Both ``normal'' galaxies and low-activities provide very rich data sets for studying phenomena that is difficult to study in the Milky Way (i.e., due to extinction) and/or is not present in the Milky Way (i.e., starburst regions with very high star formation rates resulting in superwinds). The X-ray emission occurs in both point sources, which tend to be the most luminous known X-ray binaries and supernovae, and in complex diffuse emission that is the result of heating of the ISM by hot star, supernovae and, in some cases, superwind outflows. The brightest X-ray binaries (IXOs) are black hole candidates that potentially have masses on the order of 101-4 M , intermediate to that of Galactic BHC and AGN. Both starburst and accretion emission tend to be observed whenever either type of activity is present, strongly supporting the notion of a starburst/AGN connection. There appears to be a natural upper-limit to the luminosity of starburst processes on the order of 1041 ergs s-1, and so starburst emission is usually overwhelmed by AGN emission that is not absorbed and exceeds LX ~ 1042 ergs s-1. Observed abundances tend to be absurdly low, but that is almost certainly due to ``contamination'' of the continuum from multiple temperature gas emission and unresolved point sources. Chandra and XMM will be able to resolve the starburst emission spatial and extract CCD resolution spectra, which will resolve this issue and allow abundance and temperature enhancements to be observed in individual regions within the galaxies.
The X-ray properties of nearby normal and low-activity galaxies suggest that they will contribute less ~ 10% of the X-ray background (c.f., Griffiths & Padovani 1990; Yaqoob et al. 1995). This fraction can increase if these galaxies become harder and more luminous at earlier epochs, i.e., due to the enhanced starburst activity associated with the peak in the star formation history of the universe at redshifts of ~ 1-2 (Hughes et al. 1998). It may be possible to directly observe evolution in low-activity galaxies with ultra-deep XMM surveys or with telescopes such as XEUS that promise effective areas on the order of m2 (see Figure 7).
Figure 7. The expected logN-logS distributions in the 2-10 keV bandpass obtained by converting the optical luminosity functions of Driver (1994) and the IR luminosity function of Saunders et al. (1990) to the X-ray bandpass using the optical and IR to X-ray correlations of David, Jones & Forman (1992) and Green, Anderson, & Ward (1992), respectively. Note that for the mean power-law spectral slope of low-activity galaxies described in Ptak et al. (1999) of ~ 1.7-1.8, F2-10 keV = F0.5-4.0 keV.