Discovery of a Very Large Structure at Z=3.78
  We report the discovery of a large-scale structure containing multiple
protoclusters at z=3.78 in the Bo\"otes field. The spectroscopic discovery of
five galaxies at z=3.783+/-0.002 lying within 1 Mpc of one another led us to
undertake a deep narrow- and broad-band imaging survey of the surrounding
field. Within a comoving volume of 72x72x25 Mpc^3, we have identified 65 Lyman
alpha emitter (LAE) candidates at z=3.795+/-0.015, and four additional galaxies
at z_spec=3.730,3.753,3.780,3.835. The galaxy distribution within the field is
highly non-uniform, exhibiting three large (~3-5x) overdensities separated by
8-14 Mpc (physical) and possibly connected by filamentary structures traced by
LAEs. The observed number of LAEs in the entire field is nearly twice the
average expected in field environments, based on estimates of the Lya
luminosity function at these redshifts. We estimate that by z=0 the largest
overdensity will grow into a cluster of mass 10^15 Msun; the two smaller
overdensities will grow into clusters of mass (2-6)x10^14 Msun. The highest
concentration of galaxies is located at the southern end of the image,
suggesting that the current imaging may not map the true extent of the large
scale structure. Finding three large protocluster candidates within a single
0.3 deg^2 field is highly unusual; expectations from theory suggest that such
alignments should occur less than 2% of the time. Searching for and
characterizing such structures and accurately measuring their volume space
density can therefore place constraints on the theory of structure formation.
Such regions can also serve as laboratories for the study of galaxy formation
in dense environments.
