A Dynamical Analysis of the Poor Galaxy Clusters A2626 and A2440

Abstract

We use 189 new radial velocities, an $R$ band CCD mosiac, archival X-ray images, and gas temperatures to study substructure, masses, luminosity functions, and mass-to-light ratios in A2626 and A2440. The symmetric X-ray emission from A2626 suggests a relaxed, single-component system; however, 159 redshifts reveals a complex, three component cluster at cz~17,500km/s. One components is a typical X-ray bright cluster, a second has a cluster-like galaxy population with lower central galaxy and gas density, and the third is a background structure with field galaxy composition. A comparison of the magnitude distributions within the two subclusters suggests that A2626 is a merger in progress. Virial masses and a composite luminosity function for a region with projected radius r=1.5h^{-1}Mpc yield a mass to light ratio M/L_R~610h. Analysis of the X-ray emission from the primary component yields a gas mass fraction of 2.2h^{-3/2}% and a baryon fraction of 3.4%. A radial infall model implies that the virial mass may be an underestimate. In A2440, deep CCD $R$ band photometry indicates a striking correspondence between the galaxy and gas distributions. The galaxy distribution has three main components, each associated with a giant elliptical galaxy. The two larger peaks in the galaxy distribution coincide with the primary peaks in the X-ray emission, and the third is associated with a significant X-ray surface brightness enhancement. We use this galaxy-gas correspondence and 48 redshifts to argue that the subclusters are bound within a single system, and that the two primary components are beginning to merge. The composite luminosity function and estimates of the subcluster virial masses indicate a mass-to-light ratio from 660h to 880h.

22 pages, to appear in the Astronomical Journal, Nov 96