Spiral Arms and a Massive Dust Disk with non-Keplerian Kinematics:
  Possible Evidence for Gravitational Instability in the Disk of Elias 2-27
  To determine the origin of the spiral structure observed in the dust
continuum emission of Elias 2-27 we analyze multi-wavelength continuum ALMA
data with a resolution of $\sim$0.2 arcsec ($\sim$23au) at 0.89, 1.3 and 3.3mm.
We also study the kinematics of the disk with $^{13}$CO and C$^{18}$O ALMA
observations in the $J=$3-2 transition. The spiral arm morphology is recovered
at all wavelengths in the dust continuum observations, where we measure
contrast and spectral index variations along the spiral arms and detect subtle
dust-trapping signatures. We determine that the emission from the midplane is
cold and interpret the optical depth results as signatures of a higher disk
mass than previous constraints. From the gas data, we search for deviations
from Keplerian motion and trace the morphology of the emitting surfaces and the
velocity profiles. We find an azimuthally varying emission layer height in the
system, large-scale emission surrounding the disk, and strong perturbations in
the channel maps, co-located with the spirals. Additionally, we develop
multigrain dust and gas SPH simulations of a gravitationally unstable disk and
compare them to the observations. Given the large scale emission and highly
perturbed gas structure, together with the comparison of continuum observations
to theoretical predictions, we propose infall-triggered gravitational
instabilities as origin for the observed spiral structure.
