Dating the stellar population in massive early-type galaxies atz∼ 1.5

We present the analysis of 10 massive early-type galaxies at z~1.5. They have been identified by means of a near-IR low resolution spectroscopic follow-up of a complete sample of 36 bright (K' $<$ 18.5) Extremely Red Objects (EROs, R-K'$>$ 5) selected from the Munich Near-IR ClusterSurvey (MUNICS; Drory et al. 2001). The low resolution near-IR spectra constrain their redshift at $1.2<z<1.7$, implying absolute magnitudes MK'<-26.0 and stellar masses well in excess of 10¹¹ M_. Under the hypothesis of pure passive evolution from z~1.5 to $z=0$, in the local universe they would have luminosities LK≥2.5L^*. Thus, they are the high-z counterparts of the local old massive (10¹¹-10¹² M_) early-type galaxies populating the bright end of the local luminosity function of galaxies. The comparison of their spectro-photometric properties with a grid of synthetic models suggests that the stellar populations in more than half of the sample are about ~3-5 Gyr old and 1-2 Gyr old in the remaining part. These ages imply formation redshift zf > 2 for all the galaxies and zf ≥ 4 for the oldest ones. The comparison of the 4000 break and of the overall spectral shape of the average spectrum of the 10 galaxies at z~1.5 with those of their local counterpartsconfirms that field massive early-type galaxies formed the bulk of their stellar mass at $2<z<4$, most likely over a short ($<$ 1 Gyr) star formation time scale, consistently with the results derived from the analysis of their individual spectro-photometric properties.