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.

MORPHOLOGY

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.

  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   *12h  25m and
declination *13ø; there is a smaller, less dense subcluster (B) around
the brightest cluster member M49, centered on right ascension *12h 27m
and declination *8ø30'. A  third, barely significant subclump  (C) has
been identified around M59, at  right ascension approximately equal to
12h 40m  and declination approximately equal to  12ø.  Although M87 is
most  often taken as the  center of the Virgo  Cluster,  it is off the
center (density peak) of A by approximately 1ø 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**.  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 ** *900  km s**,  whereas early-type  (E,  SO, dE, dSO)
galaxies  show a narrow distribution with  *550 km s**. 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  10* 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
5x10** 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
(**=200 km  s**).  However, as  a result of "dynamical  friction," the
subclumps will rapidly  merge. We may, in  fact,  be living in  a very
special time, shortly (10* 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***
5x10** 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), H*. In fact, almost
all determinations of H* 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 <***>*1000   km s**; if referred  to  the Sun, it  is
(v*>*1100 km s**. The Local Group is falling toward (but will not fall
into!) the Virgo  Cluster with 200-300  km s** (the value is debated),
so the  true,     cosmic  expansion velocity    of  the   cluster   is
(v**>*1200-1300  km s**. As    the distance estimates  for   the Virgo
Cluster  range from 15-22 Mpc,  one arrives at   a value of the Hubble
constant  (H*) between  50-100   km   s**  Mpc**.  Thus the    present
uncertainty in  H* 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.

      Additional Reading

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.

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.

Richter, O.-G., and Binggeli, B., eds.(1985). The Virgo Cluster,
  ESO Conference and Workshop Proceedings 20.
  European Southern Observatory, Garching.

Sarazin, C.L.(1988). X-Ray Emission from Clusters of Galaxies.
   Cambridge University Press, Cambridge.

Tully, R.B. and Fisher, J.R.(1987). Nearby Galaxies Atlas.
  Cambridge University Press, Cambridge.

See also Clusters of Galaxies, all entries; Galaxies, Local Group;
Galaxies, Local Supercluster.