Cosmic Streaming Field at Low Redshift

Abstract

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 $(l_0, b_0) = (312^{\rm o}.0^{13.5}_{-7.4}, 25^{\rm o}.7^{8.0}_{-9.2})$, 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 ($\sim 0.007$) 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 $z \sim 0.025$. 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 $\sim$ 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 $χ^2$ 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.