About a decade ago, measurements of galaxy streaming motions (peculiar velocities) revealed the presence of a large concentration of Galaxies dubbed the "Great Attractor" (GA), which was thought to create a large fraction of the drift of our Local Group of galaxies with respect to the cosmic microwave background (CMB). Further peculiar velocity surveys however reveal a more complex picture. There is now evidence that the GA, itself, is moving with respect to the CMB, as part of a global flow of galaxies out to at least 6000 km/s from the Local Group. This flow has an amplitude of roughly 350 km/s within that sphere. Going out further, Tod Lauer (NOAO) and Marc Postman (STScI) used telescopes at KPNO and CTIO in an attempt to find the termination of flows at distances well-beyond the GA, but instead measured a bulk flow of high amplitude (750 km/s) extending out to three times the distance previously surveyed. These large amplitudes, and bulk flows in general, are a strong test of cosmological models, and thus require confirmation. The figure, however, shows that there is still no consensus as to the amplitude or direction of the flow---especially at distances beyond 6000 km/s. Reiss, Press, and Kirshner, for example, using Type Ia Supernovae as distance indicators, argue against the existence of large bulk galaxy flows at large distances.
Caption:Compilation of the bulk flow of galaxies on various scales from the literature. Error bars are taken from the papers quoted.
Stephane Courteau (NOAO/DAO), Michael Strauss (Princeton), David Schlegel
(Durham), Marc Postman (STScI), and Jeff Willick (Stanford) are using NOAO
facilities to make a definitive measurement of the bulk flow at 6000 km/s
with the aim of clarifying the confusing picture of galaxy flows at large
distances. This new project relies entirely on KPNO and CTIO facilities.
Distance estimates are based on the Tully-Fisher relation between rotation
velocity and luminosity of spiral galaxies. Their it full-sky
sample includes
300 late-type spirals between 4500 and 7000 km/s. With the exception of the
Lauer-Postman survey, most peculiar velocity surveys carried out to date do
not cover the entire sky uniformly, and are thus limited by systematics in
the photometry and spectroscopy of
galaxies in different parts of the sky. The Courteau et al.
survey will be
the first well-defined full-sky survey to sample the velocity field at 6000
km/s, free of uncertainties from matching heterogeneous datasets. Because of
the overlap with existing surveys at comparable depth, this new compilation
will also be of fundamental importance in tying these datasets together in a
uniform way, which will greatly increase their usefulness for global
analyses of mass fluctuations in the universe. Photometry and long-slit
H
spectroscopy of the galaxies in the sample was started in spring
1996, and should be completed in roughly one year.
This all-sky new survey will be pivotal in establishing with high confidence whether a bulk flow extends beyond the shell of matter at 6000 km/s. Moreover, it will provide a firm pillar in building a bridge between the nearby well-sampled universe and surveys to much larger distances.