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

Abstract

We present spatial correlations of galaxies and IGM neutral hydrogen H i in the COSMOS/UltraVISTA 1.62 deg 2 field. Our data consist of 13,415 photo- z galaxies at z  ∼ 2–3 with and the Ly α forest absorption lines in the background quasar spectra selected from SDSS data with no signature of damped Ly α system contamination. We estimate a galaxy overdensity δ gal in an impact parameter of 2.5 (proper) Mpc, and calculate the Ly α forest fluctuations whose negative values correspond to the strong Ly α 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 Ly α 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 Ly α 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.