The Hubble Flow around the Centaurus A/M83 Galaxy Complex

We present HST/ACS images and color-magnitude diagrams for 24 nearby galaxies in and near the constellation of Centaurus with radial velocities VLG < 550 km/s. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 3. 0 Mpc to 6. 5 Mpc. The galaxies are concentrated in two spatially separated groups around Cen A (NGC 5128) and M 83 (NGC 5236). The Cen A group itself has a mean distance of 3. 76 +/-0. 05 Mpc, a velocity dispersion of 136 km/s, a mean harmonic radius of 192 kpc, and an estimated orbital/virial mass of (6. 4 - 8. 1) x 10¹2 Mₛun. This elliptical dominated group is found to have a relatively high mass-to-light ratio: M/LB = 125 Mₛun/Lₛun. For the M 83 group we derived a mean distance of 4. 79 +/-0. 10 Mpc, a velocity dispersion of 61 km/s, a mean harmonic radius of 89 kpc, and estimated orbital/virial mass of (0. 8 - 0. 9) x 10¹2 Mₛun. This spiral dominated group is found to have a relatively low M/LB = 34 Mₛun/Lₛun. The radius of the zero-velocity surface around Cen A lies at R₀ = 1. 40 +/-0. 11 Mpc, implying a total mass within R₀ of MT = (6. 0 +/-1. 4) x 10¹2 Mₛun. This value is in good agreement with the Cen A virial/orbital mass estimates and provides confirmation of the relatively high M/LB of this elliptical-dominated group. The centroids of both the groups, as well as surrounding field galaxies, have very small peculiar velocities, < 25 km/s, with respect to the local Hubble flow with H₀ = 68 km/s/Mpc.