Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS View

[abridged] We present observations of the dust and atomic gas phase in sevendwarf irregular galaxies of the M81 group from the SINGS and THINGS surveys.The Spitzer observations provide a first glimpse of the nature of thenon-atomic ISM in these metal-poor (Z~0.1 Z_sun), quiescent (SFR~0.001-0.1M_sun/yr) dwarf galaxies. Dust emission is detected in five out of the seventargets. Most detected dust emission is restricted to HI column densities>1x10^21 cm^-2. Spitzer spectroscopy of two regions in the brightest galaxies(IC 2574 and Holmberg II) show distinctly different spectral shapes. Thespectrum of IC 2574 shows aromatic features that are less luminous (relative tothe FIR luminosity) compared to an average SINGS spiral galaxy by a factor of\~7 . The aromatic features in Holmberg~II (which has only a slightly lowergas-phase metallicity) are fainter than in IC 2574 by an order of magnitude.This result emphazises that the strength of the aromatic features is not asimple linear function of metallicity. We estimate dust masses of ~10^4-10^6M_sun for the M81 dwarf galaxies, resulting in an average dust--to--gas ratio(M_dust/M_HI) of ~3x10^-4 (1.5x10^-3 if only the HI that is associated withdust emission is considered); this is an order of magnitude lower than thetypical value derived for the SINGS spirals. The dwarf galaxies areunderluminous per unit star formation rate at 70um as compared to the moremassive galaxies in SINGS by a factor of ~2. However, the average 70um/160umratio in the sample dwarf galaxies is higher than what is found in the othergalaxies of the SINGS sample. This can be explained by a combination of a lowerdust content in conjunction with a higher dust temperature in the dwarfs.