Misc,
The CALYMHA survey: Ly$\alpha$ escape fraction and its dependence on galaxy properties at $z=2.23$
(2016)cite arxiv:1602.02756Comment: Accepted for publication by MNRAS.
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
($łambda_c$ = 3918 \AA, $\Deltałambda$= 52 \AA). We find 17 dual
H$\alpha$-Ly$\alpha$ emitters ($f_Ly\alpha >5\times10^-17$ 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$_esc$, measured with 3$"$
apertures) from $2$\%$-30$\%. These galaxies have masses from $3\times10^8$
M$_ødot$ to 10$^11$ M$_ødot$ and dust attenuations E$(B-V)=0-0.5$.
Using stacking, we measure a median escape fraction of $1.6\pm0.5$\%
($4.0\pm1.0$\% without correcting H$\alpha$ for dust), but show that this
depends on galaxy properties. The stacked f$_esc$ tends to decrease with
increasing SFR 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. Ly$\alpha$ is significantly more extended than H$\alpha$
and the UV. f$_esc$ 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$_esc$.
