The CALYMHA survey: Ly$\alpha$ escape fraction and its dependence on galaxy properties at $z=2.23$

Abstract

We present the first results from our CAlibrating LYMan-$\alpha$ with H$\alpha$ (CALYMHA) pilot survey at the Isaac Newton Telescope. We measure Ly$\alpha$ emission for 488 H$\alpha$ selected galaxies at $z=2.23$ from HiZELS in the COSMOS and UDS fields with a specially designed narrow-band filter (${\lambda }_c$ = 3918 Å, $\Delta \lambda$= 52 Å). We find 17 dual H$\alpha$-Ly$\alpha$ emitters ($f_{\mathrm{Ly}\alpha }>5\times 10^{-17}$ erg s${}^{-1}$ cm${}^{-2}$, of which 5 are X-ray AGN). For star-forming galaxies, we find a range of Ly$\alpha$ escape fractions (f${}_{\mathrm{esc}}$, measured with 3$"$ apertures) from $2$\%$-30$\%. These galaxies have masses from $3\times 10^8$ M${}_{\odot }$ to 10${}^{11}$ M${}_{\odot }$ and dust attenuations E$(B-V)=0-0.5$. Using stacking, we measure a median escape fraction of $1.6\pm 0.5$\% ($4.0\pm 1.0$\% without correcting H$\alpha$ for dust), but show that this depends on galaxy properties. The stacked f${}_{\mathrm{esc}}$ tends to decrease with increasing SFR and dust attenuation. However, at the highest masses and dust attenuations, we detect individual galaxies with f${}_{\mathrm{esc}}$ much higher than the typical values from stacking, indicating significant scatter in the values of f${}_{\mathrm{esc}}$. Relations between f${}_{\mathrm{esc}}$ and UV slope are bimodal, with high f${}_{\mathrm{esc}}$ for either the bluest or reddest galaxies. We speculate that this bimodality and large scatter in the values of f${}_{\mathrm{esc}}$ is due to additional physical mechanisms such as outflows facilitating f${}_{\mathrm{esc}}$ for dusty/massive systems. Ly$\alpha$ is significantly more extended than H$\alpha$ and the UV. f${}_{\mathrm{esc}}$ continues to increase up to at least 20 kpc (3$\sigma$, 40 kpc [2$\sigma$]) for typical SFGs and thus the aperture is the most important predictor of f${}_{\mathrm{esc}}$.

Accepted for publication by MNRAS

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