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The link between superbubbles in the interstellar medium and diffuse X-ray emission has been clearly demonstrated (e.g. Chu & Mac Low 1990) and even some larger bubbles have been shown to be filled with hot gas (e.g. Wang et al. 1991, Bomans et al. 1994). The detailed physics of such bubbles is still ill observed. We do not know the metallicity of the hot gas, the ionization condition (equilibrium or non-equilibrium), the details of the interaction between hot gas and cool shell, or the presence and properties of cold cloudlets inside the hot cavity. In dwarf galaxies, the presence of extended hot gas could be proven and even that parts of this gas is located far away from its possible sites of creation (e.g. Heckman et al. 1995, Bomans et al. 1997). The number of such studies is still very small (see Tab. 1) and limited to galaxies with recent strong star formation. No dwarf galaxy with diffuse hot halo is known up to now. Likewise no dwarf galaxy with extended X-ray emission but currently low star formation rate has been found up to now. Clearly we do not know from observation, what the fate of the hot gas will be. Using the kinematics of the warm ionized gas together with the HI rotation curve one can estimate if hot gas inside a bubble will escape the potential well of a dwarf galaxy (e.g. Martin 1998), but the uncertainties are large, partly due to the big uncertainties in determining the dark matter potential (e.g. Swaters 1999), partly due to the current inability to measure the velocity of the faintest shells and the warm ionized gas with highest velocities (see Section 4).

The determination of the physical parameters of the hot gas are hampered by the quality of the X-ray spectra and the contamination with point sources. The new X-ray telescopes (XMM-NEWTON and CHANDRA) are now in orbit and are working well. Especially XMM-NEWTON promises large improvements in the quality of the spectra due to its unmatched sensitivity and good spatial resolution of ~ 15". CHANDRA is especially well suited for the study of the point source population due to its very good spatial resolution of ~ 0.5", but lower sensitivity. Unfortunately, most of the diffuse X-ray emission is expected to be in the very soft X-ray regime (see Tab. 1), where both satellites are hard to calibrate. Still, at least for the dwarf galaxies with brighter diffuse X-rays the new instruments should allow to analyze the plasma conditions and measure metallicity of the hot gas. This will provide a big step forward in testing the current dwarf galaxy evolution scenarios.

For the diffuse warm gas, the near futures looks also promising, with several 8-10m class telescopes currently coming online. This will enable us to study the kinematics of the outflows even at low surface brightness, hunt for very high velocity gas and presumably extremely faint outer halo gas using emission lines and quasar absorption lines. These methods should also help to better determine the mass distribution and total mass of dwarf galaxies, leading to improved estimates on the escape fraction of the hot gas and therefore the metals.

While we started to answer the question if diffuse warm and especially hot gas is a common constituent of the interstellar matter in dwarf galaxies, the detailed physics of its creation, and evolution, as well as the links to global evolution of dwarf galaxies and the intergalactic medium have only slightly been touched yet. There should be exiting years to come!


The author is very grateful to K. Weis for critically and thoroughly reading the manuscript and many stimulating discussions. Furthermore, the author likes to thank Y.-H. Chu, M. Dahlem, K. Dennerl, R.-J. Dettmar, W.J. Duschl, G. Hensler, U. Hopp, N. Junkes, M.-M. Mac Low, P. Papaderos, G. Richter, E.D. Skillman, R. Tüllmann, M. Vogler, and M. Urbanik for discussions on many facets of dwarf galaxies and outflows. This work was supported by Verbundforschung grant 50 OR 99064 and the Bonn-Bochum Graduiertenkolleg "Magellanic Clouds and other Dwarf Galaxies". This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work has also made use of the NASA's Astrophysics Data System Abstract Service and the LEDA database (

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