We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M 81 group .
The far–infrared data have been obtained as part of the ‘ Spitzer Infrared Nearby Galaxies Survey SINGS ’ .
Maps of the distribution of atomic hydrogen ( HI ) have been obtained through ‘ The H i Nearby Galaxy Survey THINGS ’ .
The Spitzer observations provide a first glimpse of the nature of the non–atomic ISM in these metal–poor ( Z \sim 0.1 Z _ { \odot } ) , quiescent ( SFR \sim 0.001–0.1 M _ { \odot } yr ^ { -1 } ) dwarf galaxies .
Dust emission is detected in five out of the seven targets ( the two systems with the lowest star formation rates are non–detections ) .
Most detected dust emission is restricted to H i column densities > 1 \times 10 ^ { 21 } cm ^ { -2 } and almost all regions of high H i column density ( > 2.5 \times 10 ^ { 21 } 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 .
The spectrum of IC 2574 shows aromatic features that are less luminous ( relative to the FIR luminosity ) compared to an average SINGS spiral galaxy by a factor ot \sim 7 .
The aromatic features in Holmberg II ( which has only a slightly lower gas–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 a simple linear function of metallicity .
Whereas the H i masses are well-constrained , model dependencies make it difficult to measure the dust masses with a high degree of confidence .
We estimate dust masses of \sim 10 ^ { 4 } – 10 ^ { 6 } M _ { \odot } for the M 81 dwarf galaxies , resulting in an average dust–to–gas ratio ( M _ { dust } /M _ { H { \sc i } } ) of \sim 3 \times 10 ^ { -4 } ( 1.5 \times 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 \mu m as compared to the more massive galaxies in SINGS by a factor of \sim 2 .
However , the average 70 \mu m/160 \mu m 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 ( likely due to the harder radiation fields in the low metallicity environments ) .