Witnessing the assembly at z=1.6 of a galaxy cluster (GMASS ultradeep
  spectroscopy of galaxies at z~2 - V)
  We present a study of a galaxy overdensity at z=1.6, Cl 0332-2742, uncovered
in the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). We
conservatively estimate the overdensity in redshift space for the spike,
containing 42 galaxies at z=1.6, to be 8.3+/-1.5 and find a velocity dispersion
of 500 km/s. A map of the surface density of galaxies at z=1.6 in the GMASS
field shows that its structure is irregular. The differences in the physical
properties of Cl 0332-2742 member and field galaxies agree with the latest
hierarchical galaxy formation models: for overdensity members, the star
formation rate (SFR), and specific SFR, is approximately 50% lower than for the
field galaxies; overdensity galaxies are twice the age, on average, of field
galaxies; and there is a higher proportion of both massive (M>10^{10.7} M_sol),
and early-type galaxies, inside Cl 0332-2742 than in the field. Among the 42
members, 7 have spectra consistent with being passively evolving, massive
galaxies. In a z-J colour-magnitude diagram, the photometric data of these
early-type galaxies are in close agreement with a theoretical red sequence of a
galaxy cluster at redshift z=1.6, which formed most of its stars in a short
burst of star formation at z~3. We conclude that the redshift spike at z=1.6 in
the GMASS field represents a sheet-like structure in the cosmic web and the
area with the highest surface density within this structure, containing already
seven passively evolving galaxies, will evolve into a cluster of galaxies at a
later time.
