Christopher J. Conselice, Alice Mortlock, Asa F. L. Bluck, Ruth Gruetzbauch
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 being measured with increasingly high certainty, the role of gas accretion from the intergalactic medium in triggering star formation still remains largely unknown. We present in this paper indirect evidence for the accretion of gas into massive galaxies with M_* > 10^{11} M_0 at redshifts 1.5 < z < 3 using results from the GOODS NICMOS Survey (GNS). Our method utilises the observed star formation rates of these massive galaxies based on UV and far-infrared observations, and the amount of stellar and gas mass added due to observed major and minor mergers to calculate the evolution of stellar mass in these systems. We show that the measured gas mass fractions 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 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, which is later turned into stars between 1.5 < z < 3.0, is = 83+/-36 M_0/yr. This is similar to what is predicted in detailed simulations of galaxy formation. We show that during this epoch, and for these very massive galaxies, 61+/-21% of stellar assembly is a result of gas accretion, while the remaining ~39% is put into place through mergers. This reveals that for the most massive galaxies at 1.5 < z < 3 gas accretion is the dominant method for instigating galaxy formation.
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http://arxiv.org/abs/1206.6995
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