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We present a study of a galaxy overdensity at z=1.6, Cl 0332-2742, uncoveredin the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). Weconservatively 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 dispersionof 500 km/s. A map of the surface density of galaxies at z=1.6 in the GMASSfield shows that its structure is irregular. The differences in the physicalproperties of Cl 0332-2742 member and field galaxies agree with the latesthierarchical galaxy formation models: for overdensity members, the starformation rate (SFR), and specific SFR, is approximately 50% lower than for thefield galaxies; overdensity galaxies are twice the age, on average, of fieldgalaxies; 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 42members, 7 have spectra consistent with being passively evolving, massivegalaxies. In a z-J colour-magnitude diagram, the photometric data of theseearly-type galaxies are in close agreement with a theoretical red sequence of agalaxy cluster at redshift z=1.6, which formed most of its stars in a shortburst of star formation at z~3. We conclude that the redshift spike at z=1.6 inthe GMASS field represents a sheet-like structure in the cosmic web and thearea with the highest surface density within this structure, containing alreadyseven passively evolving galaxies, will evolve into a cluster of galaxies at alater time.

Witnessing the assembly at z=1.6 of a galaxy cluster (GMASS ultradeep spectroscopy of galaxies at z~2 - V)