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Carbon monoxide observations in external galaxies and gas mass determination
Abstract
We investigate three aspects in the determinations of molecular gas masses from $\sp{12}$CO, $\sp{13}$CO, and dust emission. Using the $\sp{12}$CO, $\sp{13}$CO J = 2 $-$ 1 and 1 $-$ 0 as well as CS J = 2 $-$ 1 data obtained with the 14m 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/submm observations. The uncertainties in the dust mass owing to inexact modelling 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 $\mu$m. The comparison between the dust masses obtained from our model calculations and the gas masses deduced from $\sp{12}$CO J = 1 $-$ 0 observations yields an average H$\sb2$-to-dust mass ratio of $181 \pm \sbsp{91}{119}$. In the determination of H$\sb2$ column density from $\sp{13}$CO 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 overestimated, 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(H$\sb2$) can be reduced by using an alternative formula to estimate the partition function.
Subject Area
Astrophysics
Recommended Citation
Xie, Shuding, "Carbon monoxide observations in external galaxies and gas mass determination" (1993). Doctoral Dissertations Available from Proquest. AAI9316728.
https://scholarworks.umass.edu/dissertations/AAI9316728