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 340⁶³₋₇₁ km/sec, aligned in the direction of (l₀, b₀) = (312ᵒ.013.5-7.4, 25ᵒ.78.0-9.2), 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.