High Velocity Dispersion in A Rare Grand Design Spiral Galaxy at
  Redshift z=2.18
  Although relatively common in the local Universe, only one grand-design
spiral galaxy has been spectroscopically confirmed to lie at z>2 (HDFX 28;
z=2.011), and may prove to be a major merger that simply resembles a spiral in
projection. The rarity of spirals has been explained as a result of disks being
dynamically 'hot' at z>2 which may instead favor the formation of
commonly-observed clumpy structures. Alternatively, current instrumentation may
simply not be sensitive enough to detect spiral structures comparable to those
in the modern Universe. At redshifts <2, the velocity dispersion of disks
decreases, and spiral galaxies are more numerous by z~1. Here we report
observations of the grand design spiral galaxy Q2343-BX442 at z=2.18.
Spectroscopy of ionized gas shows that the disk is dynamically hot, implying an
uncertain origin for the spiral structure. The kinematics of the galaxy are
consistent with a thick disk undergoing a minor merger, which can drive the
formation of short-lived spiral structure. A duty cycle of < 100 Myr for such
tidally-induced spiral structure in a hot massive disk is consistent with their
rarity.
