The bulk flow motion and the Hubble-Lema\^{\i}tre law in the Local Universe with the ALFALFA survey

Abstract

The knowledge of the main features of the bulk flow in the Local Universe is important for a better determination of the relative motions there, an information that would contribute to a precise calculation of the Hubble-Lema\^{\i}tre law at very low redshifts. We study how to obtain the Hubble-Lema\^{\i}tre law in two sky regions using the catalog of HI sources of the ALFALFA survey, with data $cz_{\odot} < 6000$ km/s. Our methodology aims to compute $H_0$ in two regions -- located in opposite galactic hemispheres -- mapped by the ALFALFA survey, and look for dependence with distance, direction, and also test for reference frame changes. We calculate the Hubble constant, in the Cosmic Microwave Background reference frame, in opposite galactic hemispheres: $H_0^N = 70.87 \pm 2.38$ and $H_0^S = 66.07 \pm 3.02$, which allows us to measure the bulk flow velocity $V_{BF} = 401.06 \pm 150.55$ km/s at the effective distance $31.3 \pm 6.26$ Mpc, a novel result found analysing the ALFALFA data at low redshift. We confirm the influence of the bulk flow on the structures of the Local Universe which manifests through a dipolar behavior of the Hubble constant in opposite hemispheres.