LEGA-C stellar populations scaling relations. II: Dissecting mass-complete archaeological trends and their evolution since z~0.7 with LEGA-C and SDSS

TL;DR

This study uses a consistent LEGA-C analysis at $z\sim0.7$ and aperture-corrected SDSS DR7 data to trace how light-weighted ages and stellar metallicities scale with mass $M_\ast$ and velocity dispersion $\sigma_\star$, separately for quiescent (Q) and star-forming (SF) galaxies. It finds a bimodal age distribution arising from distinct Q/SF age–mass sequences, while the metallicity–mass relation shows no bimodality but a persistent Q–SF offset and a high-metallicity sequence spanning both populations. Evolution since $z\sim0.7$ to $z\sim0.1$ indicates only modest aging and metallicity changes, with low-mass SF galaxies enriching notably, and a significant role for both rejuvenation/mergers and quenching in shaping the massive galaxy population. The results stress the need for consistent modeling and aperture treatment across redshifts to robustly interpret galaxy evolution and the relative importance of passive aging versus ongoing star formation and mergers.

Abstract

With a sample of 552 galaxies at z~0.7 from the LEGA-C survey, we investigate how current star formation influences light-weighted mean stellar ages and metallicities, and their median trends with stellar mass or velocity dispersion. The bimodality in the global age-mass relation stems from the different age distributions in the quiescent (Q) and star-forming (SF) populations. A bimodality is not observed in the stellar metallicity-mass relation, although Q and SF galaxies have different distributions in this parameter space. We identify a high-metallicity sequence populated by both Q and weakly SF galaxies. At masses below logM/Msun=10.8 the median stellar metallicity-mass relation of SF galaxies steepens, as a consequence of increasing scatter toward lower stellar metallicities for galaxies with increasing specific star formation rate at fixed mass. With a consistent analysis of SDSS DR7 spectra, accounting for aperture corrections, we quantify the evolution of the stellar age and stellar metallicity scaling relations between z=0.7 and the present. We find negligible evolution in the stellar metallicity-mass relation of Q galaxies and for logM/Msun>11 galaxies in general. Lower mass SF galaxies, instead, have typically lower metallicities than their local counterparts, indicating significant enrichment since z~0.7 in the low-mass regime. The median of the stellar ages of both the general population and Q galaxies has changed by only 2 Gyr between z=0.7 and z=0.1, less than expected from cosmic aging. Some Q galaxies must evolve passively to reach the old boundary of the local population. However, in order to explain the evolution of the median trends, both individual evolution, through rejuvenation and/or minor merging impacting the outer galaxy regions, and population evolution, through quenching of massive, metal-rich star-forming galaxies, are required. (Abridged)

Figures (10)

