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

We present an analysis of 10 massive early-type galaxies at z similar to 1.5. They have been identified by means of a near-infrared (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 Cluster Survey (MUNICS) of Drory et al. 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 circle dot. Under the hypothesis of pure passive evolution from z similar to 1.5 to z = 0, in the Local Universe they would have luminosities L-K >= 2.5L*. Thus, they are the high-z counterparts of the local old massive (10(11)-10(12)M circle dot) early-type galaxies populating the bright end of the local luminosity function of galaxies. The comparison of their spectrophotometric properties with a grid of synthetic models suggests that the stellar populations in more than half of the sample are about similar to 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) >= 4 for the oldest ones. The comparison of the 4000-angstrom break and of the overall spectral shape of the average spectrum of the 10 galaxies at z similar to 1.5 with those of their local counterparts confirms 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 spectrophotometric properties.