More is better: Strong constraints on the stellar properties of LEGA-C z~1 galaxies

In this study, we present the stellar properties of 2908 galaxies at 0.6 < z < 1.0 from the LEGA-C survey. One key aspect of our study is that we stress the importance of high signal-to-noise, high spectral resolution spectroscopy in the inference of stellar population properties of galaxies (see Figure 1). We estimated the galaxy properties with the SED fitting code Prospector, by fitting spectroscopy and broadband photometry together, drawn from the LEGA-C DR3 and UltraVISTA catalogs respectively.

Figure 1: Joint posterior distributions of various physical quantities, best-fit spectrum, best-fit SED, and retrieved SFH, for an example star-forming galaxy at z = 0.611. The contours enclose 20%, 50% and 80% of the total data. The vertical black dashed line indicates the median value of each physical quantity. The inset in the right corner of the figure contains three panels. Top panel: comparison of the observed spectrum (gray line) with the fitted model (blue line). Middle panel: comparison of the observed photometry (red points) with the best-fit SED (blue line). The shaded gray region covers the wavelength range of the corresponding LEGA-C spectrum. The subpanels associated with the top and middle panels show the absolute residuals, defined as ΔFν = log obs/mod. Bottom panel: the SFH posterior.

Spectroscopy greatly improves stellar population measurements and is required to provide meaningful constraints on age, metallicity, and other properties. Pairing spectroscopy with photometry helps resolving the dust-age-metallicity degeneracy, yielding more accurate mass- and light-weighted ages, with ages inferred from photometry alone suffering such large uncertainties. 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 (see Figure 2).

Figure 2: Comparison of the main stellar population properties of galaxies with duplicate observations in LEGA-C DR3. Galaxies are color-coded by their UVJ diagram classification as star forming (blue stars) and quiescent (red points).

From our analysis, we were able to derive the physical properties of galaxies at z ~ 1. We report that on average, quiescent galaxies are characterized by high Z⋆, they are ~ 1.1 Gyr older, less dusty, with steeper dust attenuation slopes 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). These trends are summarized in Figure 3.

Figure 3: Mean stellar population and dust properties in the SFR100Myrs–M⋆ plane of our primary sample. Panels from left to right display: mass-weighted stellar age (t⋆,mw), stellar metallicity (Z⋆), dust optical depth in the V band (τdust,2), and dust attenuation slope index (n). Top row: Individual data points, with typical error bars shown in the bottom-right corner of each panel. Middle row: Average trends of the physical properties across the SFR100Myrs–M⋆ plane. Bottom row: Average upper uncertainties for each property. Each hexbin includes a minimum of five galaxies. The SFS at z = 0.60 is indicated by the solid black line, following the definition of Leja et al. (2022), while the dashed black line represents the boundary between star-forming and quiescent galaxies (sSFR = 10−11 yr−1). In the middle row, black markers indicate the median SFR100Myrsvalues in bins of M⋆: open stars for star-forming galaxies and open circles for quiescent galaxies (see Table 2). Additionally, results from previous studies at a similar redshift range are overplotted for comparison.

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