Mapping the Cosmic Web with the Sloan Digital Sky Survey

Wide-angle, moderately deep redshift surveys such as that conducted as part of the Sloan Digital Sky Survey (SDSS) allow study of the relationship between the structural elements of the large-scale distribution of galaxies - including groups, cluster, superclusters, and voids - and the dependence of galaxy formation and evolution on these enviroments. We present a progress report on mapping efforts with the SDSS and discuss recently constructed catalogs of clusters, voids, and void galaxies, and evidence for a 420h 1 Mpc supercluster or "Great Wall." Analysis of multi-band photometry and moderate-resolution spectroscopy from the SDSS reveals environmental dependence of the star formation history of galaxies that extends over more than a factor of 100 in density, from clusters all the way to the deep interiors of voids. On average, galaxies in the rarified environments of voids exhibit bluer colors, higher specific star formation rates, lower dust content, and more disk-like morphology than objects in denser regions. This trend persists in comparisons of samples in low vs. high-density regions with similar luminosity and morphology, thus this dependence is not simply an extension of the morphology-density relation. Large-scale modulation of the halo mass function and the temperature of the intergalactic medium might explain this dependence of galaxy evolution on the large-scale environment. 1. Introduction: Cities, Suburbs, and Countryside of the Universe The varied geography and demography of human civilization provides a colorful anal- ogy for the dependence of galaxy formation and evolution on the large-scale environment of the universe. The dense population of cities promotes a frenetic lifestyle and facilitates many fleeting interactions. Only the very richest get richer (cD's?), while many, drawn to the bright lights, are quickly stripped of all that they arrived with. At the other extreme lie the rural stretches, where interactions are few but which tend to be tightly-bound when they occur. Without the competition of others, a few can live surprisingly well off the steady, albeit sometimes meager, offerings of the land. This meeting has been called to study the suburbs of the universe, including the outskirts of clusters. "Intense life in the suburbs" results from living where interactions are frequent, but lasting enough to form families (groups?) and the city's pull is always felt. This environment is of partic- ular interest because this is the lowest density in which halos with multiple galaxies are common. Galaxy clusters, groups, and filaments surround the voids in a cosmic web that large galaxy redshift surveys reveal to be a ubiquitous pattern in the universe. Below we report on progress in mapping the nearby universe using the SDSS (§2) and describe results of objective identification of individual structures in the survey. We then discuss analyses of photometric and spectroscopic data for galaxy samples that are now large enough to statistically reveal dependence of galaxy evolution on environment from clusters all the way to voids (§3). We conclude by reviewing physical mechanisms that may explain this dependence (§4).