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

Abstract

We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 Local Group and nearby dwarf galaxies. Our sample spans a range of more than one dex in nebular metallicity and over three orders of magnitude in current star formation rate, allowing us to examine the dependence of the diffuse 8 micron emission, originating from hot dust and PAHs, on these parameters. We detect prominent diffuse 8 micron emission in four of the most luminous galaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightness emission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuse emission from the remaining objects. We observe general correlations of the diffuse 8 micron emission with both the current star formation rate and the nebular metallicity of the galaxies in our sample. However, we also see exceptions to these correlations that suggest other processes may also have a significant effect on the generation of hot dust/PAH emission. These systems all have evidence for old and intermediate age star formation, thus the lack of diffuse 8 micron emission cannot be attributed to young galaxy ages. Also, we find that winds are unlikely to explain the paucity of diffuse 8 micron emission, since high resolution imaging of the neutral gas in these objects show no evidence of blowout. Additionally, we propose the lack of diffuse 8 micron emission in low-metallicity systems may be due to the destruction of dust grains by supernova shocks, assuming the timescale to regrow dust grains and PAH molecules is long compared to the destruction timescale. The most likely explanation for the observed weak diffuse 8 micron emission is at least partly due to a general absence of dust (including PAHs), in agreement with their low metallicities.