  We combine ultradeep optical spectroscopy from the GMASS project ("Galaxy
Mass Assembly ultradeep Spectroscopic Survey") with GOODS multi-band photometry
and HST imaging to study a sample of passive galaxiesat 1.39<z<1.99 selected at
4.5 microns. A stacked spectrum with an equivalent integration time of ~500
hours was obtained is publicly released. The spectral and photometric SED
properties indicate very weak or absent star formation, moderately old stellar
ages of ~1 Gyr (for solar metallicity) and stellar masses in the range of
10^{10-11} solar masses, thus implying that the major star formation and
assembly processes for these galaxies occurred at z>2. 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 ~1
dex if compared to spheroids at z~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~1, a larger fraction of
passive galaxies follows the z~0 size -- mass relation. Superdense relics with
R_e~1 kpc are extremely rare at z~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.
