A kinematic study of the neutral and ionised gas in the irregular dwarf
  galaxies IC 4662 and NGC 5408
  Abridged. 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 ionised 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 nearby irregular dwarf galaxies: IC 4662 and NGC 5408. The HI line
data were obtained with the ATCA and are part of the Local Volume HI Survey.
They are complemented by optical images and spectroscopic data obtained with
the ESO NTT and the ESO 3.6m telescope. Our main aim is to study the kinematics
of the neutral and ionised gas components in order to search for outflowing gas
structures and to make predictions about their fate. We find the HI gas
envelopes of both galaxies to extend well beyond the optical discs. 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 NGC 5408 shows a fairly regular HI velocity
field, which allows us to derive a rotation curve, IC 4662 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 Halpha spectra of both
galaxies, sometimes with HI counterparts of similar velocity. We suggest the
existence of expanding superbubbles, especially in NGC 5408. This is also
supported by the detection of FWHMs as high as 70 km/s in Halpha. In case of
NGC 5408, 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 NGC 5408. This result is
consistent with studies of other dwarf galaxies.
