2. The sample
F97 present the fits of the surface brightness profiles for a large sample of early type galaxies observed with WFPC1; they modeled the data with a double power law with a break radius, derived the inner surface brightness slope (the parameter) and gave the classification into core or power law galaxy accordingly. This classification is given by the same authors for nine more early type galaxies in Magorrian et al. (1998). Crane et al. (1993), Ferrarese et al. (1994), Forbes et al. (1995) and Quillen et al. (1999) also present HST central profiles for a number of early type galaxies, and fit them with double power laws which govern the inner and the outer slopes. For all these galaxies, following F97, I have adopted the criterion of classifying them as power law galaxies when the central brightness profile does not show an inner break, and as core galaxies when it does. In the first case the inner slope always turned out to be larger than 0.3, and in the second case smaller than 0.3.
I have cross-correlated the sample of Es and S0s with inner profile measured by HST with existing catalogs of X-ray emission, namely the Einstein based catalog (Fabbiano et al. 1992), the ROSAT all-sky survey based catalog (Beuing et al. 1999), and the list of 61 early type galaxies observed with the ROSAT PSPC by Irwin & Sarazin (1998). So, I have collected a sample of 59 galaxies with information on both the X-ray emission (detection or upper limit) and the central surface brightness properties 2. 39 of these objects come from F97, 9 from Magorrian et al. (1998), 5 from Forbes et al. (1995), 3 from Quillen et al. (1999), 2 from Ferrarese et al. (1994), and 1 from Crane et al. (1993). The order of priority for taking the X-ray information has been as follows: first the Beuing et al. catalog, then the Irwin & Sarazin list, and finally the Einstein based catalog. In the final sample, the X-ray information comes from the ROSAT all-sky survey catalog for 43 objects, from the Irwin & Sarazin list for 5 objects, from the Einstein based catalog for 9 objects, and for 2 Coma galaxies (NGC4874 and NGC4889) from the ROSAT study of the Coma cluster by Dow & White (1995).
2.1. The X-ray luminosities
Two corrections need to be applied to the X-ray data of the literature. The first is a correction for the different passbands: Einstein based luminosities refer to the 0.2-4 keV energy band, while the ROSAT based data refer to 0.5-2 keV for the Irwin & Sarazin list, to 0.64-2.36 keV for the all-sky survey list, and to 0.4-2.4 keV for Dow & White (1995). I have transformed all X-ray luminosities to the (0.2-2) keV band values. This correction has been applied by taking into consideration the different spectral parameters used to derive the X-ray luminosities: thermal emission had been used with temperatures from 0.7 to 1 keV, Galactic neutral hydrogen absorption, and metallicities from 0.5 solar to solar. The amount of correction on log ranges from -0.030 for the Einstein luminosities, to +0.072 and +0.071 for the luminosities in the Irwin & Sarazin and all-sky survey lists, and to +0.025 for the two Coma galaxies.
The second correction is the rescaling of the luminosities to the same distance scale. To maximize the number of galaxies with the same source for the distance, I have adopted the distances given by Beuing et al. (1999). For eight galaxies not present in their catalog I have adopted the distances given by F97, after rescaling (they refer to km s-1 Mpc- 1, while the distances of Beuing et al. refer to km s- 1 Mpc-1). Table 1 summarizes the observational information used in this paper.
Table 1. Galaxy properties
Table 1. (continued)
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999