A new measurement of the expansion history of the Universe at z = 1.26 with cosmic chronometers in VANDELS

Aims. We aim to derive a new constraint on the expansion history of the Universe by applying the cosmic chronometers method in the VANDELS survey, studying the age evolution of high-redshift galaxies with a full-spectral-fitting approach. Methods. We selected a sample of 39 massive (log(M- /M- ) > 10.8) and passive (log(sSFR/yr- 1) < - 11) galaxies from the fourth data release of the VANDELS survey at 1 < z < 1.5. To minimise the potential contamination by star-forming outliers, we selected our sample by combining different selection criteria, considering both photometric and spectroscopic information. The analysis of the observed spectral features provides direct evidence of an age evolution with redshift and of mass-downsizing, with more massive galaxies presenting stronger age-related features. To estimate the physical properties of the sample, we performed full spectral fitting with the code BAGPIPES, jointly analysing spectra and photometry of our sources without any cosmological assumption regarding the age of the population. Results. The derived physical properties of the selected galaxies are characteristic of a passive population, with short star formation timescales ( ¨