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

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 𝛼 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 𝛼 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 ∼ 7 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 𝛼 spectroscopy of eight UV-bright (M UV ∼ − 22) galaxies at 𝑧 (cid:39) 7 selected from the ALMA REBELS survey. We detect Ly 𝛼 in 4 of 8 galaxies and use the [CII] systemic redshifts to investigate the Ly 𝛼 velocity profiles. The Ly 𝛼 lines are significantly redshifted from systemic (average velocity offset=223 km/s) and broad (FWHM ≈ 300–650 km/s), with two sources showing emission extending to ≈ 750 km/s. We find that the broadest Ly 𝛼 profiles are associated with the largest [CII] line widths, suggesting a potential link between the Ly 𝛼 FWHM and the dynamical mass. Since Ly 𝛼 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 𝛼 visibility of the most UV-luminous reionization-era galaxies.