D'Eugenio, C.C.D'EugenioDaddi, E.E.DaddiGobat, R.R.GobatSTRAZZULLO, VERONICAVERONICASTRAZZULLOLustig, P.P.LustigDELVECCHIO, IVANIVANDELVECCHIOJin, S.S.JinPuglisi, A.A.PuglisiCALABRO, ANTONELLOANTONELLOCALABROMANCINI, CHIARACHIARAMANCINIDickinson, M.M.DickinsonCimatti, A.A.CimattiOnodera, M.M.Onodera2021-11-302021-11-3020202041-8205http://hdl.handle.net/20.500.12386/31186We have obtained spectroscopic confirmation with Hubble Space Telescope WFC3/G141 of a first sizeable sample of nine quiescent galaxies at 2.4 < z < 3.3. Their average near-UV/optical rest-frame spectrum is characterized by low attenuation (A<SUB>V</SUB> ˜ 0.6 mag) and a strong Balmer break, larger than the 4000 Å break, corresponding to a fairly young age of ˜300 Myr. This formally classifies a substantial fraction of classically selected quiescent galaxies at z ˜ 3 as post-starbursts, marking their convergence to the quenching epoch. The rapid spectral evolution with respect to z ˜ 1.5 quiescent galaxies is not matched by an increase of residual star formation, as judged from the weak detection of [O II]λ3727 emission, pointing to a flattening of the steep increase in gas fractions previously seen from z ˜ 0 to 1.8. However, radio 3 GHz stacked emission implies either much stronger dust-obscured star formation or substantial further evolution in radio-mode AGN activity with respect to z ˜ 1.5.STAMPAenArticle10.3847/2041-8213/ab7a96https://iopscience.iop.org/article/10.3847/2041-8213/ab7a962020ApJ...892L...2DFIS/05 - ASTRONOMIA E ASTROFISICA