Gas Accretion Traced by Blue-dominated Ly<i>α</i> Emission and Redshifted UV Absorption Lines in Protocluster Galaxies at <i>z</i> = 2.3 from the KBSS-KCWI Survey*†

Abstract Ly α emission with a dominant blueshifted peak can probe gas flowing through the circumgalactic medium as it accretes onto galaxies and fuels new star formation, although it has seldom actually been observed. Here we present new Keck Cosmic Web Imager observations of the extended Ly α halos surrounding Q1700-BX710 and Q1700-BX711, a pair of UV continuum-selected Keck Baryonic Structure Survey galaxies at z = 2.3 in the HS1700+643 protocluster. We find that BX710's and BX711’s Ly α halos are aligned with a large-scale galaxy filament consisting of 13 spectroscopically identified protocluster galaxies. By measuring the peak separation and blue-to-red peak flux ratio of the Ly α emission profiles throughout these galaxies’ Ly α halos, we have obtained measurements of their spatially varying velocity structure. The prevalence of blue-dominated Ly α emission profiles throughout BX711’s Ly α halo suggests actively accreting gas. We fit a clumpy, multiphase Monte Carlo Radiative Transfer model, which assumes a radially varying clump velocity to the spatially resolved Ly α emission throughout BX710's and BX711’s Ly α halos and simultaneously fit these galaxies’ average down-the-barrel UV absorption profile with a radially varying velocity model. The results of these models are consistent with a combination of H i and higher-metallicity gas accretion for both galaxies, especially BX711, which exhibits inflow-driven kinematics throughout most of its Ly α halo. We consider various accretion scenarios to explain these findings, including accretion of metal-enriched gas from the cosmic web, galaxy interactions, and recycled gas from the circumgalactic medium, all of which are compatible with our current observations.