Quiescent Galaxies at z ≥ 2.5: Observations versus Models

The presence of massive quiescent galaxies at high redshifts is still a challenge for most models of galaxy formation. The aim of this work is to compare the observed number density and properties of these galaxies with the predictions of state-of-the-art models. The sample of massive quiescent galaxies has been selected from the COSMOS2015 photometric catalog with z(phot) >= 2.5, log(M-*/M-circle dot) >= 10.5, and a specific star formation rate (sSFR) of log(sSFR [yr(-1)]) <= -11. The photometric spectral energy distributions (SEDs) of the selected galaxies have been thoroughly analyzed based on different stellar population synthesis models. The final sample includes only those galaxies qualified as quiescent in all SED fitting runs. The observed properties have been compared to theoretical models: the number density of quiescent galaxies with 10.5 <= log(M-*/M-circle dot) < 10.8 is reproduced by some models, although there is a large scatter in their predictions. Instead, very massive log(M-*/M-circle dot) >= 10.8 are underpredicted by most of the current models of galaxy formation: some of them, built on the CARNage simulation, are consistent with data up to z similar to 4, while at higher redshifts the volume of the considered simulation is too small to find such rare objects. Simulated galaxies that match the observed properties in the sSFR-M-* plane at z similar to 3 have been analyzed by reconstructing their evolutionary paths: their merger trees suggest that AGN feedback could be the key process allowing for a rapid quenching of the star formation at z greater than or similar to 4 and that its treatment should be improved in models.