The ALMA REBELS Survey: Efficient Ly$α$ Transmission of UV-Bright z$\simeq$7 Galaxies from Large Velocity Offsets and Broad Line Widths

Ryan Endsley, Daniel P. Stark, Rychard J. Bouwens, Sander Schouws, Renske Smit, Mauro Stefanon, Hanae Inami, Rebecca A. A. Bowler, Pascal Oesch, Valentino Gonzalez, Manuel Aravena, Elisabete da Cunha, Pratika Dayal, Andrea Ferrara, Luca Graziani, Themiya Nanayakkara, Andrea Pallottini, Raffaella Schneider, Laura Sommovigo, Michael Topping, Paul van der Werf, Anne Hutter

Published 2022-02-02Version 1

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly$\alpha$ transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly$\alpha$ profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at $z\sim7$ remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [CII]-based redshifts from ALMA. Here we report MMT/Binospec Ly$\alpha$ spectroscopy of eight UV-bright ($\mathrm{M_{UV}}^{}\sim-22$) galaxies at $z\simeq7$ selected from the ALMA REBELS survey. We detect Ly$\alpha$ in 4 of 8 galaxies and use the [CII] systemic redshifts to investigate the Ly$\alpha$ velocity profiles. The Ly$\alpha$ lines are significantly redshifted from systemic (average velocity offset=223 km/s) and broad (FWHM$\approx$300$-$650 km/s), with two sources showing emission extending to $\approx$750 km/s. We find that the broadest Ly$\alpha$ profiles are associated with the largest [CII] line widths, suggesting a potential link between the Ly$\alpha$ FWHM and the dynamical mass. Since Ly$\alpha$ photons at high velocities transmit efficiently through the $z=7$ IGM, our data suggest that velocity profiles play a significant role in boosting the Ly$\alpha$ visibility of the most UV-luminous reionization-era galaxies.