AN X-RAY MINISURVEY OF NEARBY EDGE-ON STARBURST GALAXIES - I - THE DATA

We have analyzed all available ROSAT PSPC and HRI and ASCA data for a small far-infrared flux-limited sample of seven nearby edge-on starbur st galaxies in order to search for hot gas in their halos. We find tha t all five normal-sized spiral galaxies (NGC 253, NGC 3079, NGC 3628, NGC 4631, and NGC 4666) have hot gas in their halos, as does the small peculiar galaxy M82. NGC 55, a nearby Magellanic irregular, shows sig ns of hot gas beyond its thin disk only near the most actively star-fo rming region that is associated with a giant Ho: bubble. All fits to j oint PSPC + ASCA spectra, except NGC 55, indicate the presence of two gas phases, one at similar or equal to 0.2-0.4 keV and another at simi lar or equal to 0.65-0.9 keV, which in general appear to dominate the emission of the hot gas. In NGC 4666, for which only ROSAT data are av ailable, we currently find only a gas component near 0.3 keV. The data of NGC 4631 hint at the presence of a third gas component in this gal axy, at a very low temperature of kT similar or equal to 0.05 keV. One of our general results is that the joint PSPC + ASCA spectra allow us to identify different spectral components. Based on the additional im aging information and on consistency checks, we could fit all spectra with similar composite models including both the above thermal plasmas and a harder power-law component. The results of these fits are stati stically good, consistent with each other, and physically meaningful. However, they are not unique, and it is extremely difficult to constra in the metallicities of the different components of hot gas. Our resul ts show that all data can be fitted by composite model spectra with ne ar-solar metallicities. Extremely low metallicities, as previously fit ted by others based on individual ROSAT or ASCA observations, are thus not required. As a by-product of our investigations, spectra of other X-ray emitters in the target galaxies were obtained. We determined th e spectral properties of the hot gas and compact sources (presumably m ostly high-mass X-ray binaries) in their disks. Integral spectra, indi cating thermal emission characteristics, of those compact sources outs ide the galaxy disks with soft hardness ratios, suggest that they migh t be associated with the halo gas.