3.4. Spatial Properties

Spatially most low-activity galaxies have 2-10 keV emission which is concentrated in the nucleus (see Ptak 1997, although the statistics are limited in many cases) and are extended over kpc scales in addition to being concentrated in multiple point sources. This tends to be particularly true of starburst galaxies (c.f., Read, Ponman, & Strickland 1997; Dahlem, Weaver, & Heckman 1998) but these phenomena are also seen in LINERs and LLAGN (c.f., the LINER NGC 4594 in Fabbiano & Juda 1997; the LINER NGC 3079 in Pietsch, Trinchieri, & Vogler 1998; the LLAGN M51 in Marston et al. 1995, and the LLAGN NGC 4258 in Cecil, Wilson, & De Pree 1998). In one of the nearest starburst galaxies NGC 253 (at ~ 2.5 Mpc), ~ 73 point sources (some of which are background QSOs) have been detectedby ROSAT (Vogler & Pietsch 1999; see Figure 5), in addition to the extended emission associated with the nuclear region, disk, and halo of NGC 253. Interestingly, the point source distribution in NGC 253 is consistent with that of M31 (with the older-population buldge sources removed) and M33 (Figure 6). A similar result is observed more generally by Roberts et al. (these proceedings), which provides motivation that a universal luminosity distribution of X-ray binaries can be found, and the high-luminosity end of which would be the IXOs.

Figure 5. ROSAT PSPC image of NGC 253, from Vogler & Pietsch (1999).

Figure 6. The luminosity distribution of point sources in NGC 253, M33 and M31 (excluding buldge sources), from Vogler & Pietsch (1999).