Galaxy accretion by cold gas streams is a way to feed gas into the disks fast enough to produce wild instabilities and clump formation. This can all happen without galaxy mergers, except for some small galaxy-like pieces that come in with the cold flows. The recognition of cold flows is a major change in thinking about how galaxies form (Murali et al. 2002, Birnboim & Dekel 2003, Semelin & Combes 2005, Dekel & Birnboim 2006, Ocvirk et al. 2008, Dekel et al. 2009a, Dekel et al. 2009b, Agertz et al. 2009, Keres et al. 2005, Keres et al. 2009, Brooks et al. 2009). Hierarchical build-up models in the cold dark matter scenario may not apply to baryons as much as they apply to cold dark matter itself. The baryons may enter a galaxy in the form of cold flows, rather than minor and major mergers of component galaxies, each with their own dark matter halo.
Ceverino, Dekel & Bournaud (2010) modeled cold and hot flows with a disk galaxy forming in the center. The model is appropriate for a redshift of z = 2.3. They follow the formation and evolution of individual clumps in the disk gas, showing how the accretion quickly makes an unstable gas disk, which forms giant clumps that migrate to the center.