Reconstructing the star formation histories of quiescent galaxies at z>1

A crucial issue in modern astrophysics is to investigate how galaxy mass assembly took place in the early Universe and how it is interlinked with the evolution of galaxy star formation over cosmic time. To address these questions , this thesis work adopts a look-back approach deriving the SFHs and the stellar population properties of a sample of 23 Early-Type Galaxies, with moderate-to-high masses, log(Mstar/Msol) > 10.4, at 1 < z < 2. The sample is provided by VANDELS, an on-going ESO's VLT spectroscopic survey, performed on the CANDELS UDS and CDFS fields (PIs R. McLure and L.Pentericci). The long integration times and the VIMOS multi-object spectrograph provided high signal-to-noise spectra with a mean spectral resolution R= 580. This work is focused on the spectral analysis of the first 23 UVJ-selected quiescent galaxies observed within the survey, exploiting their optical spectra (0.48-1.0 μm, rest-frame UV). The fitting procedure is carried out by means of the public code STARLIGHT (Cid Fernandes et al. 2007), which allows a decomposition of the observed spectra into contributions from simple stellar populations. These are selected from two sets of evolutionary synthesis models at various ages and metallicities, Maraston & Strömbäck (2011) and Bruzual & Charlot (2003) respectively. We aimed at retrieving the mass- and light-weighted ages of our sample galaxies, their metallicities, dust extinction and velocity dispersions. We first tested the reliability of STARLIGHT in retrieving these outcomes by simulating spectra of increasing complexity as a function of different signal-to-noise ratios. Thereafter we studied and interpreted the retrieved properties of our sample galaxies. The reconstructed SFHs and the detection of a [OII]λ3727 emission line suggest the presence of young populations (∼100 Myr) in ∼30% of our targets. The bulk of their stellar mass is however dominated by old components with ages in the range of 2-5 Gyr and formation redshifts zf>2.