The Virgo cluster
The Virgo cluster is a massive cluster of galaxies which dominates the Virgo supercluster. There are roughly 2000 galaxies in this cluster (although ninety percent of them are dwarf galaxies). This cluster has a diameter of approximately 15 million light years which is not much larger than our Local Group but it contains fifty times the number of galaxies.
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 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 appeq 12h 25m and declination appeq 13°; there is a smaller, less dense subcluster (B) around the brightest cluster member M49, centered on right ascension appeq 12h 27m and declination appeq 8°30′. A third, barely significant subclump (C) has been identified around M59, at right ascension approximately equal to 12h 40m 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.
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 <vsmsun> appeq 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 sigmav appeq 900 km s-1, whereas early-type (E, SO, dE, dSO) galaxies show a narrow distribution with sigmav appeq 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 Msmsun, 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 (Deltav appeq 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 (appeq 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 Mtotal geq 5 x 1014 Msmsun, and a mass-to-light ratio of M/L geq 450 in solar units – which clearly indicates the dominance of dark matter.
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 <vLG> appeq 1000 km s-1; if referred to the Sun, it is (vsmsun> appeq 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 (vcos> appeq 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.