Cosmic Streaming Field at Low Redshift

We study the expansion of the nearby Universe using a sample of Type Ia supernovae at redshifts below 0. 08. These supernovae allow peculiar velocities to be measured at unprecedented precision. We have investigated in detail the possibility of a varying Hubble constant with redshift and found no evidence of a monopole term for the nearby Universe. A large scale streaming motion is found at an amplitude of about 340^63-₇₁ km/sec, aligned in the direction of (l₀, b₀) = (312^ o. 0^13. 5-₇. ₄, 25^ o. 7^8. 0-₉. ₂), which is close to the direction of the center of Shapley supercluster of galaxies. The large scale streaming motion is best fit by a function involving a strong bipolar term. The streaming velocity field extends from the lowest redshift (0. 007) to beyond 0. 025 and likely out to even higher redshifts. The velocity field at redshift below 0. 01 can be equally well described by a dipole field or by the same bipolar streaming velocity field that reaches out to beyond z 0. 025. We are also able to deduce a robust estimate of the random velocity component of the peculiar velocity field. Within the volume of redshift below 0. 01 (weighted average redshift of 0. 067), this thermal component is found to be about 270 km/sec. After correcting this smooth streaming motion, we are able to significantly improve the Hubble expansion fits of these supernovae. The CMAGIC method gives a dramatic decrease of χ² from 90 to 63 for 69 degrees of freedom, and yields a residual scatter of only 0. 12 magnitude; the maximum light method gives also a moderate improvement.