DISCOVERY OF A VERY LARGE STRUCTURE AT<i>Z</i>= 3.78

Abstract

We report the discovery of a large-scale structure containing multiple\nprotoclusters at z=3.78 in the Bo\\"otes field. The spectroscopic discovery of\nfive galaxies at z=3.783+/-0.002 lying within 1 Mpc of one another led us to\nundertake a deep narrow- and broad-band imaging survey of the surrounding\nfield. Within a comoving volume of 72x72x25 Mpc^3, we have identified 65 Lyman\nalpha emitter (LAE) candidates at z=3.795+/-0.015, and four additional galaxies\nat z_spec=3.730,3.753,3.780,3.835. The galaxy distribution within the field is\nhighly non-uniform, exhibiting three large (~3-5x) overdensities separated by\n8-14 Mpc (physical) and possibly connected by filamentary structures traced by\nLAEs. The observed number of LAEs in the entire field is nearly twice the\naverage expected in field environments, based on estimates of the Lya\nluminosity function at these redshifts. We estimate that by z=0 the largest\noverdensity will grow into a cluster of mass 10^15 Msun; the two smaller\noverdensities will grow into clusters of mass (2-6)x10^14 Msun. The highest\nconcentration of galaxies is located at the southern end of the image,\nsuggesting that the current imaging may not map the true extent of the large\nscale structure. Finding three large protocluster candidates within a single\n0.3 deg^2 field is highly unusual; expectations from theory suggest that such\nalignments should occur less than 2% of the time. Searching for and\ncharacterizing such structures and accurately measuring their volume space\ndensity can therefore place constraints on the theory of structure formation.\nSuch regions can also serve as laboratories for the study of galaxy formation\nin dense environments.\n