The Mass of the Centaurus A Group of Galaxies

The mass M, and the radius R_h, of the Centaurus A group are estimated from the positions and radial velocities of 30 probable cluster members. For an assumed distance of 3.9 Mpc it is found that R_h \sim 640 kpc. The velocity dispersion in the Cen A group is 114 \pm 21 km/s. From this value, and R_h = 640 kpc, the virial theorem yields a total mass of 1.4 \times 10^{13} M_{\sun} for the Cen A group. The projected mass method gives a mass of 1.8 \times 10^{13} M_{\sun}. These values suggest that the Cen A group is about seven times as massive as the Local Group. The Cen A mass-to-light ratio is found to be M/L_B = 155-200 in solar units. The cluster has a zero-velocity radius R_0 = 2.3 Mpc.