The effects of peculiar velocities in SN Ia environments on the local $H_0$ measurement

Abstract

The discrepancy between estimates of the Hubble Constant (H 0 ) measured from local (z 0.1) scales and from scales of the sound horizon is a crucial problem in modern cosmology. Peculiar velocities of standard candle distance indicators can systematically affect local H 0 measurements. We here use 2MRS galaxies to measure the local galaxy density field, finding a notable z < 0.05 underdensity in the SGC-6dFGS region of 27 ± 2 %. However, no strong evidence for a 'Local Void' pertaining to the full 2MRS sky coverage is found. Galaxy densities are used to measure a density parameter, ∆φ +− , which acts as a proxy for peculiar velocity (v pec ) by quantifying density gradients along a line-of-sight. ∆φ +− is found to correlate strongly with local H 0 estimates from Union 2.1 Type Ia SNe (0.02 < z < 0.04). Density structures on scales of ∼ 50 Mpc are found to correlate most strongly with H 0 estimates in both the observational data and in mock data from the MDPL2-Galacticus simulation. Interpolating SNIa H 0 estimates to their ∆φ +− = 0 values, we can correct for the effects of density structure on the local H 0 estimates, even in the presence of biased peculiar velocities. For these particular observational data, we reveal a < 0.1km s −1 Mpc −1 difference in the sample mean estimate compared to the value uncorrected for peculiar velocities. Our best estimate is then 74.9 km s −1 Mpc −1 . Using the mock data, the systematic uncertainty from these peculiar velocity corrections is estimated to be 0.3 km s −1 Mpc −1 . The dominant source of uncertainty in our estimate instead relates to Cepheid-based calibrations of distance moduli (1.7 km s −1 Mpc −1 ) and SN photometry (0.7 km s −1 Mpc −1 ).