2025MNRAS.539...34S

Rest-utraviolet (UV) spectroscopy can constrain properties of the stellar populations, outflows, covering fractions, and can indirectly constrain the Lyman continuum escape fraction of galaxies. Many works have studied the rest-UV spectra of more massive star-forming galaxies and low-mass galaxies selected via strong nebular line emission or via Ly $\alpha$ emission. However, studies of rest-UV spectroscopy have yet to be done on an unbiased sample at low mass during the epoch of peak star formation ($z\sim 2$). We present a stacked rest-UV spectrum of a complete sample of 16 dwarf galaxies ($\rm \langle log({\it M}^{*}/\mathrm{M}_\odot)\rangle _{median} = 8.2$) at $z\sim 2$. The rest-UV Keck Low Resolution Imaging Spectrometer (Keck/LRIS) spectroscopy is complemented by rest-optical Keck/MOSFIRE spectroscopy and Hubble photometry. We find generally larger Ly $\alpha$ equivalent widths ($\rm EW_{Ly\alpha } = 11.2\,\,$ Å) when compared with higher mass ($\rm \langle log({\it M}^{*}/\mathrm{M}_\odot)\rangle _{median} = 10.3$) composites from Keck Baryonic Structure Survey (KBSS, $\rm EW_{Ly\alpha } = -5\,\,$ Å). The average low- and high-ionization absorption line equivalent widths (EWs; $\rm EW_{LIS}$ and $\rm EW_{HIS}$, respectively) are weaker ($\rm EW_{LIS}$ = -1.18 Å, $\rm EW_{HIS}=$ -0.99 Å) in dwarf galaxies than in higher mass galaxies ($\rm EW_{LIS}$ = -2.04 Å, $\rm EW_{HIS}=$ -1.42 Å). The low-ionizatioz absorption lines (LIS) are optically thick and is thus a good tracer of the neutral hydrogen covering fraction. Both higher $\rm EW_{Ly\alpha }$ and lower $\rm EW_{LIS}$ measurements imply that the escape fraction of ionizing radiation is larger in lower mass galaxies at $z\sim 2$.