Star formation and molecular clouds in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the largest close neighbor of our own galaxy, at a distance of only 160,000 light-years – less than twice the diameter of the Milky Way itself. Its proximity makes it a very useful object to study in connection with the process of star formation, which is generally assumed to occur mainly in giant molecular clouds (GMCs). (The use of the term “cloud” in both cases is coincidental, not indicative of a close similarity. There was other very recent research on GMCs in M33, and the writeup on that provides some background on the topic.)

The diameter of the LMC is about 14,000 light-years, and its mass is only about 1% of the mass of the Milky Way. But it’s still quite a bit larger than the Milky Way’s other satellite galaxies. It’s actually the fourth largest galaxy in the Local Group, in which only the Milky Way, M31, and M33 are larger. It even has some traces of a barred spiral structure, rather than being a pure irregular galaxy.

The LMC hosts active star formation, in spite of its small size compared to the Milky Way, and the research discussed here is the most detailed study so far of the assumed close relationship between star formation and GMCs.

One of the main difficulties of studying star formation is that even though very young stars can be quite hot and luminous, they are also usually enveloped in thick clouds of gas and dust. The typical sign of a hot young star is strong ultraviolet emissions (the Lyman series of hydrogen). These emissions are hidden from view because of their absorption by the gas clouds, yet this same interaction also disperses the clouds. Consequently, by the time very young stars are detectable, the clouds are mostly gone.

However, the dust contained in the clouds is heated by the ultraviolet light that is absorbed, and so there is a characteristic infrared signal from the formation of new stars. Such presumed nascent stars are called “young stellar objects” (YSOs). They can be detected long before the clouds have dispersed. The Spitzer Space Telescope has been the instrument of choice for such studies. A much better correlation between the locations of GMCs and YSOs is to be expected from observations in the infrared than in ultraviolet. That is in fact what has been found, and further supported, in the present study.

GMCs can be detected by millimeter-wavelength emissions of carbon monoxide (CO). The research here represents the highest resolution survey to date of GMCs in the LMC. It was able to identify probable GMCs as small as 45 light-years in size. GMCs are recognized as regions of contiguous CO emissions having the expected structure. Since GMCs can have diameters up to 300 light-years, the obtainable resolution allows for some detection of substructure.

One of the main findings in the present research is that most large GMCs have evidence of active star formation. This implies that star formation begins soon after the formation of the GMC itself. However, there’s still uncertainty about the size and age of observable YSOs and the evidence from CO emissions of GMC characteristics. So there’s more work to do in order to relate the detailed time sequences of GMC and YSO formation. The availability of powerful new millimeter-wave instruments (Atacama Large Millimeter Array) will be a big help.

Another interesting finding is that smaller, less luminous GMCs occur more often than expected.

Astronomers look to neighboring galaxy for star formation insight

Conventional wisdom states that most of the molecular gas mass in a galaxy is apportioned to a few large clouds. However, Wong’s team found many more low-mass clouds than they expected – so many, in fact, that a majority of the dense gas may be sprinkled across the galaxy in these small molecular clouds, rather than clumped together in a few large blobs.

“We thought that the big clouds hog most of the mass,” Wong said, “but we found that in this galaxy, it appears that the playing field is more level. The low-mass clouds are quite numerous and they actually contribute a significant amount of the mass. This provides the first evidence that the common wisdom about molecular clouds may not apply here.”

The large numbers of these relatively low-mass clouds means that star-forming conditions in the LMC may be relatively widespread and easy to achieve. The findings raise some interesting questions about why some galaxies stopped their star formation while others have continued it.

Wong, T., Hughes, A., Ott, J., Muller, E., Pineda, J., Bernard, J., Chu, Y., Fukui, Y., Gruendl, R., Henkel, C., Kawamura, A., Klein, U., Looney, L., Maddison, S., Mizuno, Y., Paradis, D., & Seale, J. (2011). THE MAGELLANIC MOPRA ASSESSMENT (MAGMA). I. THE MOLECULAR CLOUD POPULATION OF THE LARGE MAGELLANIC CLOUD The Astrophysical Journal Supplement Series, 197 (2) DOI: 10.1088/0067-0049/197/2/16

Further reading:


The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud

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