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

Shiro Mukae, Masami Ouchi, Koki Kakiichi, Nao Suzuki, Yoshiaki Ono, Zheng Cai, Akio K. Inoue, Yi Kuan Chiang, Takatoshi Shibuya, Yuichi Matsuda

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16 Citations (Scopus)


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.

Original languageEnglish
Article number281
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 2017 Feb 1
Externally publishedYes


  • galaxies: formation
  • intergalactic medium
  • large-scale structure of universe
  • quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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