  One of the main questions of galaxy formation is how and when galaxies
assembled their mass. Observations suggest that most mass assembly and star
formation occured at 1 < z < 3. However, in the same redshift range, fully
assembled old and massive galaxies co-exist with star-bursting systems still in
their assembling stage. In order to explore galaxy evolution in this critical
redshift range, we started a project called GMASS (Galaxy Mass Assembly
ultra-deep Spectroscopic Survey) based on an 145 hours ESO Large Program and
aimed at doing ultradeep spectroscopy with the VLT of a sample of high redshift
galaxies slected with IRAC/Spitzer m(4.5mu) < 23, z(phot) > 1.4) from the
GOODS-South/HUDF region. The selection at 4.5 micron is crucial to select high
redshift galaxies based on the stellar mass or objects affected by strong dust
extinction. The spectroscopic deep integration times (15 - 50 hours) are needed
to identify unambiguously the redshift and nature of these galaxies, which
redshifts are mostly in the "desert" where spectroscopy is traditionally
difficult due to the absence of strong emission lines at optical wavelengths.
The survey is almost complete and some preliminary results are presented here.
