VLT/X-Shooter Near-infrared Spectroscopy and HST Imaging of Gravitationally Lensed z ~ 2 Compact Quiescent Galaxies
Résumé
Quiescent massive galaxies at z ~ 2 are thought to be the progenitors of present-day massive ellipticals. Observations revealed them to be extraordinarily compact. Until now, the determination of stellar ages, star formation rates, and dust properties via spectroscopic measurements has been feasible only for the most luminous and massive specimens (~3 × Msstarf). Here we present a spectroscopic study of two near-infrared-selected galaxies that are close to the characteristic stellar mass Msstarf (~0.9 × Msstarf and ~1.3 × Msstarf) and whose observed brightness has been boosted by the gravitational lensing effect. We measure the redshifts of the two galaxies to be z = 1.71 ± 0.02 and z = 2.15 ± 0.01. By fitting stellar population synthesis models to their spectrophotometric spectral energy distributions we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr and 1.7 ± 0.3 Gyr, respectively, which implies that the two galaxies have higher mass-to-light ratios than most quiescent z ~ 2 galaxies in other studies. We find no direct evidence for active star formation or active galactic nucleus activity in either of the two galaxies, based on the non-detection of emission lines. Based on the derived redshifts and stellar ages we estimate the formation redshifts to be z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased spatial resolution due to the gravitational lensing to derive constraints on the morphology. Fitting Sérsic profiles to the de-lensed images of the two galaxies confirms their compactness, with one of them being spheroid-like and the other providing the first confirmation of a passive lenticular galaxy at a spectroscopically derived redshift of z ~ 2. Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile, under programs 087.B-0812 (PI: Toft) and 073.A-0537 (PI: Kneib).