Gas Accretion as a Dominant Formation Mode in Massive Galaxies from the
  GOODS NICMOS Survey
  The ability to resolve all processes which drive galaxy formation is one of
the most fundamental goals in extragalactic astronomy. While star formation
rates and the merger history are now measured with increasingly high certainty,
the role of gas accretion from the intergalactic medium in supplying gas for
star formation still remains largely unknown. We present in this paper indirect
evidence for the accretion of gas into massive galaxies with initial stellar
masses M_*>10^{11} M_sol and following the same merger adjusted co-moving
number density at lower redshifts during the epoch 1.5 < z < 3, using results
from the GOODS NICMOS Survey (GNS). We show that the measured gas mass
fractions of these massive galaxies are inconsistent with the observed star
formation history for the same galaxy population. We further demonstrate that
this additional gas mass cannot be accounted for by cold gas delivered through
minor and major mergers. We also consider the effects of gas outflows and gas
recycling due to stellar evolution in these calculations. We argue that to
sustain star formation at the observed rates there must be additional methods
for increasing the cold gas mass, and that the likeliest method for
establishing this supply of gas is by accretion from the intergalactic medium.
We calculate that the average gas mass accretion rate into these massive
galaxies between 1.5 < z < 3.0, is \dot{M} = 96+/-19 M_sol/yr after accounting
for outflowing gas. We show that during this epoch, and for these very massive
galaxies, 49+/-20% of baryonic mass assembly is a result of gas accretion and
unresolved mergers. However, 66+/-20% of all star formation in this epoch is
the result of gas accretion. This reveals that for the most massive galaxies at
1.5< z< 3 gas accretion is the dominant method for instigating new stellar mass
assembly.
