Starburst driven outflows are needed to reconcile the observed star formation histories of dwarf galaxies with their current (low) metallicity (e.g. Marconi et al. 1994). They also provide an attractive mechanism to create gas free dwarf spheroidal galaxies as an evolutionary product of starbursts in intermediate redshift dwarf galaxies (Babul & Ferguson 1996). This mechanism could contribute significantly the large numbers of faint, blue galaxies in deep fields.
Still, observational data are limited to a small number of objects, which is especially true for the detection of hot gas in the halos of low mass galaxies. The only search for extended hot halos of dwarf galaxies failed to detect hot halos in their three target galaxies (Bothun et al. 1994), questioning at least some assumptions of the picture described above.
In a first analysis pass we overlayed the broad (0.1-2.4 keV) and soft (0.1-0.4 keV) band ROSAT images with B band images of the galaxies and searched for extended diffuse emission in a 10 kpc radius around the target galaxy. To test the feasibility of our project we calculated the X-ray luminosity (0.1-2.4 keV) of a column through a hot halo as function of the mean electron density. We adopted a plasma temperature of 3·106 K (typical for diffuse hot gas). Further, we assumed a foreground H I column density of log NH =20.5 and an integration time of 10 ks, both representative values for our sample. The calculations showed that we should be able to detect hot halos in our galaxy sample, if the mass in a sphere with 10 kpc radius is not smaller than 106 Msun. We should also detect patchy halos of much lower mass, if the X-ray emitting blobs have diameters larger than ∼1 kpc.
In addition to seven cases documented in the literature, we found six more galaxies in our sample which show possible extended X-ray emission. In all galaxies of our sample the prospective diffuse emission is located within 2 kpc of the galaxy (Bomans 1998). We did not detect any large hot halo.
While `extended' hot halos appear to be rare, a lot can be learned from detailed studies of the diffuse hot gas in the `lower' halo:
When comparing the X-ray emission with the Halpha map, it becomes clear that many filaments extend perpendicular from H II regions in the main body into the X-ray emitting supergiant shell (Fig. 1b). One may speculate that these filaments are chimneys, through which hot gas is vented upward, as envisaged by Norman & Ikeuchi (1989). This idea receives strong support by the dynamical data which show high velocity gas associated with the brightest of the chimneys (Bomans et al. 1997). Apparently the supergiant shell at the west side of NGC 4449 is currently energized by the star formation regions in the main body of NGC 4449, which are not a genuine starburst. This implies that less extreme star formation events may create (or at least maintain) hot gas in the lower halos of low mass galaxies.
At a distance of ∼10 Mpc, I Zw 18 is well suited to test the assumptions of the galactic wind model. Recent HST observations showed that I Zw 18 has an abnormally high carbon abundance, which implies that it is not experiencing its first burst of star formation. An older population must be present to elevate the carbon abundance (Garnett et al. 1997). Therefore the extremely low oxygen abundance may be explained by a galactic wind, transporting the metals created by massive stars and type II supernovae (e.g. O, but not C) away. Support for this idea comes from deep Halpha imaging which reveals kpc-sized shells around I Zw 18, and long-slit echelle spectroscopy showing an expanding bubble in the central region of I Zw 18 (Martin 1996; Bomans et al. 1998). I Zw 18 is detected in X-rays with the ROSAT PSPC and the spectrum can be interpreted as hot gas, but also due to high mass X-ray binaries. The interpretation is not unambiguous especially due to the fact that the point-spread function of the PSPC (30") is larger than the galaxy.
To locate and analyse the source of the X-ray emission, we observed I Zw 18 with the ROSAT HRI, which provides a spatial resolution of 5", at the expense of higher background and missing energy resolution. I Zw 18 was detected and the source is clearly not point-symmetric. At least two components are visible and they correspond to the central bubble and the area of supersonic expansion (Fig. 2). The X-ray emission also appears to fill the large shell in the south-west (Fig. 2).
It appears that we detected hot gas moving out of I Zw 18, in concordance with the idea, that metal-loss into the halo is responsible for the low metallicity of the star forming regions in the main body. While this qualitatively supports the link between metal abundance of dwarf galaxies and galactic scale outflows, a larger sample and especially higher quality data of I Zw 18 (better sensitivity, spatial and spectral resolution) are needed to make a quantitative analysis. These data can be supplied by the upcoming X-ray missions.
Not everything falls into place. Our X-ray survey for extended halos (Bomans 1998) showed, that there are no large 106 K halos around dwarf galaxies, only the local effects of recent starbursts. This is roughly consistent with the observations on NGC 4449, but only marginally consistent with the results of NGC 1569. Of all low mass galaxies only M 82 has an X-ray halo more extended than its diffuse Halpha emission.
Either the hot gas travels much larger distances in a short time and drops therefore below detection surface brightness, as implied by Mac Low & Ferrara (1998), or the gas cools rapidly out of the ROSAT band into the EUV domain. The latter idea is less likely due to EUV emission limits of star forming galaxies (Reid & Ponman 1996). One problem in constraining these possibilities is that the important temperature region around 105 K is largely unexplored, except for the Magellanic Clouds. The recent detection of a 105 K halo of the LMC (Wakker et al. 1998) underlines that we do no yet fully understand the interplay of gas and stars in low mass galaxies.
| First version: | 16th | August, | 1998 |
| Last update: | 25th | September, | 1998 |