2001ApJ...555..558R

Using a large redshift survey covering 95 deg², we demonstrate that the infall region of Abell 2199 contains Abell 2197, one or two X-ray-emitting groups, and up to five additional groups identified in redshift surveys. Our survey shows that the X-ray-emitting systems, located at projected radii of 1°.4, 1°.9, and 5°.1 (2.2, 3.1, and 8.0 h^–1 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 outermost group is well outside the virial region of A2199, and it distorts the infall pattern in redshift space. The two X-ray-emitting groups are roughly colinear, suggesting the existence of an extended (8.0 h^–1 Mpc) filament. The identification of these infalling groups provides direct support of hierarchical structure formation; studies of these systems will provide insights into structure evolution. Groups in the infall regions of nearby clusters may offer a unique probe of the physics of the warm/hot ionized medium (WHIM), which is difficult to observe directly with current instruments.