Detection of mid-infrared aromatic hydrocarbon emission features from the Small Magellanic Cloud

The mid-infrared (5–16 μm) spectral energy distribution for an individual quiescent molecular cloud in the Small Magellanic Cloud (SMC) was observed using ISOCAM. The spectrum is dominated by broad emission bands at 6.2, 7.7, and 11.3 μm, with weaker bands at 8.6 and 12.7 μm. As these are the same bands, with similar shape and relative strengths, as observed in the ISM of our Galaxy, the same carriers must exist in both galaxies. The carriers are widely considered to be large molecules or clusters of aromatic hydrocarbons, which absorb ultraviolet and visible photons and emit mid-infrared photons during high-temperature pulses. Based on the brightness of the mid-infrared emission and the estimated strength of the radiation field in the SMC, the absorption by aromatic hydrocarbons is of order 10% of total dust absorption, comparable to the case for Galactic dust. Ultraviolet observations of extinction of most SMC stars have shown that dust in the SMC does not absorb in the 2175 Å feature that is so prominent in Milky Way extinction. If aromatic hydrocarbons and featureless extinction curves were ubiquitous in the SMC, then we would conclude that aromatic hydrocarbons are not the carriers of the 2175 Å feature. However, SMC extinction curve measurements are biased toward hot, luminous stars, where aromatic hydrocarbons are destroyed, so that the absence of the 2175 Å bump may not be typical of SMC dust. The presence of aromatic hydrocarbons in the SMC further demonstrate that these molecules exist even in an interstellar medium with an order-of-magnitude lower metallicity than in the disk of the Milky Way.