The ALMA REBELS Survey: Efficient Ly$\alpha$ Transmission of UV-Bright
  z$\simeq$7 Galaxies from Large Velocity Offsets and Broad Line Widths
  Recent work has shown that UV-luminous reionization-era galaxies often
exhibit strong Lyman-alpha emission despite being situated at redshifts where
the IGM is thought to be substantially neutral. It has been argued that this
enhanced Ly$\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.
