$\vec{K}$–band luminosity (mass) segregation in AC 118 at $\vec {z}$ = 0.31

Abstract

Using new observations of the galaxy cluster A118 at intermediate redshift (z=0.31) in the Ks band, we were able to detect the cluster from the center to half the Abell radius (1.5 Mpc, Ho=50 km/s/Mpc) and possibly to 2.0 Mpc. The analysis of both the spatial distribution of galaxies of various luminosities and of the luminosity function (LF) of galaxies in different cluster locations strongly confirms and extends to larger clustercentric radii the luminosity segregation found in a previous analysis of this cluster restricted to a smaller cluster area: there is an excess of bright galaxies in the cluster core (inside 250 Kpc) or a deficit of dwarfs in the remain part of the cluster. Outside the cluster core and as far as 1.5 or even 2 Mpc, the giant-to-dwarf ratio is constant. Because of the luminosity segregation, the LF of the AC118 shows a larger number of bright galaxies per unit dwarf in the core than in other cluster locations. All non-core LFs, computed at several cluster locations, are compatible each other. These results hold both including or excluding the galaxies located in an overdensity found in the far South of AC118 and in the second clump in galaxy density at the cluster North-West. Since the near-infrared emission is a good tracer of the stellar mass, we interpret the segregation found as a mass segregation.