Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

5.4. Large-Scale Structure

Redshift surveys of the nearby universe indicate that groups of galaxies are good tracers of large-scale structure (e.g. Ramella et al 1989). The presence of a hot intragroup medium in many groups suggests that X-ray observations can also be used to map out the distribution of mass in the universe. Recent ROSAT results demonstrate the great potential of large area X-ray surveys. Mullis et al (2000) have recently completed an optical follow-up survey of the ~ 500 X-ray sources detected in the ROSAT All-Sky Survey in a 9 × 9 square-degree region around the north ecliptic pole. They identify 65 galaxy systems, ~ 30% of which are poor groups. Remarkably, some 23% of the galaxy systems found in this field belong to a single wall-like structure at z = 0.088. Although a supercluster consisting of six Abell clusters had previously been identified in this region (Batuski & Burns 1985), the X-ray data reveal that this supercluster is significantly larger than implied by the optical data alone. Furthermore, the X-ray data show that the massive Abell clusters are linked together by groups and poor clusters. The supercluster spans the entire area surveyed by Mullis et al (2000), suggesting that the true extent of this structure could be larger still. Numerical simulations imply that future X-ray missions such as CHANDRA and XMM will be able to map out even lower-density regions such as filaments (Pierre et al 2000). Such X-ray studies will be very important because many current models suggest that the majority of baryons occur in these filaments (Miralda-Escude et al 1996, Cen & Ostriker 1999).