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Abstract: The BL Lac object RGB 1745+398 lies in an environment that makes it possibleto study the cluster around it more deeply than the environments of other BLLac objects. The cluster centered on the BL Lac works as a strong gravitationallens, forming a large arc around itself. The aim of this paper is to study theenvironment and characteristics of this object more accurately than theenvironments of other BL Lac objects have been before.We measured the redshiftsof galaxies in the cluster from the absorption lines in their spectra. Thevelocity dispersion was then obtained from the redshifts. The gravitationallensing was used for measuring the mass at the center of the cluster. The massof the whole cluster could then be estimated using the softened isothermalsphere mass distribution. Finally, the richness of the cluster was determinedby counting the number of galaxies near the BL Lac object and obtaining thegalaxy-BL Lac spatial covariance function, $B {gb}$. The redshifts of ninegalaxies in the field were measured to be near the redshift of the BL Lacobject, confirming the presence of a cluster. The average redshift of thecluster is 0.268, and the velocity dispersion $470^{+190} {-110}$ kms$^{-1}$. The mass of the cluster is $M {500}=4^{+3} {-2}\times10^{14}M {\sun}$ which implies a rather massive cluster. The richness measurement alsosuggests that this is a rich cluster: the result for covariance function is$B {gb}=600\pm200$ Mpc$^{1.77}$, which corresponds to Abell richness class 1and which is consistent with the mass and velocity dispersion of the cluster.

Autor: H. Lietzen, K. Nilsson, L. O. Takalo, P. Heinamaki, P. Nurmi, P. Keinanen, S. Wagner


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