Infrared dark clouds (IRDCs) are dark patches in the sky seen against the continuous, bright infrared background produced by our galaxy.
IRDCs are rich in molecules and relatively dense, cool gas, and they are natural sites for future star birth. Studies of IRDCs to date have
emphasized those candidates that already have star formation underway within them, but astronomers are increasingly interested in probing younger, colder clouds to probe earlier stages in the star formation process.
One tool to use is the gas, ammonia. In 1969, astronomers discovered that ammonia (NH3) was present in large quantities in interstellar gas clouds. The species was most apparent in regions of star formation where the density and temperature of the gas enabled it to emit bright radio-wavelength radiation. Since then, ammonia has become one of the staple diagnostic probes of the regions where new stars are forming. One issue has been that radio telescopes capable of detecting ammonia have relatively poor spatial discrimination; this means many IRDCs appear as point sources, without structure.
CfA astronomer David Wilner and two colleagues teamed up to use a combined set of telescopes: one group in New Mexico and the second one in West Virginia. Operating together coherently, the combination is able to see small sub-structures in IRDCs, including regions within the clouds where new stars may be forming.
The group reports on a set of six relatively young IRDCs in the new issue of the Astrophysical Journal. They find strong signals from ammonia, and calculate from them that the gas temperatures are only about ten degrees above absolute zero. Although a few of their sample of IRDCs are known to contain young stars, the ammonia gas in all cases was cold - apparently the new stars have not heated up the cloud. Of particular importance is their conclusion that the density of the gas is high (none has frozen out onto dust, for example, as can happen to molecules like carbon monoxide). The paper concludes from the overall physical conditions that these clouds are probably resistant to collapsing into new stars. New stars do form in them, they argue, largely because of pressure from the much warmer gas reservoir surrounding them in their galactic neighborhoods.