Using ISOCAM observations of a molecular cloud in the SMC, we found that the mid-infrared emission of its low-metallicity interstellar medium is dominated by broad aromatic hydrocarbon features at 6.2, 7.7, 8.6, 11.3, and 12.7 µm. The locations of the features are similar to those found in the Milky Way interstellar medium, but the brightness ratios among the features are different. The 11.3 µm feature is relatively brighter in the SMC, suggesting relatively more emission from C-H bonds compared to C-C bonds, which we can explain as a difference in the aromatic hydrocarbons forming in a lower metallicity (higher H/C abundance ratio) environment in the SMC. The abundance of aromatic hydrocarbons, relative to the dust that makes the rest of the absorption, is similar in the SMC B1#1 cloud and typical Milky Way dark clouds. While the properties, and the total abundance, of dust in the SMC and Milky Way differ significantly, the aromatic hydrocarbons and larger grains are likely to play similar roles in the interstellar media of the two galaxies.
Previous studies of the mid-infrared emission from SMC suggested that its dust lacks the 2175 Å feature and may be deficient in aromatic hydrocarbons. These effects may both be due to the fact that massive star forming regions dominate the infrared emission and provide the beacons toward which extinction curves are measured. Aromatic hydrocarbons are destroyed in such high-radiation environments (Ryter, Puget and Perault 1987). Our results suggest that aromatic hydrocarbons exist in the SMC with an abundance (relative to that of large grains) comparable to that in the Milky Way. These results probably apply to other low-metallicity or high-redshift galaxies.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000