GreatAttractor-1


2. Galaxies in the "Zone of Avoidance" towards the "Great Attractor"

PR Photo 46c/99 covers a field of 0.5° x 0.5° in the Southern
constellation of Norma (The Level) and in the direction of the "Great
Attractor". This region is at an angular distance of about 7° from the
main plane of the Milky Way, i.e. less than 15 times the width of the
image shown. In this colour composite, the foreground stars in the
Milky Way mostly appear as whitish spots (the "crosses" around some of
the brighter stars are caused by relections in the telescope
optics). Many background galaxies are also seen. They form a huge
cluster (ACO 3627) with a number of bright galaxies near the center -
they stand out by their larger size and yellowish colour. In order to
facilitate transport over the Web, this image has been compressed by a
factor of four from its original size (8500 x 8250 pixels). North is
up and East is left.
  


The Milky Way Galaxy is one of the largest obstacles to the detailed
study of large-scale structures in the Universe - an observational
approach with the aim of ultimately revealing the physical conditions
at the time of the Big Bang. The interstellar dust and high density of
stars in our home galaxy severely obscure the view towards more
distant objects.

For instance, an intensive search on earlier photographic plates
covering the field shown in PR Photo 46c/99 revealed only about 70
background galaxies, much less than what can be expected for such a
rich cluster if it were seen in a sky field outside the Milky Way
band. This led astronomers in the last century to introduce the term
"Zone of Avoidance" for the Milky Way band in the sky.

Modern electronic detectors are able to register much fainter objects
than those seen on the photographic plates. In the present field,
cf. PR Photo 46c/99, it is now possible to detect previously unseen
dwarf galaxies in this rich cluster, in addition to some 200,000 stars
in our own Galaxy. This illustrates well the recent instrumental
progress for this type of observations.

The sky region of PR Photo 46c/99 is of particular interest, as
studies of the velocities of galaxies located in the cosmological
neighbourhood of the Milky Way have indicated an unusual streaming
motion in this direction. This can be explained by the presence of a
large-scale gravitational pull, and the existence of a huge
concentration of mass, named the "Great Attractor", has been inferred.

Indeed, PR Photo 46c/99 shows the central region of an enormous
cluster of galaxies (ACO 3627) in this direction. At a distance of
about 250 million light-years, it is likely to be associated with (or
to form the central part of) this "Great Attractor".

Evolution of galaxies in clusters

ESO PR Photo 46d/99 

PR Photo 46d/99 shows the central region of the cluster in PR Photo
46c/99, now at the original resolution of the WFI. Note the strong
warping of the galaxy to the left of the center, which may be caused
by gravitational interaction with one or both of the bright galaxies
that are seen above and below it. The field shown measures about 12 x
12 arcmin2; the area is about 1/7 of the full field shown in PR Photo
46c/99.
  
ESO PR Photo 46e/99 

PR Photo 46e/99 displays a small area (about 2.4 x 2.4 arcmin2)
centered on the bright galaxy in the lower left quadrant of PR Photo
46d/99. The dark lane probably indicates that it has already
cannibalized one or more of its companions. During the course of
hundreds of millions of years or more, others are likely to suffer the
same fate.
  


PR Photos 46d/99 and 46e/99 display smaller areas from PR Photo
46c/99. They include some typical examples of the dynamical evolution
of galaxies in clusters under the influence of their mutual
gravitational forces.

The effects range from cases of severe distortion of the shape (PR
Photo 46d/99) to "cannibalisation" by a giant elliptical galaxy of its
smaller companions (PR Photo 46e/99).



Technical information for PR Photos 46c-e/99: Five exposures each were
made in blue (B-band filter; 5 x 300 sec), red (R-band filter; 5 x 180
sec) and near-infrared (narrow-band filter centered at 816 nm; 5 x 240
sec) light and combined into a false-colour composite by using blue,
green, and red colour for the three images, respectively. A
logarithmic intensity scale is used to better show the inner as well
as the outer regions of the galaxies in this field.

This is the caption to ESO PR Photos 46a-j/99. They may be reproduced,
if credit is given to the European Southern Observatory.