We present HST/ACS images and color-magnitude diagrams for 24 nearby galaxies in and near the constellation of Centaurus with radial velocities V _ { LG } < 550 km s ^ { -1 } .
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 \pm 0.05 Mpc , a velocity dispersion of 136 kms ^ { -1 } , a mean harmonic radius of 192 kpc , and an estimated orbital/virial mass of ( 6.4 - 8.1 ) \cdot 10 ^ { 12 } M _ { \sun } . This elliptical dominated group is found to have a relatively high mass-to-light ratio : M / L _ { B } = 125 ~ { } M _ { \odot } / L _ { \odot } .
For the M 83 group we derived a mean distance of 4.79 \pm 0.10 Mpc , a velocity dispersion of 61 km s ^ { -1 } , a mean harmonic radius of 89 kpc , and estimated orbital/virial mass of ( 0.8 - 0.9 ) \cdot 10 ^ { 12 } M _ { \sun } .
This spiral dominated group is found to have a relatively low M / L _ { B } = 34 ~ { } M _ { \odot } / L _ { \odot } .
The radius of the zero-velocity surface around Cen A lies at R _ { 0 } = 1.40 \pm 0.11 Mpc .
implying a total mass within R _ { 0 } of M _ { T } = ( 6.0 \pm 1.4 ) \cdot 10 ^ { 12 } M _ { \sun } .
This value is in good agreement with the Cen A virial/orbital mass estimates and provides confirmation of the relatively high M / L _ { B } 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 ^ { -1 } , with respect to the local Hubble flow with H _ { 0 } = 68 km s ^ { -1 } Mpc ^ { -1 } .