Cosmic Galaxy-IGM HI Relation at z similar to 2-3 Probed in the COSMOS/UltraVISTA 1.6 Deg(2) 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 ${K}_{s}\lt 23.4$ 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 ${\delta }_{\langle F\rangle }$ whose negative values correspond to the strong Lyα forest absorption lines. We identify weak evidence of an anti-correlation between δ gal and ${\delta }_{\langle F\rangle }$ 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 sime 0.8 (−1) and ${\delta }_{\langle F\rangle }\simeq 0.1(-0.4)$, three of which, B obs–D obs, significantly depart from the δ gal–${\delta }_{\langle F\rangle }$ correlation, and weaken the correlation signal. We perform cosmological hydrodynamical simulations and compare with our observational results. Our simulations reproduce the δ gal–${\delta }_{\langle F\rangle }$ 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