New Observational H(z) Data from Full-spectrum Fitting of Cosmic Chronometers in the LEGA-C Survey

Abstract

In this work, we perform a full-spectrum fitting of 350 massive and passive galaxies selected as cosmic chronometers from the LEGA-C ESO public survey to derive their stellar ages, metallicities, and star formation histories. We extensively test our results by assessing their dependence on the possible contribution of dust, calibration of noise and signal, and use of photometric data in addition to spectral information; we also identify indicators of the correct convergence of the results, including the shape of the posterior distributions, the analysis of specific spectral features, and the correct reproduction of the observed spectrum. We derive a clear age–redshift trend compatible with the aging in a standard cosmological model showing a clear downsizing pattern, with more massive galaxies being formed at higher redshift (z f ∼ 2.5) with respect to less massive ones (z f ∼ 2). From these data, we measure the differential aging of this population of cosmic chronometers to derive a new measurement of the Hubble parameter, obtaining H ( z = 0.8 ) = 113.1 ± 15.1 ( stat . ) − 11.3 + 29.1 ( syst . ) . This analysis allows us to compare for the first time the differential ages of cosmic chronometers measured on the same sample with two completely different methods, the full-spectrum fit (this work) and the analysis of Lick indices, known to correlate with the age and metallicity of the stellar populations. Albeit an understood offset in the absolute ages, the differential ages have proven to be extremely compatible between the two methods, despite the very different data, assumptions, and models considered, demonstrating the robustness of the method.

Checking the second law at cosmic scales

“…To begin with, we undertake a Gaussian process reconstruction of the Hubble parameter H(z) and its derivative H ′ (z) from the recent 32 CC and combined 32 CC + 8 BAO data. The CC Hubble data depend on the differential ages of galaxies and do not assume any particular cosmological model [11][12][13][14][15][16][17][18]. We also take into account the systematic uncertainties reported in Moresco et al [19].…”

Cosmic chronometers with photometry: a new path to H(z)

“…Until now, the CC method has been applied to high signal-to-noise spectroscopic surveys [5][6][7][8][9][10][11] of passively evolving galaxies. The reason is that selecting the CC (objects that populate the red envelope, or the edge of the age-redshift relation) and measuring differential ages robustly, precisely and accurately, is a very challenging task due to the JCAP11(2023)047 effects of other (stellar) parameters that also determine the integrated light of the stellar population of a galaxy.…”