2010MNRAS.407..113V

The feedback between massive stars and the interstellar medium is one of the most important processes in the evolution of dwarf galaxies. This interaction results in numerous neutral and ionized gas structures that have been found both in the disc and in the halo of these galaxies. However, their origin and fate are still poorly understood.

We here present new Hi and optical data of two Magellanic irregular dwarf galaxies in the Local Volume: IC4662 and NGC5408. The HI line data were obtained with the Australia Telescope Compact Array and are part of the `Local Volume HI Survey'. They are complemented by optical images and spectroscopic data obtained with the European Southern Observatory (ESO) New Technology Telescope and the ESO 3.6-m telescope. Our main aim is to study the kinematics of the neutral and ionized gas components in order to search for outflowing gas structures and to make predictions about their fate. Therefore, we perform a Gaussian decomposition of the HI and Hα line profiles.

We find the HI gas envelopes of IC4662 and NGC5408 to extend well beyond the optical discs, with HI to optical diameter ratios of above 4. The optical disc is embedded into the central HI maximum in both galaxies. However, higher resolution HI maps show that the HI intensity peaks are typically offset from the prominent HII regions.

While NGC5408 shows a fairly regular HI velocity field, which allows us to derive a rotation curve, IC4662 reveals a rather twisted HI velocity field, possibly caused by a recent merger event. We detect outflows with velocities between 20 and 60 km/s in our Hα spectra of both galaxies, sometimes with HI counterparts of similar velocity. We suggest the existence of expanding superbubbles, especially in NGC5408. This is also supported by the detection of full width at half-maxima as high as 70 km/s in Hα, which cannot be explained by thermal broadening alone. In the case of NGC5408, we compare our results with the escape velocity of the galaxy, which shows that the measured expansion velocities are in all cases too low to allow the gas to escape from the gravitational potential of NGC5408. This result is consistent with studies of other dwarf galaxies.