The Rest-UV Spectral Properties of Dwarf Galaxies at $z\sim 2$

Abstract

Rest-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 ($⟨\log ({\mathrm{M}}^{\ast }/{\mathrm{M}}_{\odot }){⟩}_{\mathrm{median}}=8.2$) at $z\sim 2$. The rest-UV Keck/LRIS spectroscopy is complemented by rest-optical Keck/MOSFIRE spectroscopy and Hubble photometry. We find generally larger Ly$\alpha$ equivalent widths ($\mathrm{E}{\mathrm{W}}_{\mathrm{Ly}\alpha }=11.2$Å) when compared with higher mass ($⟨\log ({\mathrm{M}}^{\ast }/{\mathrm{M}}_{\odot }){⟩}_{\mathrm{median}}=10.3$) composites from KBSS ($\mathrm{E}{\mathrm{W}}_{\mathrm{Ly}\alpha }=-5$Å). The average low- and high-ionization absorption line EWs ($\mathrm{E}{\mathrm{W}}_{\mathrm{LIS}}$ and $\mathrm{E}{\mathrm{W}}_{\mathrm{HIS}}$, respectively) are weaker ($\mathrm{E}{\mathrm{W}}_{\mathrm{LIS}}$=-1.18 Å, $\mathrm{E}{\mathrm{W}}_{\mathrm{HIS}}=$-0.99 Å) in dwarf galaxies than in higher mass galaxies ($\mathrm{E}{\mathrm{W}}_{\mathrm{LIS}}$=-2.04 Å, $\mathrm{E}{\mathrm{W}}_{\mathrm{HIS}}=$-1.42 Å). The LIS absorption lines are optically thick and is thus a good tracer of the neutral hydrogen covering fraction. Both higher $\mathrm{E}{\mathrm{W}}_{\mathrm{Ly}\alpha }$ and lower $\mathrm{E}{\mathrm{W}}_{\mathrm{LIS}}$ measurements imply that the escape fraction of ionizing radiation is larger in lower-mass galaxies at $z\sim 2$.

9 pages, 8 figures, 1 table. To be published in MNRAS