, imprecision in the observed data, and the presence of a correlation between grain size and temperature are examined. The dust masses are estimated from the derived dust temperature distributions in 12 galaxies making use of the available data at 60, 100, 345, and 761 microns. The comparison between the dust masses obtained from our model calculations and the gas masses deduced from 12CO J = 1 - O observation yields an average H2-to-dust mass ratio of 181+/-11991. In the determination of H2 column density from 13CO emission the clumpiness of material in molecular clouds has not been considered in the evaluation of the uncertainties. We take this effect into account to examine the uncertainties owing to the use of the LTE approximation. The clumpy cloud model proposed by Kwan and Sanders (1986) is used over a wide range of parameters. Our calculations indicate that the largest uncertainty arises from the assumption that all levels possess a common excitation temperature in estimating the partition function. The partition function can be either over estimated, owing to subthermal excitations in high J levels, or underestimated, probably owing to an underestimate of the mean excitation temperature when the volumn filling factor is so small that the surface coverage factor of antenna beam is less than one. The uncertainties in N(H2) can be reduced by using an alternative formula to estimate the partition function.
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