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Fossil Alignments

BCGs are often well-aligned with the distribution of the cluster galaxies (Sastry 1968; Carter el al. 1980; Porter, Schneider & Hoessel 1991)[53][14][46] as well as more extended large-scale clustering features (Binggeli 1982)[7] and it has been suggested that this is due to filamentary collapse expected in hierarchical structure formed from Gaussian random noise (e.g. Rhee & Roos 1990; West 1994; Bond, Kofman, & Pogosyan 1996)[47][61][9]. The alignment of the central galaxy is seen as the consequence of an anisotropic collapse remembered from the initial random density field. Filamentary collapse leads to nearly head-on collisions of galaxies which create prolate merger remnants aligned with the initial collision trajectory (Villumsen 1982).

The simulated BCG examined here shows the alignment effect as seen in previous work. The shape and orientation of the BCG is indeed nearly congruent with the galaxy distribution in the cluster. The angle between the long axis of the central galaxy and the cluster galaxy population as measured by its moments is only 15tex2html_wrap_inline907. Furthermore, the orientation of the BCG is closely aligned with the primordial filament delineated by the 3 large galaxies which make up most of the mass of the BCG. We should emphasize that this alignment effect only works for the central, giant elliptical where the kinematics and morphology are dependent on the large-scale convergence of the flow of matter into the cluster's forming potential well. Other galaxies falling into the cluster that avoid merging with the BCG will have random alignments dependent on their merging history and tidal interactions with the cluster core and other galaxies. This simulation strongly supports the hypothesis that the shape and orientation of the central galaxy are fossils of the filamentary initial conditions of the cluster collapse, although further simulations should be done to confirm this result.

John Dubinski
Tue Feb 17 16:03:05 EST 1998