Astronomy and Astrophysics, volume 302, 21-36 (1995/10-1)
The Virgocentric velocity from bright cluster galaxies.
Abstract (from CDS):
By referencing Weedman's nuclear magnitudes of bright cluster galaxies to the microwave background radiation (MBR) frame and correcting them for cluster richness by means of new CCD observations, I find a Virgocentric velocity (the actual difference between the Virgo cluster peculiar velocity and the peculiar velocity of the Local Group (LG) in the direction of Virgo) of -38±72km/s (inflow). The same analysis applied to the brightest cluster galaxy data of Sandage & Hardy yields a Virgocentric motion of 22±151km/s (outflow). From the weighted averages of several distance indicators for 17 selected clusters, the Virgocentric motion is -52±67km/s (inflow). There is thus little evidence for a significant Virgocentric motion, a result in agreement with previous studies using m* from cluster galaxy luminosity functions, and rg from the fundamental planes. Because the distance indicators span the range from nearby to very distant clusters (where peculiar velocities are proportionally small), these Global Method results are independent of any assumed model for flow within the Local Supercluster (LSC). An estimate of the collective Virgocentric velocity in the LSC at the position of the LG yields a slight outflow, i.e., 39±100km/s. The Weedman data give a Virgo peculiar velocity (difference between the MBR frame velocity and the Hubble flow) of 401±71km/s. Sandage & Hardy's data give 462±151km/s, and the selected clusters give 387±66km/s. A large peculiar velocity of Virgo is associated with a small Hubble parameter. The predicted MBR frame velocities of the Virgo cluster from these methods are 1073±60km/s, 1012±146km/s, and 1087±54km/s, respectively. These velocities are considerably smaller than those computed by simply adding the sun's projected MBR motion to Virgo's heliocentric velocity. If a true distance modulus of 31.70 (long scale) is assumed for Virgo, H0=49, 46, and 50km/s/Mpc, respectively. On the other hand, for a true distance modulus of 30.84 (short scale), H0=73, 69, and 74km/s/Mpc, respectively. The velocity of the LG relative to the MBR, undisturbed by the infall determined from the selected clusters, is 584km/s. Its direction lies about 4deg away from the LG's original direction and 8deg away from the Hydra-Centaurus supercluster. If Virgo also has this same space velocity, then its component in the radial direction from the LG is 387km/s. Remarkably, this is the same as its peculiar velocity. Similar agreement is found between the velocity component of the Hydra I cluster and its measured radial peculiar velocity. These results indicate a high degree of consistency not found in previous studies.
galaxies: clusters: individual: Virgo cluster - galaxies: distances and redshifts - local group - galaxies: photometry - cosmic microwave background - distance scale
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