Cosmic Galaxy-IGM H i Relation at z ∼ 2-3 Probed in the COSMOS/UltraVISTA 1.6 Deg² Field

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 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 whose negative values correspond to the strong Lyα forest absorption lines. We identify weak evidence of an anti-correlation between δ _gal and 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 , three of which, B _obs-D _obs, significantly depart from the δ _gal- correlation, and weaken the correlation signal. We perform cosmological hydrodynamical simulations and compare with our observational results. Our simulations reproduce the δ _gal- 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.