Dust Properties of NGC 147 and NGC 185
  We present new mid- to far-infrared images of the two dwarf compact
elliptical galaxies that are satellites of M31, NGC 185 and NGC 147, obtained
with the Spitzer Space Telescope. Spitzer's high sensitivity and spatial
resolution enable us for the first time to look directly into the detailed
spatial structure and properties of the dust in these systems. The images of
NGC 185 at 8 and 24 micron display a mixed morphology characterized by a
shell-like diffuse emission region surrounding a central concentration of more
intense infrared emission. The lower resolution images at longer wavelengths
show the same spatial distribution within the central 50" but beyond this
radius, the 160 micron emission is more extended than that at 24 and 70micron.
On the other hand, the dwarf galaxy NGC 147 located only a small distance away
from NGC 185 shows no significant infrared emission beyond 24 micron and
therefore its diffuse infrared emission is mainly stellar in origin. For NGC
185, the derived dust mass based on the best fit to the spectral energy
distribution is 1.9e3 Msol, implying a gas mass of ~3.0e5 Msol. These values
are in agreement with those previously estimated from infrared as well as from
CO and HI observations and are consistent with the predicted mass return from
dying stars based on the last burst of star formation ~1.0e9 yr ago. Based on
the 70 to 160micron flux density ratio, we estimate a temperature for the dust
of ~17K. For NGC 147, we obtain an upper limit for the dust mass of 4.5e2 Msol
at 160 micron (assuming a temperature of ~20K), a value consistent with the
previous upper limit derived using ISO observations of this galaxy. In the case
of NGC 185, we also present full 5-38 micron low-resolution (R~100) spectra of
the main emission regions.
