Physics
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10429b&link_type=abstract
HST Proposal ID #10429
Physics
Hst Proposal Id #10429 Cosmology
Scientific paper
The 600 km/s motion of the Local Group {LG} with respect to the cosmic microwave background {CMB} is now known to high accuracy. However, its precise origin remains poorly understood. The contribution to the motion from the pull of the rich Shapley supercluster at z = 0.048 is particularly controversial. This extreme mass concentration contains more than 20 Abell clusters within 35 Mpc of its very rich central cluster A3558, and is recognized as both the optically richest and the most X-ray luminous structure in the local {z < 0.1} universe. Yet, published values for the mass of Shapley continue to differ by an order of magnitude, and recent estimates of its pull on the LG range from negligible {20 km/s} to highly significant {300 km/s or more}. Here we propose to resolve this key issue by using ACS to measure high-precision surface brightness fluctuation {SBF} distances in order to make a direct measurement of the infall towards Shapley. We will target three Shapley foreground clusters where the infall is expected to be high {possibly 1000 km/s or more}, as well as the Shapley core, in order to test the assumption that it is at rest in the CMB. Prior to ACS, the Shapley region was unreachable for SBF, but ACS doubles the distance range of the SBF method with HST, enabling the distances to be measured to the required accuracy. The proposed measurements will place a firm limit on the largest mass fluctuation in the nearby universe and finally determine its contribution to the observed CMB dipole.
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