• Figure 1: Distribution in U-V versus V-J (left panel) and SSFR versus $M_\ast$ (right panel) for the parent LEGA-C sample (small dots) and for the silver (empty symbols) and high-S/N golden samples (filled symbols) used in this work. In the left panel, galaxies are distinguished into Q (magenta) and SF (blue) based on their distance from the star-forming main sequence, our default classification (i.e. Q galaxies lie below the dashed line in the right panel). In the right panel, red/blue symbols identify Q/SF galaxies according to their location in the UVJ plane (left and right of the dashed line in the right-hand panel, respectively).
• Figure 2: Luminosity-weighted mean age (left panels) and mean stellar metallicity (right panels) as a function of stellar mass and of SSFR for LEGA-C galaxies at $z\approx 0.7$. Upper panels: the data points are color coded by SSFR after applying LOESS-smoothing. Black vectors in the lower right corner are proportional to the partial correlation coefficients computed as in ScholzDiaz24, and the red vector indicates the direction of maximal increase of SSFR. The grey vectors correspond to a reference correlation coefficient of 0.7. Middle panels: Quiescent galaxies (circles) and star-forming galaxies (stars), color coded by their SSFR, without smoothing. Filled symbols highlight galaxies in the golden subsample. Filled squares show the median age and median metallicity in bins of stellar mass 0.2 dex wide and with at least 5 galaxies (grey for the weighted silver sample, black for the non-weighted golden galaxies). The errorbars indicate the 16-84 percentile range of each distribution (errors on the median are smaller). The median age-mass trends are fit with a linear relation (Eqn.\ref{['eqn:age_mass']}), while the median metallicity-mass trends are fit with the functional form as in Eqn. \ref{['eqn:met_mass']}. Grey solid lines show the fit to the silver median relations, with extrapolation shown by the dashed lines. Bottom panels: direct comparison of the volume and completeness-weighted median relations for quiescent (magenta) and star-forming (blue) galaxies in the silver sample.
• Figure 3: Top panel: SSFR versus stellar mass for silver galaxies, divided into bins of distance from the relation fit to $UVJ$ star-forming galaxies (dashed line, Eq. \ref{['eqn:ssfr_mass']}). Bottom panel: Luminosity-weighted mean stellar metallicity as a function of stellar mass for LEGA-C silver sample (grey points). Filled squares show the median stellar metallicity in bins of stellar mass (0.3-dex wide and with at least 5 galaxies), weighted by ${\tt Tcor}\times {\tt w\_spec\_silver}$, for galaxies in different bins of distance from the Main Sequence (as illustrated in the top panel). The errorbars represent the weighted $\rm16^{th}$ and $\rm84^{th}$ percentiles. The unweighted median trends would be very similar to those shown here, but with narrower inter-percentile ranges, in particular for the intermediate-SSFR sub-sample.
• Figure 4: Luminosity-weighted mean age (left panels) and mean stellar metallicity (right panels) as a function of stellar velocity dispersion for LEGA-C silver galaxies. Upper panels: the data points are color-coded by their SSFR after applying LOESS-smoothing. Black vectors in the bottom-right corner are proportional to the partial correlation coefficients computed as in ScholzDiaz24, and the red vector indicates the direction of maximal increase of SSFR. Grey vectors indicate a reference correlation coefficient of 0.7. Middle panels: Quiescent galaxies (circles) and star-forming galaxies (stars), color-coded by their individual SSFR. Filled symbols highlight galaxies in the golden subsample. Median trends in bins of $\sigma_\ast$ (0.1 dex wide and with at least 5 galaxies) are shown by grey (black) squares for the weighted silver (non weighted golden) sample. Linear fits to the median silver points are shown by solid lines (dashed lines for the extrapolation outside the median data points). Bottom panels: comparison of the median trends for quiescent (magenta) and star-forming galaxies (blue) in the silver sample. The errorbars represent the 16-84 percentile range of each distribution.
• Figure 5: Comparison of the light-weighted age versus stellar mass relation for LEGA-C and SDSS galaxy samples, for the whole population (left panels) and for quiescent and star-forming galaxies separately (middle and right panels, respectively). The SDSS data include corrections for aperture effects, as well as weights for spectroscopic and volume completeness (see text for details). Top row: Data points show LEGA-C golden (black filled and dark color points) and silver samples (grey and light color points). The contours show the distribution for SDSS DR7 $S/N>10$ samples, tracing number density levels that enclose 16, 50, 84, 97.5, 99.8, 99.9% of the total density. Middle row: median values of light-weighted age in bins of stellar mass for LEGA-C golden and silver sample (weighted for volume and spectroscopic completeness) and for SDSS, with the same color coding as in the top row. The errorbars indicate the (weighted) 16-84 percentiles of the distribution. The solid curves show the fit to the median silver points, with the same functional form adopted in Paper I for the whole population (as Eq.\ref{['eqn:met_mass']} for metallicity) and with a linear function (Eq.\ref{['eqn:age_mass']}) for Q and SF galaxies. The dashed lines show the extrapolation beyond the range of the data. Bottom row: Difference in (linear) age between SDSS median points and LEGA-C ( golden and silver) median points, with errorbars computed from the error on the medians; the solid lines show the differences between the fitted functions in the middle row. The y-axis range extends to 5 Gyr, which corresponds to the cosmic time elapsed between z=0.7 and z=0.1, i.e. roughly the expected age difference under pure passive evolution.