The Effect of Star Formation on Molecular Clouds in Dwarf Irregular Galaxies: IC 10 and NGC 6822

Facilities: JCMT (RxA, RxB)
Collaborator: Dr. C. D. Wilson
Status: Published in The Astrophysical Journal (see Petitpas & Wilson 1998)

Summary: We have observed CO emission from the dwarf irregular galaxies IC 10 and NGC 6822. We model line ratios to determine the physical conditions of the molecular gas in these low metallicity environments. We find that these galaxies are more porous to ultravoilet radation, allowing hot stars to heat molecular clouds more effectively than is possible in other higher metallicity galaxies. In IC 10 we find large gravitationally bound molecular clouds which may be resposible for it's high star formation rate.

Dwarf irregular galaxies are rather simple systems, free from the complicated dynamics that accompany bars or spiral density waves. This simplicity makes them useful for studying the interactions between star formation and the interstellar medium (ISM). Temperature, density, and abundance may all affect the star formation process, and thus, determining the physical conditions inside molecular clouds is important for understanding how the properties of the molecular gas affect the type and amount of stars formed. Unfortunately, extragalactic molecular clouds have low filling factors even within beams as small as 15" and so their true temperatures cannot be deduced from the observed peak temperature of the spectral line. Instead, observations of many rotational transitions of CO and its isotopomers can be combined with radiative transfer models to constrain the physical conditions inside these clouds. As it is necessary to observe some of the rarer isotopomers such as \thco, these techniques have been primarily applied to starburst galaxies (e.g. Tilanus et al. 1991).

In this work we studied the physical conditions and dynamics of molecular gas near areas of active star formation. Using the James Clerk Maxwell Telescope, we measured a variety of different rotational (J) transitions of CO and its isotopomers. We have observed the 12CO J=2-1 and J=3-2 lines at a few locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James Clerk Maxwell Telescope. In addition, we have observed the 13CO J=2-1 line for IC 10 and the first detection of the 13CO J=3-2 transition in a Local Group galaxy. The CO line ratios in IC 10 are uniform and are consistent with the average line ratios observed in M33. These low metallicity environments appear to be porous to UV radiation and allow for more efficient heating of molecular gas by nearby HII regions. The spectral line ratios suggest that the CO emission is optically thin in NGC 6822. The line ratio in one of the molecular clouds in IC 10 indicate that the gas must have a rather high kinetic temperature of about 100 K. In IC 10 we observe structures on a variety of size scales that all appear to be gravitationally bound. This effect may help explain the rather high star formation rate in IC 10.



References

Petitpas, G. R., & Wilson, C. D., 1998, ApJ, 496, 226

Tilanus, R. P. J., Tacconi, L. J., Sutton, E. C., Zhou, S., Sanders, D. B., Wynn-Williams, C. G., Lo, K. Y., & Stephens, S. A., 1991, ApJ, 376, 500