The Onset of Spiral Structure in the Universe
  The onset of spiral structure in galaxies appears to occur between redshifts
1.4 and 1.8 when disks have developed a cool stellar component, rotation
dominates over turbulent motions in the gas, and massive clumps become less
frequent. During the transition from clumpy to spiral disks, two unusual types
of spirals are found in the Hubble Ultra Deep Field that are massive, clumpy
and irregular like their predecessor clumpy disks, yet spiral-like or sheared
like their descendants. One type is ``woolly'' with massive clumpy arms all
over the disk and is brighter than other disk galaxies at the same redshift,
while another type has irregular multiple arms with high pitch angles, star
formation knots and no inner symmetry like today's multiple-arm galaxies. The
common types of spirals seen locally are also present in a redshift range
around z~1, namely grand design with two symmetric arms, multiple arm with
symmetry in the inner parts and several long, thin arms in the outer parts, and
flocculent, with short, irregular and patchy arms that are mostly from star
formation. Normal multiple arm galaxies are found only closer than z~0.6 in the
UDF. Grand design galaxies extend furthest to z~1.8, presumably because
interactions can drive a two-arm spiral in a disk that would otherwise have a
more irregular structure. The difference between these types is understandable
in terms of the usual stability parameters for gas and stars, and the ratio of
the velocity dispersion to rotation speed.
