2017ApJ...835..281M

We present spatial correlations of galaxies and IGM neutral hydrogen H I in the COSMOS/UltraVISTA 1.62 deg² field. Our data consist of 13,415 photo-z galaxies at z ∼ 2-3 with K_s 23.4 and the Lyα forest absorption lines in the background quasar spectra selected from SDSS data with no signature of damped Lyα system contamination. We estimate a galaxy overdensity δ_gal in an impact parameter of 2.5 (proper) Mpc, and calculate the Lyα forest fluctuations δ_<F>_ whose negative values correspond to the strong Lyα forest absorption lines. We identify weak evidence of an anti-correlation between δ_gal and δ_<F>_ with a Spearman's rank correlation coefficient of -0.39 suggesting that the galaxy overdensities and the Lyα forest absorption lines positively correlate in space at the ∼90% confidence level. This positive correlation indicates that high-z galaxies exist around an excess of H I gas in the Lyα forest. We find four cosmic volumes, dubbed A_obs, B_obs, C_obs, and D_obs, that have extremely large (small) values of δ_gal ≃ 0.8 (-1) and δ_<F>_≃0.1(-0.4), three of which, B_obs-D_obs, significantly depart from the δ_gal-δ_<F>_ correlation, and weaken the correlation signal. We perform cosmological hydrodynamical simulations and compare with our observational results. Our simulations reproduce the δ_gal-δ_<F>_ correlation, agreeing with the observational results. Moreover, our simulations have model counterparts of A_obs-D_obs, and suggest that the observations pinpoint, by chance, a galaxy overdensity like a proto-cluster, gas filaments lying on the quasar sightline, a large void, and orthogonal low-density filaments. Our simulations indicate that the significant departures of B_obs-D_obs are produced by the filamentary large-scale structures and the observation sightline effects.