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

Abstract

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 $z\sim 0.7$. 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 CaII 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 $[Z/H]=0.08\pm 0.18$ and $[\alpha /Fe]=0.13\pm 0.11$. Finally, we find positive correlations between $\log \mathrm{age}$, $[Z/H]$, $[\alpha /Fe]$ and the stellar velocity dispersion, with slopes of ($0.48\pm 0.14$), ($0.26\pm 0.17$), and ($0.23\pm 0.11$), 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.

28 pages, 13 figures, 6 tables (including citations and appendices). Accepted for publication in ApJ

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