More is better: Strong constraints on the stellar properties of LEGA-C

Aims. We present the stellar properties of 2908 galaxies (1208 quiescent and 1700 star forming) at 0.6 < z < 1.0 from the Large Early Galaxy Astrophysics Census (LEGA-C) survey. We emphasize the importance of high signal-to-noise, high spectral resolution spectroscopy in the inference of stellar population properties of galaxies. Methods. We estimated the galaxy properties with the Bayesian spectral energy distribution (SED) framework <ns0:monospace>Prospector</ns0:monospace>. We fit spectroscopy and broadband photometry together, drawn from the LEGA-C DR3 and UltraVISTA catalogs, respectively. Results. We report a positive correlation between light-weighted ages and stellar velocity dispersion (σ⋆). The trend with σ⋆ is weaker for the mass-weighted ages and stellar metallicity (Z⋆). At fixed σ⋆, we find a tentative correlation between Z⋆ and stellar age. On average, quiescent galaxies are characterized by high Z⋆; they are ∼1.1 Gyr older, less dusty, and have steeper dust attenuation slopes (due to a lower optical depth) compared to star-forming galaxies. Conversely, star-forming galaxies are characterized by significantly higher dust optical depths and shallower (grayer) attenuation slopes. Low-mass (high-mass) star-forming galaxies have lower (higher) Z⋆, while their stellar populations are on average younger (older). A key pragmatic result of our study is that a linear-space metallicity prior is preferable to a logarithmic-space one when using photometry alone, as the latter biases the posteriors downward. Conclusions. Spectroscopy greatly improves stellar population measurements and is required to provide meaningful constraints on age, metallicity, and other properties. Pairing spectroscopy with photometry helps to resolve the dust–age–metallicity degeneracy. Spectroscopic data yield more accurate mass- and light-weighted ages, with ages inferred from photometry alone suffering such large uncertainties that their utility is limited. Stellar metallicities are constrained by our spectroscopy, but precise measurements remain challenging (and impossible with photometry alone), particularly in the absence of Mg and Fe lines redward of 5000 Å in the observed spectrum.