Ages and metallicities of quiescent galaxies: confronting broadband ($UVJ$) colours with stellar absorption lines

TL;DR

This study measures ages and metallicities of quiescent galaxies at redshifts 0.6-1.0 using spectroscopy, revealing different behaviors of age and metallicity in the UVJ colour diagram and highlighting limitations of current stellar population models.

Contribution

It provides empirical measurements of ages and metallicities of distant quiescent galaxies and compares them with model predictions, uncovering significant discrepancies.

Findings

Age correlates with reddening in colours, consistent with models.

Metallicity trends differ from model expectations, showing complex behaviour.

Data-model disparities suggest non-solar abundance patterns or model limitations.

Abstract

For decades, studying quiescent galaxies beyond $z\sim1$ has been challenging due to the reliance on photometric spectral energy distributions, which are highly susceptible to degeneracies between age, metallicity, dust, and star-formation history. Only recently has deep, rest-frame, optical spectroscopy made robust metallicity and age measurements possible, allowing us to empirically assess their effects on continuum shapes. To this end, we measure ages and metallicities of $\sim700$ massive ($10.2\lesssim\log(M_*/M_\odot)\lesssim11.8$), quiescent galaxies at $0.6\lesssim z\lesssim1.0$ from the Large Early Galaxy Astrophysics Census (LEGA-C) via continuum-normalized, absorption-line spectra, and compare with independent rest-frame $U-V$ and $V-J$ colours. Age increases along the quiescent sequence as both colours redden, consistent with stellar population synthesis (SPS) model predictions. Metallicity increases perpendicularly to the age trend, with higher metallicities at redder $U-V$ and bluer $V-J$ colours. Thus, age and metallicity behave differently in the $UVJ$ diagram. Moreover, this trend conflicts with SPS model expectations of increasing metallicity approximately along the quiescent sequence. Independent dynamical mass-to-light ratio trends also differ dramatically from SPS model predictions. These results demonstrate that relying on model fits to continuum shapes alone may lead to systematic biases in ages, metallicities, and stellar masses. The cause of these data-model disparities may stem from non-solar abundance patterns in quiescent galaxies or the treatment of evolved stellar phases in the models. Resolving these discrepancies is crucial, as photometric data remain central even with JWST.