The origin of the observed Ly α EW distribution of dwarf galaxies at z ∼ 2

Abstract

ABSTRACT We present a rest-UV-selected sample of 32 lensed galaxies at z ∼ 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 ($-19 \le {M_{\rm UV}} \le -17$) at this redshift. The fraction of dwarf galaxies identified as Ly α emitters ($\rm EW \ge 20$ Å) is ${X_{\rm LAE}}=33^{+17}_{-12}\,{\rm per\, cent}$. We use the Balmer lines and UV continuum to estimate both the intrinsic EW and the Ly α escape fraction to determine their relative importance in dictating the observed EW distribution. Fainter galaxies (MUV > −19) show larger intrinsic EWs and escape fractions than brighter galaxies. Only galaxies with intrinsic EWs greater than 40 Å have escape fractions larger than 5  per cent. We find little to no anticorrelation between the escape fraction and dust in dwarf galaxies. The volumetric escape fraction of our sample is $f_{\rm esc}^{\rm Ly\alpha } = 4.6^{+2.0}_{-1.4}\,{\rm per\, cent}$ in agreement with measurements found elsewhere in the literature. 60  per cent of the total integrated Ly α luminosity density comes from galaxies with EWobs > 20 Å.