A Re-examination of the Origins of the Peculiar Velocity Field within the Local Supercluster


The recent re-evaulation of the peculiar velocity field outside the Local Supercluster (Dekel et al. 1999) has permitted a re-examination of the origins of the peculiar velocity field within the Local Supercluster using the Mark III Catalog of Galaxy Peculiar Velocities. It is shown that the large-scale coherent pattern of peculiar velocities within the LSC are well-fit by a combination of the Outside-Region-(generated)-Motions (O-R-M) from the Potent model with a Virgocentric infall pattern that produces 220 km s of Virgocentric infall at the Local Group towards the Virgo cluster moving at 88 km s towards the Local Group. The part of the LG CMB motion this model cannot fit is that perpendicular to the Supergalactic plane. On what size scale this SGZ motion of the Local Group is shared by neighboring galaxies cannot be determined from the present data set, but can be found if we can accurately measure galaxy distances close to the Galactic plane. 1. The Four Issues to be Addressed In the Mark III Catalog (Willick et al. 1995, 1996) the distance of the Virgo cluster (VC) is DVC = 1330± 52 km s . The heliocentric radial velocity of the VC (VVC(helio)) from Huchra (1996) is 1094±35 km s . Combine this with the heliocentric peculiar motion of the Sun in the Cosmic Microwave Background (CMB) frame of 368.6 km s towards l = 264.7, b = 48.2, yields a CMB radial velocity for the VC (VVC(CMB)) of 1420 ± 35 km s . If we combine VVC(CMB) with the peculiar velocity of the Local Group (LG) in the CMB frame (372 km s) in the direction of the VC, we find the LG and the VC move towards each other in the CMB reference frame with a mutual velocity of Vpec(LG : VC) = 282 ± 63 km s . Four issues can be addressed from these data: 1) How much of the CMB motion between the LG and the VC is generated by the VC itself (i.e., classic “Virgocentric infall” motion), how much is generated by masses on larger size scales? 2) The peculiar velocities of the galaxy groups defined to be within the Coma-Sculptor (C-S) Cloud by Tully (1987) can be demonstrably shown to share most of the LG CMB peculiar velocity (e.g. Faber & Burstein 1988; Tully


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