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4. Early type galaxies and groups

The large quantities of gas observed in this class of sources have now been studied in some detail and several general characteristics of the large scale emission are now relatively well established. However, little is known of the emission at intermediate to small scales. New observations have now shown that the gas distribution in the inner regions of early type galaxies can be quite spectacular. At small radii the emission is often disturbed, and arcs-tails-peculiar/unexpected shapes are coupled with inhomogeneous spectral characteristics of the gas. I will concentrate on a few examples from the literature that will illustrate how gas distribution in early type galaxies can be just as interesting as in later types, and rather more unexpected!

4.1. NGC 5846

This galaxy is at the center of a small group, and optically is a rather unperturbed object. The X-ray emission is very extended, roughly azimuthally symmetric and regular, with a large total X-ray luminosity of LX(0.2 - 2 keV) 7 × 1041 erg s-1 within r = 10' [Finoguenov et al. 1999]. The emission on the sub-arcminute scale instead is much more spectacular and complex: as shown by Fig. 4, an arc structure ("hook"), a tail pointing S and several knots are evident in the soft energy band, and their relative strengths are clearly energy-dependent.

Figure 4

Figure 4. Adaptively smoothed X-ray data of the central region of NGC 5846 at different energies, as indicated in each panel. The red continuum image of the same region (data from the ESO 3.6m + EFOSC) is shown in the bottom right cannel. The comparison between the different images is a clear example of the complexity of the X-ray morphology, and shows how little it reproduces the stellar light distribution.

There is also a curious correspondence with the optical line emission and with the dust distribution, both on scales of 1' - 2', as already discovered with the ROSAT HRI [Trinchieri et al. 1997], and on much smaller scales of 2" - 4" (300-600 kpc) discovered with Chandra [Trinchieri &Goudfrooij, 2002]. The correspondence is extremely good, but the physical link between the two emissions is still rather puzzling:

Why are their morphologies so similar then (Fig 5)? A possible link between Halpha [+dust] and X rays is through thermal conduction [Cowie & McKee, 1977, Sparks, Macchetto & Golombek, 1989, de Jong et al. 1990]. Cold gas+dust are acquired from the outside at the same time; dust/cold gas act simultaneously on the hot coronal gas, inducing local cooling + excitation into emission; this produces an expected LHalpha ~ 1.5 × 1039 T3/27 n0.01 erg s-1 [Cowie & McKee, 1977]. For the temperature and densities derived from the X-ray observation e.g. in the "hook", of T ~ 6 × 106 K and n ~ 0.35 cm-3 respectively, the Halpha luminosity should be ~ 2.4 × 1040 erg s-1, to be compared to a measured LHalpha ~ 1.3 × 1040 erg s-1. Too good to be true!

Figure 5

Figure 5. Halpha + [NII] image of the inner regions of NGC 5846 (LEFT) on the same scale as the 0.2-2.0 keV emission observed with Chandra (MIDDLE). The Halpha + [NII] contours on the X-ray smoothed image (RIGHT) indicate that both morphology and scales are very closely related in this region (from Trinchieri & Goudfrooij [2002]).

Can we then assume that this mechanism is responsible for the complex morphologies at small radii seen in other galaxies? The number of cases with both good X-ray and Halpha data is limited, but there are a few examples - unfortunately so far NGC 5846 remains the best and perhaps only example for which such close link is observed. For other objects there seem to be a need of other explanations for the small scale morphological perturbation, and unfortunately again each object appears so far to be "unique" (but the cases studies are very few!)

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