SHADOW OF A COLOSSUS: A z=2.44 GALAXY PROTOCLUSTER DETECTED IN 3D Lyα FOREST TOMOGRAPHIC MAPPING OF THE COSMOS FIELD

Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at z similar to 2.3-3 within a 11'.5 x 13'.5 area of the COSMOS field (similar to 1200 deg(-2) projected area density or similar to 2.4 h(-1) Mpc mean transverse separation), we reconstruct a 3D tomographic map of the foreground Ly alpha forest absorption at 2.2 < z < 2.5 with an effective smoothing scale of is an element of(3D) approximate to 2.5 h(-1) Mpc comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased toward regions with enhanced intergalactic medium (IGM) absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at z = 2.45 associated with a recently discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be M-dm(z = 2.45) = (1.1 +/- 0.6) x 10(14) h M-1(circle dot), and argue based on this mass and absorption strength that it will form at least one z similar to 0 galaxy cluster with M(z = 0) = (3 +/- 1.5) x 10(14) h(-1)M(circle dot), although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at z = 2.30 within a r similar to 1 h(-1) Mpc radius and Delta z similar to 0.006, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Ly alpha forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters or other features of large-scale structure.