The Origin of the Observed Lyman alpha EW Distribution of Dwarf Galaxies at z~2

Abstract

We present a rest-UV selected sample of 32 lensed galaxies at $z\sim 2$ observed with joint Keck/LRIS rest-UV and Keck/MOSFIRE rest-optical spectra behind the clusters Abell 1689, MACS J0717, and MACS J1149. The sample pushes towards the faintest UV luminosities observed ($-19\le {\mathrm{M}}_{\mathrm{UV}}\le -17$) at this redshift. The fraction of dwarf galaxies identified as Ly$\hat{I}\pm$ emitters ($\rm EW \ge 20\ \overset{\lower.5em\circ}{\mathrm{A}}$) is ${\mathrm{X}}_{\mathrm{LAE}}=25_{-10}^{+15}\%$. We use the Balmer lines and UV continuum to estimate the intrinsic EW allowing us to distinguish the effects of the ionizing spectra and Ly$\hat{I}\pm$ escape fraction on the observed EW distribution. Fainter galaxies (${\mathrm{M}}_{\mathrm{UV}}>-19$) show larger intrinsic EWs and escape fractions than brighter galaxies. Only galaxies with intrinsic EWs greater than 40$\ \overset{\lower.5em\circ}{\mathrm{A}}$ have escape fractions larger than 0.05. We find an anti-correlation between the escape fraction and ${\mathrm{A}}_{\mathrm{V}}$ as well as UV spectral slope. The volumetric escape fraction of our sample is $f_{\mathrm{esc}}^{\mathrm{Ly}\hat{\mathrm{I}}\pm }=4.59_{-1.4}^{+2.0}\%$ in agreement with measurements found elsewhere in the literature. About half of the total integrated Ly$\hat{I}\pm$ luminosity density comes from galaxies with ${\rm EW}_{\rm obs}>20\ \overset{\lower.5em\circ}{\mathrm{A}}$.

11 pages, 13 figures