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C.J. Conselice (STScI), M.A. Bershady (U. Wisconsin-Madison), M Dickinson (STScI)
It is currently not understood how and when large galaxies form. Is there an era of rapid assembly followed by passive evolution, or do large galaxies assemble gradually by merging throughout the history of the universe? By using a physical morphological system on NIR and optical images of galaxies in the Hubble Deep Field North, we show that in the past a higher number of galaxies are undergoing mergers than in the nearby universe. We can characterize this merger fraction evolution as (1+z)2.3+/-0.2 out to z ~ 3.5. The increase of mergers with redshift is broadly consistent with hierarchical galaxy assembly models, and can potentially account for a host of other galaxy evolution phenomenon.