  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.
