VIRGO CLUSTER

Bruno Binggeli

The Virgo Cluster is the closest and best-studied great cluster of galaxies, lying at a distance of approximately 20 Mpc in the constellation of Virgo. Cosmographically, the Virgo Cluster is the nucleus of the Local Supercluster of galaxies, in whose outskirts we (in the Milky Way, in the Local Group) are situated. As early as 1784, Charles Messier noted an unusual concentration of ``nebulae'' in Virgo; 15 out of the 109 ``Messier'' objects are, in fact, Virgo Cluster galaxies, the most famous of which is Messier 87, the giant elliptical galaxy with the mysterious jet. After Edwin P. Hubble's 1923 discovery of Cepheids in M31, the true nature of the group of nebulae in Virgo as a self-gravitating system of hundreds of galaxies was soon realized, and the first systematic investigations of the Virgo Cluster (as it was subsequently called) were carried out by Harlow Shapley and Adelaide Ames. Ever since, the Virgo Cluster has been, and still is, of primary importance for extragalactic astronomy: Large numbers of equidistant galaxies of all types and luminosities can be observed here in great detail, rendering the cluster: (1) an ideal laboratory for the study of the systematic properties of galaxies and (2) a fundamental stepping stone for the cosmological distance scale.

The Virgo Cluster is a fairly poor, loosely concentrated, irregularly shaped (see Fig. 1) cluster of galaxies with a high abundance of spiral galaxies among the bright cluster members (see Table 1). It is representative of the most common class of galaxy clusters, which is characterized by these properties. Rich, dense, regularly shaped clusters of predominantly E and SO galaxies are much rarer. Nevertheless, owing to its proximity, the ``mediocre'' Virgo Cluster could be mapped to an unsurpassed level of depth and morphological detail, rendering it presently the richest cluster of galaxies in terms of the number of known member galaxies. As Table 1 shows, dwarf galaxies, the dwarf elliptical (dE) types in particular, numerically dominate the cluster population. These stellar systems of low surface brightness are hard to detect, even in nearby Virgo. There must be thousands more extremely faint and diffuse cluster members that are still awaiting discovery.

Figure 1. 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 magnitude scale (blue total apparent magnitudes) is given at the top of the figure. This map should be a fair representation of how the cluster appears in the sky. The two brightest galaxies, at right ascension approximately equal to 12h28m and declination approximately equal to 12°40' and right ascension approximateli equal to 12h27m and declination approximately equal to 8°17', are M87 and M49, respectively. [Reproduced by permission from Binggeli, Tammann, and Sandage (1987), Astron. J. 94 251.]

The distribution of the presently known Virgo Cluster members is shown in Fig.1. The cluster covers a large, roughly circular sky area of approximately 10° diameter. Several subconcentrations can be distinguished. There is a major subcluster (A) of galaxies around the giant E galaxy M87, centered on right ascension 12^h 25^m and declination 13°; there is a smaller, less dense subcluster (B) around the brightest cluster member M49, centered on right ascension 12^h 27^m and declination 8°30'. A third, barely significant subclump (C) has been identified around M59, at right ascension approximately equal to 12^h 40^m and declination approximately equal to 12°. Although M87 is most often taken as the center of the Virgo Cluster, it is off the center (density peak) of A by approximately 1° in the direction toward C. With respect to morphological type, the elliptical and SO member galaxies are the most strongly clustered species; they constitute the ``skeleton'' of the cluster. The E types, in particular, are distributed preferentially (almost chain-like) along the axis A-C. Remarkably, even the jet of M87 is aligned with this fundamental cluster axis. Spiral and (dwarf) irregular galaxies, on the other hand, are scattered over the whole face of the cluster, almost without noticeable concentration.

DYNAMICS

As its irregular structure suggests, the Virgo Cluster is not in a state of dynamical equilibrium - not even in the central region, which is more surprising. There is evidence that the cluster is still in the making.

