Evolution of Cold Streams and Emergence of the Hubble Sequence
  A new physical framework for the emergence of the Hubble sequence is
outlined, based on novel analyses performed to quantify the evolution of cold
streams of a large sample of galaxies from a state-of-the-art ultra-high
resolution, large-scale adaptive mesh-refinement hydrodynamic simulation in a
fully cosmological setting. It is found that the following three key physical
variables of galactic cold inflows crossing the virial sphere substantially
decrease with decreasing redshift: the number of streams N_{90} that make up
90% of concurrent inflow mass flux, average inflow rate per stream dot M_{90}
and mean (mass flux weighted) gas density in the streams n_{gas}. Another key
variable, the stream dimensionless angular momentum parameter lambda, instead
is found to increase with decreasing redshift. Assimilating these trends and
others leads naturally to a physically coherent scenario for the emergence of
the Hubble sequence, including the following expectations: (1) the predominance
of a mixture of disproportionately small irregular and complex disk galaxies at
z>2 when most galaxies have multiple concurrent streams, (2) the beginning of
the appearance of flocculent spirals at z~1-2 when the number of concurrent
streams are about 2-3, (3) the grand-design spiral galaxies appear at z<1 when
galaxies with only one major cold stream significantly emerge. These expected
general trends are in good accord with observations. Early type galaxies are
those that have entered a perennial state of zero cold gas stream, with their
abundance increasing with decreasing redshift.
