In part II several different classes of X-ray sources have been introduced, which can be observed within the Milky Way. One of the largest uncertainties in their study is the distance of the more luminous sources, which are often known to not better than a factor of two. This results in an order of magnitude uncertainty in the X-ray luminosity and prevents the determination of the luminosity function of the various types of sources. In addition to these problems, the absorption within the plane of the Galaxy strongly suppresses soft X-ray emission. Therefore, supersoft X-ray sources (SSS) and the hot interstellar medium (ISM) can only be observed in the solar neighborhood.
Observations of the X-ray source population and the ISM in nearby galaxies help us to overcome these difficulties. All X-ray sources in a galaxy are - to first order - seen at the same distance. For the study of SSS and the hot ISM, galaxies are of specific interest which are observed in directions with low Galactic foreground absorption; late type galaxies seen face-on are best suited for studies of the source population and the ISM in the galaxy disk; galaxies seen edge-on allow us to resolve emission from the hot ISM in the galaxy halo and the interaction region from point sources and the ISM in the disk.
In this chapter we will discuss the different X-ray emission components of nearby galaxies leaving aside the emission from active galactic nuclei (AGN), which are present in some of the galaxies and which will be dealt with in Chap. 22.
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