Hot Dust and PAH Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies

摘要

We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 LocalGroup and nearby dwarf galaxies. Our sample spans a range of more than one dexin nebular metallicity and over three orders of magnitude in current starformation rate, allowing us to examine the dependence of the diffuse 8 micronemission, originating from hot dust and PAHs, on these parameters. We detectprominent diffuse 8 micron emission in four of the most luminous galaxies inthe sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightnessemission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuseemission from the remaining objects. We observe general correlations of thediffuse 8 micron emission with both the current star formation rate and thenebular metallicity of the galaxies in our sample. However, we also seeexceptions to these correlations that suggest other processes may also have asignificant effect on the generation of hot dust/PAH emission. These systemsall have evidence for old and intermediate age star formation, thus the lack ofdiffuse 8 micron emission cannot be attributed to young galaxy ages. Also, wefind that winds are unlikely to explain the paucity of diffuse 8 micronemission, since high resolution imaging of the neutral gas in these objectsshow no evidence of blowout. Additionally, we propose the lack of diffuse 8micron emission in low-metallicity systems may be due to the destruction ofdust grains by supernova shocks, assuming the timescale to regrow dust grainsand PAH molecules is long compared to the destruction timescale. The mostlikely explanation for the observed weak diffuse 8 micron emission is at leastpartly due to a general absence of dust (including PAHs), in agreement withtheir low metallicities.