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Large Scale Structure from Brightest Cluster...(1Dec93)
Galaxies
(from NOAO Highlights!, NOAO Newsletter No. 36, 1 December 1993)

We measure how our local group of galaxies is drifting through space
from radial velocities and distances of distant clusters of galaxies in
several directions.  Such studies have suggested the existence of a
"Great Attractor," a concentration of mass thought to lie in the
direction of the Hydra-Centaurus supercluster, whose gravity causes a
bulk flow of material toward itself. Bulk flows of galaxies are
expected as a natural consequence of the large scale structure of the
Universe.  The amplitude of the flows as a function of spatial scale
offers information on the power spectrum of the mass distribution in
the Universe, and thus can potentially discriminate between the
various theories of formation for both large scale structure and
galaxies themselves. For many years bulk flow associated with the Great
Attractor defined the largest structural scale known in the Universe.

                        [Figure not included]

More recently astronomers have begun to examine larger volumes of space
to search for structure on even larger spatial scales.  Work using the
KPNO 4-m and 2.1-m telescopes and the CTIO 1.5-m telescope by T.R.
Lauer (NOAO) and M. Postman (STScI) suggests that bulk flows continue
over distance scales nearly three times larger than the previously
known limit of 6000 km s-1. They used photometry of brightest cluster
galaxies (such as the one in A3526, shown on the cover) to derive
distances for 119 clusters of galaxies and to measure the space
velocity of the Local Group with respect to an inertial frame
comprising all Abell galaxy clusters within 15,000 km s-1 redshift. The
goal of the Lauer-Postman survey was to see if the motion of the Local
Group measured with respect to the Abell clusters was consistent with
that inferred from the dipole anisotropy of the cosmic microwave
background. This is thus a direct test to see if the large-scale bulk
flows of galaxies seen by other investigators on smaller scales finally
damp out at much larger distances. Surprisingly, Lauer and Postman find
that the Local Group motion inferred from the Abell cluster sample
strongly differs from that inferred from the dipole of the cosmic
microwave background. An obvious interpretation is that the entire
volume probed by the clusters is itself streaming with a velocity of
nearly 700 km s-1. In short, Lauer and Postman did not detect the flow
convergence scale, and conclude that bulk flows of galaxies continue to
much larger scale than previously expected. The result of the
Lauer-Postman survey is intriguing as it implies more power on larger
scales than is predicted by any theoretical model, and thus offers a
powerful probe into the formation of the largest structures in the
Universe.

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