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

Abstract

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 mum. Results: A stacked spectrum with an equivalent integration time of ~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 ≈1 Gyr (for solar metallicity) and stellar masses in the range of 1010-11 Mo, 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 mum. 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 Re < 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. Based on the ESO VLT Large Program 173.A-0687.