Toward a Better Understanding of Cosmic Chronometers: Stellar Population Properties of Passive Galaxies at Intermediate Redshift

We take advantage of the publicly available LEGA-C spectroscopic survey to measure the stellar population properties of 140 individual massive and passive galaxies at z similar to 0.7. We develop and publicly release PyLick, a flexible Python code to measure UV to near-IR spectral indices. With PyLick we study the H/K ratio as a new diagnostic based on the pseudo-Lick Ca ii H and K indices and find that a cut in H/K < 1.1 can be used jointly with other criteria to select (or verify the purity of) samples of passive galaxies. By combining photometric and spectroscopic criteria, we select a reliable sample of passively evolving galaxies. We constrain single-burst stellar ages, metallicities [Z/H], and [alpha/Fe] with an optimized set of Lick indices, exploring in detail the robustness of our measurement against different combinations. Even without imposing cosmological priors, the derived ages follow a clear trend compatible with the expected cosmological aging of the universe. We observe no significant redshift evolution for the metal abundance with respect to the values derived at z = 0, with median [Z/H] = 0.08 +/- 0.18 and [alpha/Fe] = 0.13 +/- 0.11. Finally, we analyze the relations between log age, [Z/H], [alpha/Fe], and the stellar velocity dispersion, finding slopes of (0.5 +/- 0.1), (0.3 +/- 0.2), and (0.2 +/- 0.1), respectively; the small scatter of <0.2 dex points to rather homogeneous and short star formation histories. Overall, these results confirm and extend low-redshift findings of a mass-downsizing evolution. This work further strengthens the possibility of selecting pure samples of passive galaxies to be exploited reliably as cosmic chronometers to place independent cosmological constraints.