GALAXIES, LOCAL SUPERCLUSTER

GIULIANO GIURICIN

A HISTORICAL PERSPECTIVE

The  large-scale surveys  of the  last  twenty years have verified the
existence of large  agglomerations  of galaxies. The  most conspicuous
groupings can contain several clusters of galaxies, which explains why
they have been given  the name "superclusters."   Some evidence of the
existence  of  our  own supercluster   (dubbed   the Local,  or Virgo,
Supercluster), where    our Galaxy and  the   Local Group  of galaxies
reside, dates back to the   earliest surveys of extragalactic  nebulae
made by William and John Herschel in the nineteenth century.  However,
the reality of the   Local Supercluster has become generally  accepted
only in recent years.

  The surveys by  the Herschels showed  an excess of bright nebulae in
the  northern galactic hemisphere, a  fact  that reflects the outlying
position of our galaxy with respect to nearby  galaxies. In the 1920s,
J.H.   Reynolds and knut Lundmark noted  a remarkable concentration of
the  largest nebulae along a  great circle of  the sphere, but drew no
conclusion from  this. (At that time several  astronomers were not yet
convinced that many nebulae were external galaxies.)

  The  French astronomer  Gerard de  Vaucouleurs of  the University of
Texas at   Austin was the  first   to describe  and define   the Local
Supercluster. He  proposed that the   concentration  of the  brightest
galaxies along a great circle  of the sphere was the  trace of a  flat
supersystem (the Local Supercluster)   centered in the Virgo  cluster,
which has a diameter  of a few megaparsecs  (Mpc) and is about 10  Mpc
away (1  Mpc  =3.26 million light-years).  [In  the present article we
adopt  a value of 100  km  s** Mpc** for  the  Hubble constant in  the
relationship   (Hubble's law)   between  the redshift    observed in a
galaxy's spectrum and the galaxy's distance; a change  in the value of
the Hubble constant alters  the spatial scale.] The Local Supercluster
was found to  contain also several tens of  galaxy  groups, along with
single galaxies scattered among them.

THE COMPONENTS OF THE LOCAL SUPERCLUSTER

A first  rough idea of the structure  of the Local Supercluster can be
an  obtained by inspecting the  two-dimensional distribution of bright
galaxies (i.e., the location  of galaxies projected  onto the globe of
the sky,   without any  indication  of  the  distance  of each).   The
distribution of -1280 galaxies   (brighter than about the   thirteenth
photographic magnitude), taken  from  the  Harvard surveys of   Harlow
Shapley  and Adelaide Ames  (published in  the 1930s), clearly reveals
the Virgo cluster (at galactic longitude  1-280ø and galactic latitude
b-75ø) and the Ursa Major cloud (which  extends from a region near the
galactic pole   to  1-140ø, b-50ø)  in the   northern  hemisphere (see
Fig. 1). Galaxies  in the region  marked by contour  A probably form a
system  that does  not belong  to  the Local Supercluster because they
have,  in general, higher  redshifts.  The axis  corresponding to  the
dense  chain of galaxies that contains  the  Virgo cluster, called the
supergalactic equator by   de  Vaucouleurs, is shown  by  a  curve  in
Fig. 1. The supergalactic longitude is counted along the supergalactic
equator from its intersection with the galactic equator at 1-137ø. The
southern galactic hemisphere is  less populous than the northern;  the
two most striking features, denoted by contours B (the so-called south
galactic chain) and C, deviate from the  supergalactic equator and may
be considered to be distant,  separate superclusters. If we omit these
structures,  there  is some  concentration  along   the  supergalactic
equator. In the zone below 20ø galactic latitude, the relative lack of
galaxies is due to   the absorption of light   by the disk of  our own
galaxy.

  A much better description of the Local Supercluster is made possible
if we  know the three-dimensional distribution  of nearby galaxies, as
derived from their positions as well as from  their redshifts. In this
case, one often  simply assumes that  the redshifts  measure distances
fairly well (according to Hubble's law); this is  true if there are no
large systematic  deviations from Hubble's  law.  Two basic sources of
information for the positions and radial velocities of bright (nearby)
galaxies in  the  whole sky    are  the catalogs of   bright  galaxies
published   in the 1980s,  the  Revised Shapley-Ames Catalog of Bright
Galaxies by Allan Sandage and Gustav A. Tammann  and the Nearby Galaxy
Catalog  (NBG) by  R.   Brent   Tully.   From   the system  of   polar
supergalactic     coordinates  [supergalactic     longitude     (SGL),
supergalactic  latitude  (SGB), distance]   it   is useful  to  derive
orthogonal Cartesian  supergalactic  coordinates (SGX,  SGY, SGZ): the
Sun is at the origin; the SGX axis, defined by the intersection of the
galactic and supergalactic  planes, is aligned  toward SGL=0ø, SGB=0ø;
the SGY axis is roughly  in the direction of the  Virgo cluster (it is
aligned toward SGL=90ø, SGB=0ø); and the  SGZ axis is perpendicular to
the supergalactic  plane and points in the  direction of  SGB=90ø. The
positive SGY axis is only 6ø removed from the north galactic pole, and
the SGX-SGZ plane  is almost coincident  with the plane of the Galaxy.
Our  galaxy thus would be seen  almost face-on from  the center of the
Local Supercluster.  Figure   2  shows the projection  of  all (-1260)
northern galaxies of   the NBC catalog  onto  the  SGX-SGZ plane.  The
viewer is located in   the plane of  the supercluster's  equator.  The
disk  of  the Local Supercluster  stretches  across the  middle of the
diagram.  The Virgo cluster is the  densest concentration of galaxies,
somewhat to the left with respect to the center of  the figure; to the
left of the Virgo cluster is the "southern extension" (also called the
Virgo II cloud) and to its right is  the Ursa Major cloud (also called
the Canes  Venatici cloud), which  is the largest  cloud in  the Local
Supercluster  and the  one in  which  we reside.  There  are also some
clouds located off the supergalactic plane. A schematic picture of the
Local Supercluster in three dimensions is  shown in Fig. 3: Some major
clouds  (Virgo III, Crater,  Canes Venatici spur, Leo  II) lie off the
supergalactic plane, which is  essentially formed by the Virgo cluster
and the Virgo II and Canes Venatici clouds.

