Dating the stellar population in massive early-type galaxies at $z\sim$1.5

Abstract

We present the analysis of 10 massive early-type galaxies at $z\sim1.5$. 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 M$_{K'}<-26.0$ and stellar masses well in excess of 10$^{11}$ M$_\odot$. Under the hypothesis of pure passive evolution from $z\sim1.5$ to $z=0$, in the local universe they would have luminosities L$_K\ge2.5$L$^*$. Thus, they are the high-z counterparts of the local old massive (10$^{11}-10^{12}$ M$_\odot$) 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 $\sim$3-5 Gyr old and 1-2 Gyr old in the remaining part. These ages imply formation redshift $z_{f} > 2$ for all the galaxies and $z_{f} \geq 4$ for the oldest ones. The comparison of the 4000\AA break and of the overall spectral shape of the average spectrum of the 10 galaxies at $z\sim1.5$ 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.