The Two-Component Cluster in Centaurus

Abstract

In a previous paper (Lucey et al 1980) we reported the existence of a double-peaked velocity distribution for the rich, nearby Centaurus cluster (α = 12h 46.9m, δ = −40° 57′). We have now obtained an enlarged data set for Centaurus consisting of 203 redshifts and GF photometry (g = IIIa−J + GG495, F = IIIa−F + RG630) for 329 galaxies which lie within 3° of the cluster’s centre. We have redshifts for all but 2 of the 99 galaxies brighter than G26.5 = 16.5 that lie within 1° of the cluster’s centre. The velocity histogram for the enlarged data set is shown (figure 1). The velocity distribution is bimodal and has best fit double gaussian parameters of: n₁ = 123, v₁ = 3041 km s⁻¹, σ₁ = 577 km s⁻¹ and n₂ = 57, v₂ = 4570 km s⁻¹, σ₂ = 262 km s⁻¹ where the velocity dispersions have been corrected for a measuring error of 100 km s⁻¹. We have labelled the lower velocity and the higher velocity components Cen30 and Cen45 respectively. The sky distributions of the two components overlap completely. The galaxy distribution in the core region of the cluster is shown in figure 2.