Astron. Astrophys. 364, 53-69 (2000)

1. Introduction

One of the observational evidences from X-ray data of early type galaxies is the large scatter in the correlation between the X-ray and optical luminosities (Fabbiano et al. 1992): at a given optical luminosity, the X-ray to optical flux ratios L may range from those observed in the bulge of M31 and in spiral galaxies, to values 50-100 times higher. The presence of at least two components, hot gas dominating at high X-ray luminosities, and the evolved stellar population at faint X-ray luminosities, can explain in part this observational evidence. The spectral characteristics of the emission also reflect the relative importance of these components: already from the limited Einstein spectral data, the average emission temperature is larger in galaxies with lower L (Kim et al. 1992), in agreement with the idea of an increasing contribution of hard, individual X-ray sources relative to the hot gas component as the L ratios decrease. An additional very soft component was also found in the lowest L class, possibly due to a 0.2-0.4 keV interstellar medium or to the collective emission of stellar sources (Pellegrini & Fabbiano 1994).

The ROSAT PSPC and ASCA data confirm the Einstein results: a soft (kT0.5-1 keV) optically thin emission due to hot gas dominates in X-ray bright objects, and an harder component, with kT keV, most likely associated with the evolved stellar population (Matsushita et al. 1994; Matsumoto et al. 1997; Buote 1999), or to nuclear activity (Allen et al. 2000; Matsumoto et al. 1997) and present in all galaxies, dominates in X-ray faint objects. In the lowest L class, a very soft component is also measured, with a temperature now well constrained to 0.2-0.3 keV (Fabbiano et al. 1994; Pellegrini 1994; Fabbiano & Schweizer 1995; Kim et al. 1996). However, the origin of the very soft component has not been properly understood yet: it could be placed in stellar sources, in X-ray binaries (Irwin & Sarazin 1998), or could be a cooler phase of the ISM: as detailed hydrodynamical simulations show (Pellegrini & Fabbiano 1994), hot gas with the required emission temperature and luminosity can be retained by the galaxies. However, the hot gas temperature is comparable to that of the very soft component only in galaxies with quite shallow potential wells, i.e., with central stellar velocity dispersion km s^-1.

More galaxies with intermediate to low L ratios still need to be investigated. We report here the results on the BeppoSAX and ASCA observations of 5 such galaxies. Their general optical properties are summarized in Table 1 together with the group in which they fall according to their soft band X-ray fluxes. The wide energy band and the good spectral resolution of both satellites are suitable to detect and measure separately the amount and spectral parameters of the different emission components. Moreover, the spatial resolution of BeppoSAX at high energies allows us to study the spatial characteristics of the hard component, to determine its extent. We discuss the observations and data analysis in Sects. 2-6, we briefly compare the spectral results with those reported in the literature from previous missions and/or different authors in Sect. 7, and we discuss the results in Sect. 9. In Sect. 8 we further discuss a detection at very high energy of a source in the field of NGC 1553, but most likely unrelated to this galaxy.

Table 1. General galaxy properties.
Notes:
a) from de Vaucouleurs et al. (1991). is the total B magnitude, corrected for Galactic and internal extinction; the size gives the apparent major and minor axes diameters at the surface brightness level of 25 mag/square arcseconds.
b) distance from Fabbiano et al. (1992), who adopt a Hubble constant of 50 km s^-1 Mpc^-1.
c) total B-band luminosity , derived using the indicated distance and .
d) central stellar velocity dispersion from McElroy (1995).
e) Group into which the galaxy has been classified on the basis of its L(0.2-4 keV)/ ratio (for Einstein data, Kim et al. 1992), or its L(0.5-2.0 keV)/ ratio (for ROSAT data, Irwin & Sarazin 1998). The X-ray faintest galaxies belong to group 1 while the X-ray brightest to group 4. The group boundaries differ in the two papers; in both works they have been chosen so that each group contains roughly the same number of galaxies of the same general spectral characteristics.

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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