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.

Automated summary

Using publicly available data, the stellar population properties of 140 massive and passive galaxies are measured. A new diagnostic based on pseudo-Lick indices is developed and used in combination with photometric and spectroscopic criteria to select a reliable sample of passively evolving galaxies. The ages, metallicities, and element abundances of the galaxies are constrained using optimized Lick indices, and their relationships with stellar velocity dispersion are analyzed. The results confirm the existence of mass-downsizing evolution.