The Virgo Cluster and the Variety of Cluster Xray Morphology

The galaxy cluster in the constellation of Virgo is with a distance of about 17 Mpc [58] the nearest of all galaxy clusters and thus a perfect object for very detailed studies. In fact our Galaxy lays in the outskirts of the cosmic web structure surrounding the Virgo cluster, which has a recession velocity of about 1000 km s-1. The galaxy field of the cluster stretches over a region of more than 10 x 10deg over the sky and the galaxy distribution and population has been studied in detail by Binggeli et al. [6]. They distinguish a major norther part, A, a smaller southern part, and several galaxy sheets partly in the background. An X-ray image of the cluster

Fig. 23.12 The Virgo galaxy cluster as observed in the ROSAT All-Sky Survey in the energy band 0.5-2.4 keV [8]. The left pannel shows the surface brightness distribution as color map, while the right panel shows the same image as contour map with individual sources marked

obtained in the ROSAT All-Sky Survey is shown in Fig. 23.12, [8]. It shows a complex surface brightness distribution, which reveals Virgo as a dynamically young cluster. We note the bright nearly symmetric X-ray halo in the northern part of the cluster centered on the giant elliptical galaxy M87, a southern part of the cluster with much lower surface brightness emission with the elliptical M49 at its center, an extension in the North-East housing the massive elliptical M60 and another X-ray halo surrounding the massive elliptical M86 about 1 degree NWW of M87. Thus, apart from the central region around M87, the matter in the Virgo cluster is still collapsing and not in a virial equilibrium.

A more detailed comparison of the galaxy and X-ray emitting ICM distribution shows that the X-ray contours follow closely the distribution of the early type galaxies (ellipticals and S0s), including the early type dwarf galaxies (which are found in large numbers in Virgo), while late type galaxies (spirals and irregulars) tend to avoid the X-ray emitting regions [128]. The good match between the early type galaxies and X-rays is particularly striking in a region of a sharp contrast at the edge South-West of M87. A study of the HI content of spirals in 21 cm by Cayatte et al. [33] shows nicely that the HI deficiency is also correlated with the density of the X-ray emitting ICM. A few galaxies closer to M87 show signs of being stripped of their ICM in the 21 cm images and the modeling of this effect is well consistent with ram pressure stripping by the cluster ICM acting on these infalling galaxies [9,33]. Overall, the Virgo cluster is considered a spiral rich cluster and the above features suggest that most of the spirals are infalling, emphasizing again the picture that the Virgo cluster consists of a smaller relaxed core and probably a much larger amount of matter on the infall, creating a cluster of the stature of the Coma cluster in the not too distant future.

One chuck of matter that is infalling with a dramatic effect at present is the galaxy M86. This galaxy is actually blueshifted as observed from Earth and thus must be falling with a high relative velocity 200 km s^1) from behind into the Virgo cluster. The substantial X-ray halo surrounding it and the velocity distribution of the dwarf galaxies in its vicinity [6] imply that M86 is actually the dominant galaxy of a small galaxy group falling into Virgo. The spectacular interaction effects of this event have recently been studied in detail with XMM-Newton by Finoguenov etal. [61].

Because of the large extent of the cluster in the sky it is difficult to cover Virgo in total with an X-ray study other than the sky survey. With the ASCA satellite this effort was undertaken; however, Shibata et al. [137] constructed a temperature map of the Virgo cluster ICM and found an interesting patchy structure with a correlation length of about 300 kpc, which they interpreted as the fossil traces of ICM heating by infalling galaxy groups from which the cluster was formed.

If we compare the X-ray image of Virgo with the dynamically much more evolved Coma cluster shown in Fig. 23.1, the difference is readily apparant. In general we find a broad range of cluster X-ray morphologies involving quite evolved and relaxed clusters, less frequent major cluster mergers, and also dynamically younger clusters which accrete matter in a more diffuse fashion than the major mergers. The gallery of X-ray images from our recently conducted systematical study of cluster structure with XMM-Newton (with targets statistically selected from the X-ray flux-limited REFLEX survey) gives a good impression of the typical range of cluster morphologies (Fig. 23.13).

Fig. 23.13 XMM-Newton image gallery of galaxy clusters selected statistically from the REFLEX sample for a systematic study of galaxy cluster structure in the frame of an XMM Large programme [19]. The clusters cover almost homogeneously a luminosity range from 0.4 —13. • 1044 erg s—1 and a redshift range from z = 0.055 — 0.183. The sample should give a nearly unbiased impression of the range of cluster morphologies of a flux limited cluster sample

Fig. 23.13 XMM-Newton image gallery of galaxy clusters selected statistically from the REFLEX sample for a systematic study of galaxy cluster structure in the frame of an XMM Large programme [19]. The clusters cover almost homogeneously a luminosity range from 0.4 —13. • 1044 erg s—1 and a redshift range from z = 0.055 — 0.183. The sample should give a nearly unbiased impression of the range of cluster morphologies of a flux limited cluster sample

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