The MOSDEF-LRIS Survey: Probing the ISM/CGM Structure of Star-forming Galaxies at z ∼ 2 Using Rest-UV Spectroscopy

Abstract

Abstract The complex structure of gas, metals, and dust in the interstellar and circumgalactic medium (ISM and CGM, respectively) in star-forming galaxies can be probed by Ly α emission and absorption, low-ionization interstellar (LIS) metal absorption, and dust reddening E ( B − V ). We present a statistical analysis of the mutual correlations among Ly α equivalent width (EW Ly α ), LIS equivalent width (EW LIS ), and E ( B − V ) in a sample of 157 star-forming galaxies at z ∼ 2.3. With measurements obtained from individual deep rest-UV spectra and spectral energy distribution modeling, we find that the tightest correlation exists between EW LIS and E ( B − V ), although correlations among all three parameters are statistically significant. These results signal a direct connection between dust and metal-enriched H i gas and that they are likely cospatial. By comparing our results with the predictions of different ISM/CGM models, we favor a dusty ISM/CGM model where dust resides in H i gas clumps and Ly α photons escape through the low H i covering fraction/column density intraclump medium. Finally, we investigate the factors that potentially contribute to the intrinsic scatter in the correlations studied in this work, including metallicity, outflow kinematics, Ly α production efficiency, and slit loss. Specifically, we find evidence that scatter in the relationship between EW Ly α and E ( B − V ) reflects the variation in the metal–to–H i covering fraction ratio as a function of metallicity and the effects of outflows on the porosity of the ISM/CGM. Future simulations incorporating star formation feedback and the radiative transfer of Ly α photons will provide key constraints on the spatial distributions of neutral hydrogen gas and dust in the ISM/CGM structure.