2016MNRAS.458..449M

We present the first results from our CAlibrating LYMan α with Hα (CALYMHA) pilot survey at the Isaac Newton Telescope. We measure Lyα emission for 488 Hα selected galaxies at z=2.23 from High-z Emission Line Survey in the COSMOS and UDS fields with a specially designed narrow-band filter (λ_c=3918 Å, Δλ=52 Å). We find 17 dual Hα-Lyα emitters [f_Lyα > 5 x 10^–17 erg/s/cm2, of which five are X-ray active galactic nuclei (AGN)]. For star-forming galaxies, we find a range of Lyα escape fractions (f_esc, measured with 3 arcsec apertures) from 2 to 30 per cent. These galaxies have masses from 3 x 10⁸ M_☉ to 10¹¹ M_☉ and dust attenuations E(B - V)=0-0.5. Using stacking, we measure a median escape fraction of 1.6±0.5 per cent (4.0±1.0 per cent without correcting Hα for dust), but show that this depends on galaxy properties. The stacked f_esc tends to decrease with increasing star formation rate and dust attenuation. However, at the highest masses and dust attenuations, we detect individual galaxies with f_esc much higher than the typical values from stacking, indicating significant scatter in the values of f_esc. Relations between f_esc and UV slope are bimodal, with high f_esc for either the bluest or reddest galaxies. We speculate that this bimodality and large scatter in the values of f_esc is due to additional physical mechanisms such as outflows facilitating f_esc for dusty/massive systems. Lyα is significantly more extended than Hα and the UV. f_esc continues to increase up to at least 20 kpc (3σ, 40 kpc [2σ]) for typical star-forming galaxies and thus the aperture is the most important predictor of f_esc.