Core radii determinations for four clusters of galaxies

Abstract

Bahcall (1975) has found that the average core radius for a group of 15 clusters of galaxies is 0.25[plus or minus]0.05 Mpc. At the suggestion of Dr. G. Welch it was decided to study four nearby clusters of galaxies (A2052, A2593, A2626, and A154) in order to determine their core radii. If it turned out that the dispersion of core radii at low redshifts is small, then these core radii could be said to be effectively constant. Any variation of the core radius at large redshifts would then be due to the geometry of the universe. Accordingly, a computer program was written that would find a core radius by fitting ring count data from the chosen clusters to an Emden isothermal gas sphere. The ring counts were made to three magnitude limits, one of which approximated that of Bahcall. Also, each magnitude limit was used to find four core radii: one using all the ring count data and a counted background density; one using half the ring count data (only the core region) and a counted background density; one using all the data but solving for a background density (among other parameters); and one using half the data and solving for the background density. These four results were compared in various ways in order to determine which method produced the “best” core radius. Then the “best” core radius for each cluster at the magnitude limit used by Bahcall was added to her results to obtain a new average and standard deviation. Several conclusions were drawn from the overall results. 1. In the course of testing the program it was found that different results were found between this and other programs using the same data. This indicates the need of a unique program to be used exclusively. 2. Better results seem to be found when the background density is counted. 3. Better results seem to be found when all data (about out to the Abell radius) is used as opposed to only the core data. 4. Two clusters show evidence of mass segregation (A2052 and A2593). 5. The spread of core radii from the four clusters of this thesis at (or more precisely, “near”) Bahcall’s magnitude limit is large enough to cast doubt on the idea of using core radii as universal geometry indicators (R[subscript c](average) = 0.20[plus or minus]0.13 Mpc for the four clusters of this thesis).