Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy
  Disks and Compact Spheroids
  We present a simple theoretical framework for massive galaxies at high
redshift, where the main assembly and star formation occurred, and report on
the first cosmological simulations that reveal clumpy disks consistent with our
analysis. The evolution is governed by the interplay between smooth and clumpy
cold streams, disk instability, and bulge formation. Intense, relatively smooth
streams maintain an unstable dense gas-rich disk. Instability with high
turbulence and giant clumps, each a few percent of the disk mass, is
self-regulated by gravitational interactions within the disk. The clumps
migrate into a bulge in ~10 dynamical times, or ~0.5Gyr. The cosmological
streams replenish the draining disk and prolong the clumpy phase to several
Gigayears in a steady state, with comparable masses in disk, bulge, and dark
matter within the disk radius. The clumps form stars in dense subclumps
following the overall accretion rate, ~100 Msun/yr, and each clump converts
into stars in ~0.5 Gyr. While the clumps coalesce dissipatively to a compact
bulge, the star-forming disk is extended because the incoming streams keep the
outer disk dense and susceptible to instability and because of angular momentum
transport. Passive spheroid-dominated galaxies form when the streams are more
clumpy: the external clumps merge into a massive bulge and stir up disk
turbulence that stabilize the disk and suppress in situ clump and star
formation. We predict a bimodality in galaxy type by z~3, involving giant-clump
star-forming disks and spheroid-dominated galaxies of suppressed star
formation. After z~1, the disks tend to be stabilized by the dominant stellar
disks and bulges. Most of the high-z massive disks are likely to end up as
today's early-type galaxies.
