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 $K_s<23.4$ 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 ${\delta }_{⟨F⟩}$ whose negative values correspond to the strong Lyα forest absorption lines. We identify weak evidence of an anti-correlation between δ _gal and ${\delta }_{⟨F⟩}$ 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 ${\delta }_{⟨F⟩}\simeq 0.1(-0.4)$, three of which, B _obs–D _obs, significantly depart from the δ _gal–${\delta }_{⟨F⟩}$ correlation, and weaken the correlation signal. We perform cosmological hydrodynamical simulations and compare with our observational results. Our simulations reproduce the δ _gal–${\delta }_{⟨F⟩}$ 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.