Distances to clusters and determination of a kinematic reference frame

For his Ph.D. thesis, Danny Dale (now at U Wyoming) conducted an all-sky observing program designed to improve the quality of the I band Tully-Fisher (TF) relation and to determine accurately the reflex motion of the Local Group with respect to intermediate redshift clusters. This work has been done in collaboration with Giovanelli, Haynes, Scodeggio, Eduardo Hardy (U Laval), and Luis Campusano (U Chile).

We obtained between 5 and 15 TF measurements per cluster on a sample of 52 clusters with redshifts 0.02 < z < 0.08. The data extends the TF study of nearby clusters of Giovanelli et al. (1997).

The two data sets are used jointly to establish a cluster inertial reference frame from which peculiar velocities and bulk flow solutions are obtained.

We find the Local Group motion in this frame to be 565+/-113 km/s in the direction (l,b)=(267o,26o)+/-10o when peculiar velocities are weighted according to their errors. After optimizing the dipole calculation to sample equal volumes equally, the vector is 509+/-195 km/s toward (255o,33o)+/-22o. Both solutions agree, to within 1 sigma or better, with the Local Group motion as inferred from the cosmic microwave background (CMB) dipole. Thus, the cluster sample as a whole moves slowly in the CMB reference frame, its bulk flow being at most 200 km/s.

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