Dust and atomic gas in dwarf irregular galaxies of the M81 group: The SINGS and things view

We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the Spitzer SINGS and VLA THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the nonatomic ISM in these metal-poor (Z ∼ 0.1 Z_⊙), quiescent (SFR ∼ 0.001-0.1 M_⊙ yr^-1) dwarf galaxies. Most detected dust emission is restricted to H I column densities >1 × 10²¹ cm^-2, and almost all regions of high H I column density (>2.5 × 10²¹ cm^-2) have associated dust emission. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes and aromatic features, although the galaxies have comparable gas-phase metallicities. This result emphasizes that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ∼10 ⁴-10⁶ M_⊙ for the M81 dwarf galaxies, resulting in an average dust-to-gas ratio (M_dust/M_{H I}) of ∼3 × 10^-4 (1.5 × 10^-3 if only the H I that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70 p,m as compared to the more massive galaxies in SINGS by a factor of ∼2. However, the average 70/160 μm ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.