A kinematic study of the neutral and ionized gas in the irregular dwarf galaxies IC 4662 and NGC 5408★

Abstract

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 H i and optical data of two Magellanic irregular dwarf galaxies in the Local Volume: IC 4662 and NGC 5408. The H i line data were obtained with the Australia Telescope Compact Array and are part of the ‘Local Volume H i 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 H i and Hα line profiles. We find the H i gas envelopes of IC 4662 and NGC 5408 to extend well beyond the optical discs, with H i to optical diameter ratios of above 4. The optical disc is embedded into the central H i maximum in both galaxies. However, higher resolution H i maps show that the H i intensity peaks are typically offset from the prominent H ii regions. While NGC 5408 shows a fairly regular H i velocity field, which allows us to derive a rotation curve, IC 4662 reveals a rather twisted H i velocity field, possibly caused by a recent merger event. We detect outflows with velocities between 20 and 60 km s−1 in our Hα spectra of both galaxies, sometimes with H i counterparts of similar velocity. We suggest the existence of expanding superbubbles, especially in NGC 5408. This is also supported by the detection of full width at half‐maxima as high as 70 km s−1 in Hα, which cannot be explained by thermal broadening alone. In the 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.

The intriguing H i gas in NGC 5253: an infall of a diffuse, low-metallicity H i cloud?★

“…Following these models, the expansion velocity of the outflows will never get close to or beyond the escape velocity due to the presence of a dark matter halo (in our case, maybe the huge amount of neutral gas that is surrounding the main body of the galaxy is also playing an important role) that slows down the gas expelled by the galactic winds. In fact, the expansion velocities of the gas computed by van Eymeren et al (2010) are 30–50 per cent the escape velocities.…”

“…We now compare the features of the neutral gas component found in NGC 5253 with the other starbursts described in the literature. Recent analyses of the neutral gas component of dwarf starburst galaxies include NGC 2366 (van Eymeren et al 2009a), NGC 4861 (van Eymeren et al 2009b), NGC 5408 and IC 4662 (van Eymeren et al 2010). All these galaxies have the common characteristic of showing outflows of neutral gas that follows the ionized gas.…”

“…van Eymeren et al (2010) reported evidence of merging in the BCDG IC 4662, as its diffuse H i gas is somewhat distorted and it shows a tail of neutral gas. Indeed, spectroscopic observations (Hidalgo‐Gámez, Masegosa & Olofsson 2001; López‐Sánchez et al, in preparation) seem to reveal important physical and chemical differences in the ionized gas within this galaxy, which also indicate that this galaxy is actually formed by two entities in interaction.…”

Atomic hydrogen, star formation and feedback in the lowest mass blue compact dwarf galaxies

“…This weakening of the central dark matter cusp occurs even if there is no escape of baryonic matter from the galaxy. On the observational front, van Eymeren et al () as well as Cannon et al () both find evidence for feedback from star formation into the neutral ISM of dwarf galaxies. But even though our galaxies represent some of the smallest mass, actively star‐forming systems, feedback between the star formation on the neutral gas distribution in two of them (d0958+66 and d1028+70) is not clearly evident.…”

Dynamics of starbursting dwarf galaxies

“…Studies of the Hα kinematics have shown that galactic winds are common in BCDs, but the velocities of the Hα gas are usually smaller than the escape velocities (Martin 1996(Martin , 1998van Eymeren et al 2009bvan Eymeren et al ,a, 2010, implying that the ionized gas is gravitationally bound to the galaxy. Similar results have been found by studies of the Na D absorption doublet (Schwartz & Martin 2004).…”