  The population of compact massive galaxies observed at z > 1 are
hypothesised, both observationally and in simulations, to be merger remnants of
gas-rich disc galaxies. To probe such a scenario we analyse a sample of 12
gas-rich and active star forming sub-mm galaxies (SMGs) at 1.8 < z < 3. We
present a structural and size measurement analysis for all of these objects
using very deep ACS and NICMOS imaging in the GOODS-North field. Our analysis
reveals a heterogeneous mix of morphologies and sizes. We find that four
galaxies (33% \pm 17%) show clear signs of mergers or interactions, which we
classify as early-stage mergers. The remaining galaxies are divided into two
categories: five of them (42% \pm 18%) are diffuse and regular disc-like
objects, while three (25% \pm 14%) are very compact, spheroidal systems. We
argue that these three categories can be accommodated into an evolutionary
sequence, showing the transformation from isolated, gas-rich discs with typical
sizes of 2-3 kpc, into compact (< 1 kpc) galaxies through violent major merger
events, compatible with the scenario depicted by theoretical models. Our
findings that some SMGs are already dense and compact provides strong support
to the idea that SMGs are the precursors of the compact, massive galaxies found
at slightly lower redshift.
