Cosmic Galaxy-IGM HI Relation at z 2-3 Probed in the COSMOS/UltraVISTA 1.6 deg² Field

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