Far-infrared observations of dust in Ly$\alpha$ emitters at z=2-6

Abstract

The bright Ly$\alpha$ line is regularly used to identify high-redshift star-forming galaxies known as Ly$\alpha$ emitters (LAEs). However, Ly$\alpha$ is affected by resonant scattering and dust absorption making interpretation of its brightness challenging without additional observations. We use SCUBA-2, PACS and SPIRE data to investigate the far-infrared emission, Ly$\alpha$ escape fraction ($f{esc}$(Ly$\alpha$)) and infrared excess (IRX=LIR/LUV) in $\sim$4000 LAEs at z=2.2-6 from SC4K. Five LAEs, all hosting AGN, are individually detected with fluxes $S_{850}$ = 3.7-5.5 mJy at 850$\mu\mathrm{m}$. Stacking is used to probe the average emission from all individually undetected LAEs, though the stacks are undetected at all wavelengths (e.g. $S_{850}$ < 0.09 mJy; 3$\sigma$). We group the sample into bins of redshift, stellar mass, Ly$\alpha$ luminosity and AGN status. Most subsets are undetected but LAEs containing AGN and that have high stellar masses ($M_{\star} = 10^{10} - 10^{12}\, M_{\odot}$; including and excluding AGN) are detected at most wavelengths, suggesting that stellar mass and AGN heating may be enhancing the dust visibility. Individually detected LAEs and detected stacks have $f{esc}$(Ly$\alpha$)=1-7%, while all undetected stacks $\geq$ 10%. All LAEs together average over > 21% and display significant scatter, suggesting a clumpy ISM dust distribution. Non-zero $f{esc}$(Ly$\alpha$) in massive and AGN-hosting LAEs suggests ionizing photons may escape even from dusty galaxies, challenging the idea that dusty galaxies are poor leakers. Examination of the IRX-$\beta_{UV}$ relation shows LAEs have higher IRX than typical star-forming galaxies at similar redshifts. However, our detections tend to favour more massive, AGN-hosting systems and deeper observations are therefore needed.