The LEGA-C galaxy survey: multiple quenching channels for quiescent galaxies at $z\sim1$

TL;DR

This study uses LEGA-C DR3 spectroscopy combined with UltraVISTA photometry to derive nonparametric star-formation histories for 2908 galaxies at $0.6\le z\le1.0$, paired with precise size measurements from HST imaging. It reveals a broad spectrum of SFHs and quenching timescales, with PSBs quenching on $\tau_\mathrm{q} \sim 0.13$ Gyr and the main quiescent population on $\tau_\mathrm{q} \sim 1.23$ Gyr. A strong link emerges between size, metallicity, and quenching: larger quiescent galaxies tend to be younger and metal-poor-ish with slower quenching, while compact quiescent galaxies are metal-rich and quench more rapidly; PSBs lie near the intermediate size regime and show the fastest quenching. The authors interpret these results as evidence for multiple, or at least a continuous distribution of, quenching channels driven by different physical mechanisms, potentially yielding similar morphologies across evolutionary histories.

Abstract

We analyzed the sizes and star-formation histories (SFHs) of 2908 galaxies with $M_\star \geq 10^9$ M$_\odot$ at $0.6 < z < 1.0$, drawn from the LEGA-C survey. The goal is to investigate the connection between galaxy sizes with SFH, stellar age, and metallicity. SFHs were derived with Prospector by fitting the high signal-to-noise, high spectral resolution spectroscopy drawn from the LEGA-C DR3 together with the broadband photometry from the UltraVISTA catalog. Galaxy sizes were measured by fitting a 2D S{é}rsic profile to the HST ACS~F814W images. We find diverse SFHs and quenching timescales ($τ_\rm{q}$). The main quiescent population quenched over $τ_\rm{q}=1.23\pm0.04$ Gyr, whereas compact post-starburst galaxies (PSBs) quenched much faster, $τ_\rm{q}=0.13\pm0.03$ Gyr. At fixed stellar mass, smaller quiescent galaxies quenched more rapidly than larger ones; at fixed size, the dependence on stellar mass is weak. Larger quiescent galaxies are marginally younger, quenched more slowly, and have near-solar metallicities, while compact quiescent galaxies are older, metal-rich, and quenched faster. PSBs formed half their mass later ($z_\rm{form}\sim1.9$) and quenched on the shortest timescales. The general trends with galaxy size, $Z_\star$, and $z_\rm{form}$ for the quiescent populations remain consistent regardless of the method used to derive the stellar properties. We conclude that compact quiescent galaxies are consistent with both early, moderately fast quenching and with more rapid, late quenching. While this may suggest the existence of multiple quenching channels, our data are also compatible with a continuous distribution of quenching timescales. These findings suggest that different physical mechanisms may drive quenching across galaxy populations, potentially leading to similar morphological outcomes despite differing evolutionary histories.

Figures (12)

• Figure 1: SFR$_\mathrm{100Myr}$ as a function of $M_\star$ for the LEGA-C galaxies at $0.6 \le z_\mathrm{spec} \le 1$. Galaxies are color-coded by their $UVJ$ diagram classification as $UVJ$ star-forming (blue stars) and $UVJ$ quiescent (red points). Post-starburst galaxies are indicated as yellow squares. The magenta line indicates the SFS at $z = 0.6$ from Leja_2022ApJ...936..165L, and the dashed black line indicates the separation between star formation and quiescence (sSFR = $10^{-11}$ yr$^{-1}$). The average uncertainties for the two main galaxy populations are shown in the bottom-right corner in their respective colors.
• Figure 2: Galaxy size as a function of $M_\star$ for the LEGA-C galaxies at $0.6 \le z_\mathrm{spec} \le 1$. Based on their sSFR, galaxies are separated into star-forming (blue stars) and quiescent (red points) populations. Post-starburst galaxies are indicated as yellow squares. For each galaxy population, we fitted a linear relation between the median sizes in each 0.1 dex mass bin and the stellar mass (gray symbols). To avoid the flattening of the relations at the low-mass end, we fit star-forming galaxies with $\log \left(M_\star / \mathrm{M}_\odot \right) \ge 9.8$, and quiescent galaxies with $\log \left(M_\star / \mathrm{M}_\odot \right) \ge 10.3$.
• Figure 3: Distribution of the galaxy size metric $\Delta \log \left(R_{e}\right)$, defined as the distance of the sizes $R_\mathrm{e}$ from the median size--mass relation at a given mass, for each galaxy population. The blue histogram shows the relative size distribution for the star-forming galaxies, while the red histogram shows the relative size distribution for the quiescent ones. The corresponding solid curves are the best-fit Gaussian distributions. The statistics of the mean offset ($\mu$) and variance ($\sigma$) of the each Gaussian distribution is shown at the top-left corner.
• Figure 4: Nonparametric SFHs of LEGA-C galaxies recovered with Prospector. The fitted SFR of each time bin is normalized by the total stellar mass formed up to that time bin. The SFHs are divided into the star-forming (blue) and quiescent (red) populations, based on their sSFR. In the background, we show the individual SFHs of our sample, while the thick lines and shaded areas denote the median and the $16^\mathrm{th}$--$84^\mathrm{th}$ percentile range for the star forming (solid blue line and shaded blue area), quiescent (solid red line and shaded red area; excluding the 84 PSBs), and PSB (dashed yellow line and shaded yellow area) galaxies. The green shaded area displays the transition boundary from star forming (dashed black line) to quiescence (solid black line). The coral point and square markers show the measurements of sSFR by Salmon_2015ApJ...799..183S and Leja_2022ApJ...936..165L respectively, at higher redshift. The dashed coral line shows the SFH of lower-mass, quiescent galaxies ($10.5 \leq \log (M_\star / {\rm M_\odot}) < 11$) from Tacchella_2022ApJ...926..134T.
• Figure 5: Median SFHs of LEGA-C galaxies separated into three stellar mass bins. From left to right: median SFHs of the full primary galaxy sample, the star-forming population, and the quiescent population. The dotted violet line in the star-forming galaxies' panel shows the SFH of high-mass, star-forming galaxies from Tacchella_2022ApJ...926..134T. The dashed and dotted coral lines in the quiescent galaxies' panel show the SFHs of lower-mass and higher-mass, quiescent galaxies from Tacchella_2022ApJ...926..134T, respectively. The green shaded area displays the transition boundary from star forming (dashed black line) to quiescence (solid black line).