The existence of a very hot component of the interstellar medium in the disk and halo of our Galaxy was predicted by Spitzer in 1956. He proposed typical temperatures of 106-7 K and electron densities of 10-(2-3) cm-3. A quarter of a century later, the Einstein satellite detected X-rays from the expected hot media in three spiral galaxies (M 82, NGC 253 and NGC 3628), galaxies with star-formation rates much higher than for "normal type" galaxies. These X-ray observations revealed the existence of hot gas, not only in the bulges and disks of these starburst galaxies, but also in their halos.
In order to search for further examples of these hot interstellar media with ROSAT, the "Nearby Galaxies Group" at the Max-Planck-Institut für extraterrestrische Physik has selected a large sample of spiral galaxies. The sample contains both normal and starburst galaxies, as well as galaxies hosting active galactic nuclei. Whereas galaxies seen edge-on allow a good separation of the disk and halo components, this separation for face-on galaxies can only be achieved by indirect arguments. Diffuse X-ray emission due to hot gas is seen in the disks and halos of several edge-on galaxies, e.g. NGC 253, NGC 3079, NGC 4565, NGC 4631. These emission components have also been established in the face-on galaxies M 51, M 83, M 100, M 101, NGC 1566, NGC 4449 and NGC 5055, and in the semi-inclined NGC 4258, using the spectral information present in the PSPC data. Further work has established the existence of diffuse X-ray emission within the disks and halos of the edge-ons NGC 891, NGC 3628 and NGC 4666, the inclined galaxy M 82 and the face-ons NGC 1068 and NGC 6946. In general, it is seen that galaxies with a higher level of star formation activity contain more hot gas in their disks and halos, and this can be explained by the star-forming processes taking place. Stellar winds from massive stars together with temporally and spatially correlated supernova explosions, can form cavities of hot (million degree) gas in the interstellar medium. These cavities (superbubbles) are usually surrounded by a dense shell of swept-up, cool interstellar medium (supershells), and the hot gas can flow from the superbubbles, into the halo of the galaxy, when the supershells "break".
The figures show the X-ray observations of two edge-on starburst galaxies, NGC 253 and NGC 3079, both galaxies showing diffuse emission from the disks and halos. In the case of the very nearby NGC 253 (2.58 Mpc), the diffuse emission has an X-ray luminosity of 3 x 1039 erg/s. For NGC 3079 (17.3 Mpc), the luminosity is 2 x 1040 erg/s. In addition, the ROSAT observations have established the existence of X-ray emission associated with nuclear superbubbles, with luminosities of 7 x 1038 erg/s (NGC 253) and 8 x 1039 erg/s (NGC 3079).
W. Pietsch, R. Supper, and A. Vogler, ROSAT Observations of X-ray Emission from Superbubbles, Proceedings of the 11th IAP Astrophysics Meeting: The Interplay between Massive Star Formation, the ISM, and Galaxy Evolution, July 3-8, 1995, Institut d'Astrophysique de Paris, Eds. D. Kunth, B. Guiderdoni, M. Heydari-Malayeri, Trinh Xuan Thuan
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