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

Abstract

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 (⟨τ⟩ = 0.28 ± 0.02 Gyr), low dust extinction (⟨AV, dust⟩ = 0.43 ± 0.02 mag), and sub-solar metallicities (⟨Z/Z⊙⟩ = 0.44 ± 0.01) compatible with other measurements of similar galaxies in this redshift range. The stellar ages, even if no cosmological constraint is assumed in the fit, show a decreasing trend compatible with a standard cosmological model, proving the robustness of the method in measuring the ageing of the population. Moreover, they show a distinctive mass-downsizing pattern, with more massive galaxies (⟨log(M⋆/M⊙)⟩ = 11.4) being older than less massive ones (⟨log(M⋆/M⊙)⟩ = 11.15) by ∼0.8 Gyr. We thoroughly tested the dependence of our results on the assumed SFH, finding a maximum 2% fluctuation on median results using models with significantly different functional forms. The derived ages are combined to build a median age–redshift relation, which we used to perform our cosmological analysis. Conclusions. By fitting the median age–redshift relation with a flat ΛCDM model, assuming a Gaussian prior on ΩM, 0 = 0.3 ± 0.02 from late-Universe cosmological probes, we obtain a new estimate of the Hubble constant H0 = 67−15+14 km s−1 Mpc−1. In the end, we derive a new estimate of the Hubble parameter by applying the cosmic chronometers method to this sample, deriving a value of H(z = 1.26) = 135 ± 65 km s−1 Mpc−1 considering both statistical and systematic errors. While the error budget in this analysis is dominated by the scarcity of the sample, this work demonstrates the potential strength of the cosmic chronometers approach up to z > 1, especially in view of the next incoming large spectroscopic surveys such as Euclid.