Speaker
Description
Young star-forming galaxies are believed to be the main contributors to the process of cosmic reionization which completed roughly $1$ billion years after the big bang, at a redshift $z\sim6$. However, it is unclear whether such galaxies actually had sufficient ionizing escape fractions, as direct measurements are impossible due to the neutral intergalactic medium at this epoch. Indirect measurements of the HI gas covering fraction via low-ionization interstellar (LIS) absorption lines offers a promising means to constrain the escape fraction. While reionization-era galaxies are typically too faint for absorption line measurements, we use stacking analysis of JWST/NIRSpec data to determine the LIS absorption properties of galaxies at $z>6$, representing the first such measurements in the epoch of reionization. We perform a comprehensive spectral stacking analysis on $> 50$ galaxies at mean $z=7.48$ to measure equivalent widths (EWs) of LIS absorption features as well as Ly$\alpha$. We infer constraints on evolution in the covering fraction of HI, and hence the ionizing escape fraction, based on the LIS measurements from our stacks. Our results also reveal the unambiguous presence of strong outflows in these early galaxies, with composite spectra exhibiting blueshifted LIS features with redshifted Ly$\alpha$ emission, which has important implications for the gas content and star formation burstiness in early galaxies. I will also discuss complementary measurements of the interstellar gas content of faint $z\sim7$ galaxies from ALMA observations. Collectively, our results constrain both the escape of ionizing photons and the nature of feedback at $z>6$, giving insight into the properties of early galaxies and their role in cosmic reionization.
