Mapping and mass measurement of the cold dust in NGC 205 with Spitzer

We present observations at 3.6, 4.5, 5.8, 8, 24, 70, and 160 μm of NGC 205, the dwarf elliptical companion of M31, obtained with the Spitzer Space Telescope. The point-source-subtracted images at 8 and 24 μm display a complex and fragmented infrared emission coming from both very small dust particles and larger grains. The extended dust emission is spatially concentrated in three main emission regions, seen at all wavelengths from 8 to 160 μm. These regions lie approximately along NGC 205's semimajor axis and range from ∼100 to 300 pc in size. On the basis of our mid-/far-infrared flux density measurements alone, we derive a total dust mass estimate on the order of 3.2 × 10⁴ M_⊙, mainly at a temperature of ∼20 K. The gas mass associated with this component matches the predicted mass returned by the dying stars from the last burst of star formation in NGC 205 (∼0.5 Gyr ago). Analysis of the Spitzer data combined with previous 1.1 mm observations over a small central or "Core" region (18″ diameter) suggests the presence of very cold (T ∼ 12 K) dust and a dust mass about 16 times higher than is estimated from the Spitzer measurements alone. Assuming a gas-to-dust mass ratio of 100, these two data sets, i.e., with and without the millimeter observations, suggest a total gas mass in the range from 3.2 × 10⁶ to 5 × 10⁷ M_⊙.