A Local Hubble Bubble from Type Ia Supernovae?

We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae(SNe Ia) to test for a local void. The sample extends from 20 to 300 Mpc/h,with distances, deduced from light-curve shapes, accurate to ~6%. AssumingOmega_m=1 and Omega_lambda=0, the most significant deviation we find from theHubble law is an outwards flow of (6.6+/-2.2)% inside a sphere of radius 70Mpc/h as would be produced by a void of ~20% underdensity surrounded by a denseshell. This shell roughly coincides with the local Great Walls. Monte Carloanalyses, using Gaussian errors or bootstrap resampling, show the probabilityfor chance occurrence of this result out of a pure Hubble flow to be ~2%. Themonopole could be contaminated by higher moments of the velocity field,especially a quadrupole, which are not properly probed by the current limitedsky coverage. The void would be less significant if Omega_m is low andOmega_lambda is high. It would be more significant if one outlier is removedfrom the sample, or if the size of the void is constrained a-priori. Thisputative void is not in significant conflict with any of the standardcosmological scenarios. It suggests that the Hubble constant as determinedwithin 70 Mpc/h could be overestimated by ~6% and the local value of Omega maybe underestimated by ~20%. While the present evidence for a local void ismarginal in this data set, the analysis shows that the accumulation of SNe Iadistances will soon provide useful constraints on elusive and important aspectsof regional cosmic dynamics.Comment: 21 pages, 3 figures. Slightly revised version. To appear in ApJ, 503, Aug. 20, 199