Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a Forest Tomographic Mapping of the COSMOS Field

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Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at z 2.3-3 within a 11.5'×13.5' area of the COSMOS field (1200deg² projected area density or 2.4h⁻¹Mpc mean transverse separation), we reconstruct a 3D tomographic map of the foreground Lyα forest absorption at 2.2<z<2.5 with an effective smoothing scale of σ₃d≈3.5h⁻¹Mpc comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased towards regions with enhanced 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) = (9±4)× 10¹³h⁻¹M, and argue based on this mass and absorption strength that it will form at least one z 0 galaxy cluster with M(z=0) = (3± 2) × 10¹⁴h⁻¹M, 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 1h⁻¹Mpc radius and Δz 0.006, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Lyα 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 and other features of large-scale structure.