From the presently known radial velocities (redshifts) of about 350, mostly bright Virgo members, one derives a mean heliocentric, systemic velocity of the cluster of <v> 1100 km s^-1. Although this mean is invariant, the velocity distribution differs substantially for different galaxy types. Late-type (spiral and irregular) galaxies have a broad velocity distribution with a dispersion (standard deviation from <v>) of _v 900 km s^-1, whereas early-type (E, SO, dE, dSO) galaxies show a narrow distribution with _v 550 km s^-1. The late types are thus more dispersed, not only in space, but also in velocity. This has been taken as evidence that spiral and irregular galaxies have only recently (in the last few 10⁹ yr) fallen, or are still in the process of falling, into the cluster from the environment: These galaxies are not yet settled down in the cluster (``dynamically relaxed'') but are streaming inward and outward in the manner of a damped oscillation. Such an infall scenario is plausible, as the Local Supercluster is indeed made up of large ``clouds'' of spiral and irregular galaxies: One such cloud seems to be falling into the Virgo cluster at this very epoch. Likewise, the southern subcluster B may be falling into the main subcluster A.

The well-concentrated early-type galaxies of subcluster A must then be viewed as the oldest cluster members that formed in the densest part(s) of the cluster or fell into it very early on. However, these galaxies do not constitute a dynamically relaxed cluster core, as one would expect. Rather, the central part of the Virgo Cluster seems to consist of a small number of subclumps of galaxies, one of which is defined by M87 alone. In spite of its enormous mass of approximately 5 x 10¹³ M, which is indicated by its large, x-ray emitting halo of hot gas, this giant galaxy is off the cluster center in space and velocity (v 200 km s^-1). However, as a result of ``dynamical friction,'' the subclumps will rapidly merge. We may, in fact, be living in a very special time, shortly ( 10⁹ yr) before the final formation of a relaxed cluster core in Virgo.

This is exciting but it also complicates the dynamical modeling of the Virgo Cluster. The virial theorem can no longer be applied to derive a cluster mass. Nevertheless, requiring simply that the cluster be gravitationally bound (total energy equal to 0), one gets M_total 5 x 10¹⁴ M, and a mass-to-light ratio of M/L 450 in solar units - which clearly indicates the dominance of dark matter.

A PROBE FOR COSMOLOGY

Bright cluster members have traditionally been used to derive the Hubble constant (expansion rate of the universe), H₀. In fact, almost all determinations of H₀ are based on the Virgo Cluster, because it is the center of a large velocity perturbation pattern that embraces the whole supercluster, including us. The velocity of the Virgo Cluster, if referred to the centroid of the Local Group (removing the motion of the Sun in the Milky Way, removing the motion of the Galaxy in the Local Group), is <v_LG> 1000 km s^-1; if referred to the Sun, it is (v> 1100 km s^-1. The Local Group is falling toward (but will not fall into!) the Virgo Cluster with 200-300 km s^-1 (the value is debated), so the true, cosmic expansion velocity of the cluster is (v_cos> 1200-1300 km s^-1. As the distance estimates for the Virgo Cluster range from 15-22 Mpc, one arrives at a value of the Hubble constant (H₀) between 50-100 km s^-1 Mpc^-1. Thus the present uncertainty in H₀ is essentially the difficulty in pinning down the distance to the Virgo Cluster.

Once this important problem is solved, attention is likely to shift back to the cluster as such, and the freed energy may be used to exploit this great galaxy mine for the sake of a better understanding of the formation and evolution of structure in the universe, rendering the Virgo Cluster a true probe for cosmology.

1. Binggeli, B., Tammann, G.A., and Sandage, A. (1987). Studies of the Virgo Cluster. VI. Morphology and kinematics of the Virgo Cluster. Astron. J. 94 251.
2. Jacoby, G.H., Ciardullo, R., Ford, H.C. (1990). Planetary nebulae as standard candles. V. The distance to the Virgo Cluster. Ap. J. 356 332.
3. Richter, O.-G., and Binggeli, B., eds. (1985). The Virgo Cluster, ESO Conference and Workshop Proceedings 20. European Southern Observatory, Garching.
4. Sarazin, C.L. (1988). X-Ray Emission from Clusters of Galaxies. Cambridge University Press, Cambridge.
5. Tully, R.B. and Fisher, J.R. (1987). Nearby Galaxies Atlas. Cambridge University Press, Cambridge.
6. See also Clusters of Galaxies, all entries; Galaxies, Local Group; Galaxies, Local Supercluster.