2020ApJ...896...45M

We present an IGM H I tomographic map in a survey volume of 16×19×131 h^–3 comoving Mpc³ (cMpc³) centered at MAMMOTH-1 nebula and three neighboring quasars at z = 2.3. The MAMMOTH-1 nebula is an enormous Lyα nebula (ELAN), hosted by a type-II quasar dubbed MAMMOTH1-QSO, that extends over 1 h^–1 cMpc with no clear physical origin. Here we investigate the H I-gas distribution around MAMMOTH1-QSO with the ELAN and three neighboring type-I quasars, making the IGM H I tomographic map with a spatial resolution of 2.6 h^–1 cMpc. Our H I tomographic map is reconstructed with H I Lyα forest absorption of bright background objects at z = 2.4-2.9: one eBOSS quasar and 16 Keck/LRIS galaxy spectra. We estimate the radial profile of H I overdensity for MAMMOTH1-QSO, and find that MAMMOTH1-QSO resides in a volume with fairly weak H I absorption. This suggests that MAMMOTH1-QSO may have a proximity zone where quasar illuminates and photoionizes the surrounding H I gas and suppresses H I absorption, and that the ELAN is probably a photoionized cloud embedded in the cosmic web. The H I radial profile of MAMMOTH1-QSO is very similar to those of three neighboring type-I quasars at z = 2.3, which is compatible with the AGN unification model. We compare the distributions of the H I absorption and star-forming galaxies in our survey volume, and identify a spatial offset between density peaks of star-forming galaxies and H I gas. This segregation may suggest anisotropic UV background radiation created by star-forming galaxy density fluctuations.