Fig. 2 is a map of the ca. 1300 galaxies in the area of the Las Campanas survey of the Virgo cluster (Binggeli et al. 1985) judged to be cluster members. The membership criteria were based on (1), the morphological appearance of the galaxies; e.g. dwarf ellipticals, which constitute the dominant population of the cluster, have a characteristically low surface brightness; and/or (2), the measured radial velocities. The velocity criterion works of course only if the cluster is sufficiently isolated in space. Fortunately, this seems to be the case, i.e. there is a small void behind the cluster (although not quite so in the case of spirals and irregulars which form a sort of filament that runs through the cluster, cf. Sect. 4 below). Velocities are available only for the brightest 400 members. However, morphology is an equally efficient tool to pick up the members; later velocity measurements have nearly always confirmed our morphological judgement (e.g. Drinkwater et al. 1996). Detailed galaxy morphology is of course limited to the most nearby clusters, such as Virgo.
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Figure 2. Map of the Virgo cluster. All cluster members are plotted with luminosity-weighted symbols. The symbol size (area) is proportional to the luminosity of the galaxy. The apparent magnitude scale is given on top of the figure (absolute magnitudes follow from m - M = 31.5 if a distance of 20 Mpc is adopted). The most prominent Messier galaxies are indicated. Figure from Binggeli et al. (1987). |
The magnitude limit of completeness of the Las Campanas survey is around BTlim = 18, or, if we assume a distance of 20 Mpc, MBTlim = -13.5. Undoubtedly, there are hundreds, if not thousands of more, extremely faint members of the Virgo cluster - analogous to the dwarf spheroidal companions of our Galaxy (Phillipps et al. 1998). However, these will unlikely alter the structural appearance of the cluster.
Let us now have a look at the structure of the Virgo cluster based on Fig. 2. The primary characteristic is certainly the overall irregularity of the cluster. Although we would not hesitate to call M87 the ``king'' of the Virgo cluster (despite the fact that it is not even first-ranked in apparent magnitude; M49 is slightly brighter), it is not the center of the cluster. But it is the center of the most massive subcluster, as we shall see below. If one naively draws density contours (isopleths) with a suitable smoothing (number or luminosity-weighted ), as in Binggeli et al. (1987), M87 is off the peak density (the cluster ``center'') by almost one degree. That peak density is closer to the M84 / M86 lump, to the NW of M87. However, our smoothing was rather like putting a mattress on the bumpy back of a camel. There is clear evidence for a secondary subcluster around M86 (see below), so we deal with a double structure, a double conglomerate of galaxies in the central part of the Virgo cluster. The two subclusters, called here the ``M87 subclump'' and the ``M86 subclump'', seem to be in a state of merging. Although this becomes clear only when we discuss the kinematics and the X-ray properties of the cluster below, the central double structure is quite obvious already from a simple plot of the galaxy postions in the sky (Fig. 2).
There is another double structure of the Virgo cluster on a larger scale, along N-S, defined by the northern M86 / M87 subclump structure (called ``cluster A'' in Binggeli et al. 1987) on the one hand, and the southern galaxy concentration around M49, called here the ``M49 subclump'' (= ``cluster B'') on the other hand. Possibly, there is a small subclump around M60 (= ``cluster C''). A number of very small bound subsystems, essentially groups of galaxies consisting of one bright galaxy plus a swarm of dwarf galaxies (as, e.g., M100 + satellites in the far North), are likely to exist, but these are difficult to identify even with kinematic data (for the general question of bound companions in the Virgo cluster, see Ferguson 1992, and Binggeli 1993).
So there are two main axes of the Virgo cluster: one N-S, i.e. M100-M86 / M87-M49, and one E-NW, i.e. M60-M87-M84 / M86. Remarkably, the former axis is nearly perfectly aligned with the position angle of the outer isophotes of M87 (e.g., Weil et al. 1997), while the latter is perfectly aligned with the jet axis of M87. The two axes appear also very prominently in the X-ray image of the Virgo cluster (Fig. 5).