Lyα emission from galaxies in the Epoch of Reionization

Abstract

The intrinsic strength of the Ly$α$ line in young, star-forming systems makes it a special tool for studying high-redshift galaxies. However, interpreting observations remains challenging due to the complex radiative transfer involved. Here, we combine state-of-the-art hydrodynamical simulations of 'Althaea', a prototypical Lyman Break Galaxy (LBG, stellar mass $M_{\star}$$\simeq$ $10^{10}{\rm M}_{\odot})$ at $z=7.2$, with detailed radiative transfer computations of dust/continuum, [CII] 158 $μ$m, and Ly$α$ to clarify the relation between the galaxy properties and its Ly$α$ emission. Althaea exhibits low ($f_α< 1\%$) Ly$α$ escape fractions and Equivalent Widths, EW $\lesssim 6$ Angstrom for the simulated lines of sight, with a large scatter. The correlation between escape fraction and inclination is weak, as a result of the rather chaotic structure of high-redshift galaxies. Low $f_α$ values persist even if we artificially remove neutral gas around star forming regions to mimick the presence of HII regions. The high attenuation is primarily caused by dust clumps co-located with young stellar clusters. We can turn Althaea into a Lyman Alpha Emitter (LAE) only if we artificially remove dust from the clumps, yielding EWs up to $22$ Angstrom. Our study suggests that the LBG-LAE duty-cycle required by recent clustering measurements poses the challenging problem of a dynamically changing dust attenuation. Finally, we find an anti-correlation between the magnitude of Ly$α$-[CII] line velocity shift and Ly$α$ luminosity.

published in MNRAS