  In general, it is possible  to distinguish  three components of  the
Local Supercluster: the Virgo cluster, the  "disk" (i.e., the galaxies
distributed along  a   thin layer  coinciding  with  the supergalactic
plane),   and  the "halo"   (galaxies    that are distributed  roughly
spherically, considerably off the plane). Of the NBG galaxies, roughly
40%  belong  to the  halo component; of   the remaining 60%, one-third
belong  to the  Virgo cluster and   two-thirds to the disk  component,
which   contains the two  extended clouds  (Canes   Venatici and Virgo
II). Taken together,    these  clouds display  a   structure  that  is
flattened in the  ratio SGX:SGY:SGZ =6:3:1.  The thickness of the disk
of the Local Supercluster (the short axis) is very  thin (-1 Mpc). The
vast majority  of  (disk and halo)  bright galaxies  reside in a small
number of  clouds; the number density of  bright galaxies falls off as
1/d2, where d  is the distance from  the Virgo cluster.  Collectively,
these clouds occupy a very  small fraction of  the volume of the Local
Supercluster (most of the space is empty).   Within the clouds, on the
smallest scale we can identify  groups (which are mostly bound systems
of galaxies) that  together contain about 70% of  the  galaxies of the
NBC catalog, if the Virgo members  are included.  Remarkably, only -1%
of the galaxies appear to be isolated (outside clouds).

  The internal motions of galaxies in most of the various large clouds
of the  Local Supercluster are small (less  than 100 km  s** along the
line  of  sight). Hence, individual member  galaxies  can have crossed
only a relatively small portion of the cloud  in the billions of years
since the galaxies came into  being, which suggests that little mixing
can  have occurred in the  supercluster  as a whole.  This fact  gives
insight into evolutionary  history  that is  simply not obtainable  at
scales smaller than  those   of  superclusters (i.e., in   groups  and
clusters,   where the original   distribution  of matter  tends to  be
smeared out by evolutionary mixing).

  Elliptical and lenticular galaxies have  a tendency to be located in
the high-density regions of the Local Supercluster, whereas early-type
spiral  galaxies tend to be  found in regions of intermediate density,
and late-type spiral and irregular galaxies in regions of low density.

MOTIONS OF GALAXIES IN THE LOCAL SUPERCLUSTER

A better approach  to the mapping of  the distribution of the galaxies
in  space is    to  estimate galaxy   distances from    a   variety of
redshift-independent distance  indicators. The   two most widely  used
distance   indicators   are the stellar    velocity   dispersion in an
elliptical galaxy and the  width of the neutral-hydrogen emission line
(observed at  the 21-cm radio wavelength)  in a spiral galaxy. The use
of  redshift- independence  distance  indicators has the  advantage of
avoiding the assumption  that galaxies strictly  obey Hubble's law and
allowing  us to map  the (non-Hubble) peculiar  motions of galaxies. A
galaxy's peculiar motion  is  found by subtracting from   its observed
radial velocity the fraction of its recession velocity attributable to
cosmic expansion (according to Hubble's law and a redshift-independent
estimate of the galaxy's distance)  within a velocity reference system
(e.g., that defined by the cosmic microwave background).

  In recent  years, spectroscopy  and  photometry of large samples  of
galaxies  showed that, in the  cosmic microwave background rest frame,
the   peculiar motions  of the    galaxies of  the Local  Supercluster
(including our Local Group) and of a few nearby superclusters can best
be fitted by a flow  (mostly at a  velocity of -600  km s**) towards a
great mass concentration  (dubbed  "The Great Attractor")  centered on
the galactic coordinates 1-310ø, b-10ø (in the  Centaurus region) at a
distance of -40 Mpc. There seems to be a large concentration of galaxy
clusters in the vicinity  of the  Great Attractor but,  unfortunately,
much of  this grouping lies behind   the obscuring dusty plane  of the
Milky Way.

     Additional Reading

Burns, J.O.(1986). Very large structures in the universe.
   Scientific American 255 (No. 1) 38.

Dressler, A.(1987). The large scale streaming of galaxies.
   Scientific American 257 (No. 3) 46.

Gregory, S.A. and Thompson, L.A.(1982). Superclusters and
   voids in the distribution of galaxies.
   Scientific American 246 (No. 3) 106.


Oort, J.H.(1983). Superclusters. Ann. Rev. Astron. Ap. 21 373.

Silk, J., Szalay, A.S., and Zeldovich, Y.B.(1983). The large scale
   structure of the universe. Scientific American 249 (No. 4) 72.