UC Riverside

Galaxy evolution is concerned with the mechanisms that determine the distributions of gas, dust, and metals, and with the efficiency with which galaxies produce stars. These components are the primary visible building blocks of a galaxy. Throughout cosmic history the escape of ionizing radiation from a galaxy out into the intergalactic medium has been a notoriously difficult measurement to make, and one that informs the ionizing background radiation. This is relevant for the development of galaxies generally, but also plays a significant role in understanding the last great phase change of the universe, reionization ($z\sim6$). It is unclear whether it is massive galaxies or dwarf galaxies which are able to expel sufficient amounts of ionizing radiation to ionize neutral hydrogen in the intergalactic medium during this epoch of the universe. Any attempts to measure the ionizing escape fraction during reionization is untenable even for newer instruments like the James Webb space telescope due to the neutral intergalactic medium obscuring large amounts of ionizing radiation. As such, lower redshift galaxies are studied both for the sake of constraining the ionizing background throughout cosmic history and as benchmarks for understanding reionization. Often, studies will select galaxies based on their Ly$\alpha$ emission as it has a possible connection with Lyman continuum escape. However, since Ly$\alpha$ is a resonant line transition it is absorbed and scattered by even small amounts of neutral hydrogen. Because of this, it is unclear what galaxy properties may be correlated with Ly$\alpha$ escape. To understand the biases behind selecting on Ly$\alpha$ we need to better understand what causes Ly$\alpha$ to escape and the distribution of Ly$\alpha$ properties in galaxy samples. Ly$\alpha$ also serves as a proxy of the neutral fraction in the intergalactic medium at high redshift. Finally, outflows and feedback are best understood by comparing empirical measures of outflows and covering fractions with models from simulations. These models can be better constrained when including empirical markers in the dwarf regime. Many studies have looked into these measurements at higher masses ($\rm log(M^*/M_\odot)>9$), or for samples biased towards strong line emitters. There has yet to be a complete sample of low mass galaxies studied at $z\sim2$.

In this work we present a sample of 32 galaxies at $z\sim2$ during the peak of star-formation in the history of the universe. We focus on low-UV luminosity ($\rm M_{UV} > -19$) and low mass ($\rm log(M^*/M_\odot)<9$) sub-samples of this parent sample. A unique characteristic of this sample is the complete spectroscopy of the galaxies. Each galaxy has rest-UV and rest-optical spectra from the Keck/LRIS and keck/MOSFIRE instruments respectively. This allows us to confirm the redshift of galaxies for which we do not observe any apparent spectral features in the rest-UV spectra. This, along with improved S/N from gravitational lensing, allows us to to produce a complete sample of dwarf galaxies from which to measure proxies of the ionizing escape fraction of galaxies. The primary proxies used in the literature and in this work are the Ly$\alpha$ EW, the Ly$\alpha$ escape fraction, and the low-ionization interstellar absorption line EW \citep{Saldana-Lopez2023}.

The Ly$\alpha$ EW we measure for individual galaxies and find that generally dwarf galaxies tend to have Ly$\alpha$ in emission in contrast to higher mass samples which show Ly$\alpha$ EWs have Ly$\alpha$ in absorption on average \citep{Du2018}. The volumetric escape fraction of dwarf galaxies at $z\sim2$ is consistent with the literature value of $5\%$ \citep{Hayes2010,Ciardullo2014,Sobral2017,Weiss2021}, placing an upper limit on the volumetric escape fraction of ionizing photons. The LIS absorption lines show about a factor of 2 weaker EWs for low mass galaxies when compared with higher mass samples \citep{jones2012,Du2018}. A lower EW of LIS absorption lines implies a lower covering fraction of neutral hydrogen and therefore greater avenues of escape on average for ionizing photons. Collectively, our measurements imply greater amounts of ionizing radiation escaping from dwarf galaxies on average when compared with higher mass samples of star-forming galaxies.