In elliptical/early-type galaxies the X-ray emission is dominated by hot gas (in some cases similar to group or cluster of galaxies X-ray emission, with a King-like radial surface brightness profile, occasionally including a cooling flow), with a temperature of 0.8-1.0 keV (Trinchieri, Fabbiano, & Kim 1997). Some early-type galaxies also contain a contribution from a LLAGN (see Di Matteo, et al.; these proceedings) and low-mass X-ray binaries (LMXB, Irwin & Bregman, 1999).
The X-ray emission of spiral galaxies is generally dominated by X-ray binaries (Fabbiano 1989). Spiral arms often contain HII regions, particularly regions of local density enhancements (i.e., knots). These HII regions are often X-ray bright as a result of the products of enhanced star formation: hot stars, supernovae (SN) and supernova remnants (SNR), high-mass X-ray binaries and black hole candidates (BHC) (c.f. NGC 1313 in Colbert et al. 1995). Not surprisingly, ``bluer'' galaxies, which tend to be later-type galaxies with higher star-formation rate (with the blue colors being a result of a higher proportion of massive stars than is observed in galaxies dominated by older stellar populations) are X-ray bright (Fabbiano, Feigelson, & Zamorani 1982). There is also a strong correlation between X-ray luminosity and IR luminosity, with the IR emission being produced by dust that has been heated by massive stars (David, Jones & Foreman 1992; Green, Anderson & Ward 1992). In cases of very high star formation rates occurring near the edge of galaxies, blow-outs can occur when the local pressure resulting from outflows from hot stars and SN exceeds the ambient interstellar medium (ISM) pressure, as observed by ROSAT in galaxies such as NGC 55 (Schlegel et al. 1997). There is also X-ray evidence that normal galaxies (Cui et al. 1996), including the Milky Way, possess hot (T 106 K) ``halos'' or ``coronae'' (Spitzer 1956).
Spiral galaxies also typically contain a bulge component, which may be counterparts to elliptical galaxies. A substantial fraction of the X-ray sources found in the nearest spiral galaxy, M31, are concentrated in the bulge. These sources are presumably mostly LMXB associated with the older stellar populations in globular clusters. The X-ray spectrum of the bulge region of M31 as observed by the ROSAT PSPC and ASCA (Irwin & Bregman 1999) and BeppoSAX (Trinchieri et al. 1999). This spectrum has a power-law component consistent with X-ray binaries in our galaxies, plus a soft component (see Figure 2) that is similar to that considered to be due to X-ray binaries in early-type galaxies. Note that it would be difficult to observe this soft component in Galactic binaries since most lie in the disk of the Milky Way and hence are highly absorbed.
|
Figure 2. ASCA + PSPC spectra from the bulge of M31. Figure taken from Irwin & Bregman (1999), with a shaded area added to delineate the region typically absorbed in Galactic X-ray sources. |
Colbert and Mushotzky
(1999)
discuss a survey of X-ray sources in normal
galaxies with luminosities in excess of 1.3 x 1038 ergs
s-1, the Eddington luminosity for accretion onto a solar-mass
object.
These intermediate-luminosity X-ray objects (IXO) have X-ray spectra consistent
with accretion onto black holes with masses of ~ 101-4
M
.
IXOs are typically significantly displaced
from the galactic centers, and hence they are not AGN. This implies that
IXOs may be cases of ``intermediate'' mass black holes, and may be precursors
to some modern-day AGN (see also M82 below and in
Ptak & Griffiths 1999).