Astron. Astrophys. 351, 487-494 (1999)

4. Relation of L_X with other basic galaxy properties

Not surprisingly, the behavior of the X-ray emission with respect to the shape of the central brightness profile is reminiscent of the trend of with respect to the other basic properties characterizing early type galaxies, already mentioned in Sect. 1: one is the deviation of the isophotal shape from a pure elliptical shape, and is described by the parameter introduced by Bender et al. (1989); the other is the degree of anisotropy in the velocity dispersion tensor, as measured by the anisotropy parameter (v is the maximum rotational velocity and is the stellar velocity dispersion in the central regions; e.g., Davies et al. 1983). Disky () objects are generally low X-ray emitters and flattened by rotation; boxy () and irregular objects show the whole range of and various degrees of velocity anisotropy (Bender et al. 1989; see also Fig. 3). The relationship between X-ray emission and has been reanalyzed by Eskridge et al. (1995) for the sample of Einstein galaxies: they confirm that the most X-ray luminous galaxies are boxy, and find that the trend of with is a wedge, with no highly disky objects at high X-ray emission. Pellegrini et al. (1997) find similar results for the relation between and (a measure of the importance of rotation), or : at high or there are no galaxies with high (see also Fig. 4).

Fig. 3. The - diagram for the early type galaxies in Fig. 1, with isophotal shape measurement available in the literature. Disky galaxies (those with ) are shown with full triangles and boxy galaxies with boxes. The data are taken from Table 1, and the dashed line is as in Fig. 1.

Fig. 4. The relation of with the ratio between the maximum rotational velocity v and the central velocity dispersion (Sect. 4). Power law galaxies are shown with open circles and core galaxies with full circles. The values of come from McElroy (1995); v has been measured for 54 galaxies in Table 1, by Davies et al. (1983), Franx et al. (1989b), Fried & Illingworth (1994), Bender et al. (1994), Fisher et al. (1995).

What is surprising is that the - relation is sharper than that between and isophotal distortion, or (these are more wedges than L-shapes, as described above). As an example, I consider here in detail the - and the - relations (Fig. 2; has been measured for 47 of the galaxies in Table 1). While the - relation shows clearly a dichotomy in the X-ray properties, the - relation looks like a wedge, because not all disky galaxies are low X-ray emitters. There are in fact both power law and core galaxies (i.e., X-ray faint and X-ray bright objects) for . This fact is explained by an inspection of Fig. 3, that is another version of Fig. 1 where disky and boxy galaxies are plotted with different symbols: disky and boxy objects are more mixed in the - relation than core and power law galaxies. So, Figs. 2 and 3 show that all power law galaxies have log , while the same is not true for galaxies with disky isophotes. One concludes that a global property such as is tightly connected with a nuclear property , such as the inner profile shape, and more so than with other characterizing galaxy properties as the isophotal shape. This occurs in spite of the fact that there are more objects in the - plot than in the - one; an effect of "blurring", because a larger sample is considered, is expected to affect the - plot more than the - one.

It must be noted that the absence of a sharp transition in the - relation might be produced, at least in part, by the uncertainties associated with the definition of . In fact, the values given in Table 1 refer to an average between 10 and 60 arcsec, i.e., between typically 0.2 and 1 optical effective radius, where the shape of the isophotes is known to vary within the same galaxy, sometimes also significantly (e.g., van den Bosch et al. 1994).

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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