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K. Rines, A. Mahdavi, M.J. Geller (CfA), A. Diaferio (U. Torino), J.J. Mohr (U. Illinois), G.A. Wegner (Dartmouth)
As part of a large redshift survey of the infall regions surrounding nearby clusters of galaxies, we show that the infall region of Abell 2199 contains Abell 2197 and one or two X-ray emitting groups. Our survey shows that these groups, located at projected radii of 1.\circ4, 1.\circ9, and 5.\circ1 (2.4, 3.2, and 8.4~h-1{\rm Mpc}), are connected kinematically to A2199. A2197 is itself an optically rich cluster; its weak X-ray emission suggests that it is much less massive than A2199. The absence of a sharp peak in the infall pattern at the position of A2197 supports this hypothesis. The outer groups have previously been identified as X-ray emitting groups. The outermost group is well outside the virial region of A2199 and, as expected, it distorts the infall pattern in redshift space. The identification of these infalling groups provides direct support of hierarchical structure formation and may provide a useful laboratory for testing models of structure formation. The groups also offer a unique probe of the physics of the intergalactic medium where it is not directly observable with current or planned instruments. If the outermost group is bound to the supercluster, the observed offset between its optical and X-ray centers could be caused by ram pressure stripping by the IGM.