The Hubble flow around the CenA / M83 galaxy complex
Abstract
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.
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