GMASS ultradeep spectroscopy of galaxies at z similar to 2 - II. Superdense passive galaxies: how did they form and evolve?

Aims. The aim of this work is to investigate the physical, structural and evolutionary properties of old, passive galaxies at z > 1.4 and to place new constraints on massive galaxy formation and evolution. Methods. We combine ultradeep optical spectroscopy from the GMASS project (Galaxy Mass Assembly ultradeep Spectroscopic Survey) with GOODS multi-band (optical to mid-infrared) photometry and HST imaging to study a sample of spectroscopically identified passive galaxies at 1.39 < z < 1.99 selected from Spitzer Space Selescope imaging at 4.5 mu m. Results. A stacked spectrum with an equivalent integration time of similar to 500 h was obtained and compared with libraries of synthetic stellar population spectra. The stacked spectrum is publicly released. The spectral and photometric SED properties indicate very weak or absent star formation, moderately old stellar ages of approximate to 1 Gyr (for solar metallicity) and stellar masses in the range of 10(10-11) M-circle dot, thus implying that the major star formation and assembly processes for these galaxies occurred at z > 2. No X-ray emission was found neither from individual galaxies nor from a stacking analysis of the sample. Only one galaxy shows a marginal detection at 24 mu m. These galaxies have morphologies that are predominantly compact and spheroidal. However, their sizes (R-e <= 1 kpc) are much smaller than those of spheroids in the present-day Universe. Their stellar mass surface densities are consequently higher by approximate to 1 dex if compared to spheroids at z approximate to 0 with the same mass. Their rest-frame B-band surface brightness scales with the effective radius, but the offset with respect to the surface brightness of the local Kormendy relation is too large to be explained by simple passive evolution. At z approximate to 1, a larger fraction of passive galaxies follows the z approximate to 0 size-mass relation. Superdense relics with R-e approximate to 1 kpc are extremely rare at z approximate to 0 with respect to z > 1, and absent if R-e < 1 kpc. Because of the similar sizes and mass densities, we suggest that the superdense passive galaxies at 1 < z < 2 are the remnants of the powerful starbursts occurring in submillimeter-selected galaxies at z > 2. The results are compared with theoretical models and the main implications discussed in the framework of massive galaxy formation and evolution.