An X‐Ray Minisurvey of Nearby Edge‐on Starburst Galaxies. I. The Data

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Abstract
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 starburst galaxies in order to search for hot gas in their halos. We find that 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 signs of hot gas beyond its thin disk only near the most actively star-forming region that is associated with a giant Hα bubble. All fits to joint PSPC + ASCA spectra, except NGC 55, indicate the presence of two gas phases, one at 0.2-0.4 keV and another at 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 available, 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 galaxy, at a very low temperature of kT 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 imaging 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 statistically good, consistent with each other, and physically meaningful. However, they are not unique, and it is extremely difficult to constrain the metallicities of the different components of hot gas. Our results show that all data can be fitted by composite model spectra with near-solar metallicities. Extremely low metallicities, as previously fitted 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 the spectral properties of the hot gas and compact sources (presumably mostly high-mass X-ray binaries) in their disks. Integral spectra, indicating thermal emission characteristics, of those compact sources outside the galaxy disks with soft hardness ratios, suggest that they might be associated with the halo gas.

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