3. LOW-ACTIVITY GALAXIES

3.1. Spectral Properties

Despite their heterogeneous optical classification, low-activity galaxies usually have a similar spectral shape (Serlemitsos, Ptak & Yaqoob 1996). Specifically, in general these galaxies exhibit at least two spectral components: a soft, thermal component with kT ~ 0.7 keV and a hard component well-modeled by either a thermal bremsstrahlung with kT ~ 5-10 keV or a power-law with ~ 1.8 (Ptak et al. 1999). The hard (soft) component is typically absorbed by a column density of ~ 10²² cm^-2 (~ 10^20-21 cm^-2). The fact that the hard component tends to be more spatially compact (see below) and absorbed than the soft component implies that the hard component is emanating from further within the galaxies (i.e., the nuclei). The fact that starburst galaxies exhibit a hard component, likely due accreting sources, and LINER and LLAGN galaxies exhibit soft emission, likely due to starburst activity, strongly supports the idea of a starburst/AGN connection.

The luminosity of the hard component tends to be on the order of 10^40-41 ergs s^-1 and the luminosity of the soft component 10^39-40 ergs s^-1, with the relative intensity varying from galaxy to galaxy (see Figure 3). It is evidently rare for starburst activity, the origin of the soft component, to achieve luminosities in excess of 10^40-41 ergs s^-1 (Halpern, Helfand, & Moran 1995; however see Moran, Lehnert & Helfand 1999 for a counter-example in NGC 3256). Accordingly, sources with X-ray luminosities > 10⁴¹ ergs s^-1 (e.g., NGC 3998 and NGC 3147) only require a power-law component since the starburst component is overwhelmed. Conversely, in Seyfert 2s where the AGN is highly absorbed, starburst emission is often observed below 2 keV (see Turner et al. 1998).

Figure 3. ASCA SIS spectra of M51 (starburst + LLAGN), M82 (starburst), NGC 253 (starburst) and NGC 4258 (LLAGN).

The abundances inferred from the soft component tend to be sub-solar (on the order of 10^-1 solar). This is surprising since starburst emission is presumably the result of massive star evolution and accordingly should be highly enriched, however many effects might be contributing to this. For example, it is probably too simple to be fitting the %probably multi-temperature starburst emission with a single component (c.f., Breitschwerdt & Komossa 1999; Dahlem, Weaver & Heckman 1998), or other sources of continuum may be present such as soft emission from X-ray binaries (c.f., Figure 2). In brighter sources (see the residuals in Figure 3 and Ptak et al. 1997), it appears that there is a deficiency of Fe relative to -process elements (e.g., Ne, Mg, Si, S, etc. produced in massive stars), although the effect is diminished somewhat when more complex models are invoked (see Dahlem, Weaver, & Heckman 1999).