SFHs OF $Z\sim1$ Galaxies in LEGA-C

TL;DR

This study reconstructs star formation histories of 607 galaxies at redshifts 0.6-1.0 using high-resolution spectra, revealing correlations between galaxy age, velocity dispersion, and star formation activity, and showing that SFHs retain a long-term memory of past activity.

Contribution

It introduces a novel full spectrum fitting method to analyze galaxy SFHs at z~1, linking progenitors and descendants across cosmic time with high-quality LEGA-C spectra.

Findings

Higher-$\sigma_*$ galaxies formed earlier and faster.

Most quiescent galaxies show passive evolution, some with recent rejuvenation.

Current star formation correlates with activity 3 Gyrs prior.

Abstract

Using high resolution spectra from the VLT LEGA-C program, we reconstruct the star formation histories (SFHs) of 607 galaxies at redshifts $z = 0.6-1.0$ and stellar masses $\gtrsim10^{10}$M$_{\odot}$ using a custom full spectrum fitting algorithm that incorporates the emcee and FSPS packages. We show that the mass-weighted age of a galaxy correlates strongly with stellar velocity dispersion ($\sigma_*$) and ongoing star-formation (SF) activity, with the stellar content in higher-$\sigma_*$ galaxies having formed earlier and faster. The SFHs of quiescent galaxies are generally consistent with passive evolution since their main SF epoch, but a minority show clear evidence of a rejuvenation event in their recent past. The mean age of stars in galaxies that are star-forming is generally significantly younger, with SF peaking after $z<1.5$ for almost all star-forming galaxies in the sample: many of these still have either constant or rising SFRs on timescales $>100$Myrs. This indicates that $z>2$ progenitors of $z\sim1$ star-forming galaxies are generally far less massive. Finally, despite considerable variance in the individual SFHs, we show that the current SF activity of massive galaxies ($>$L$_*$) at $z\sim1$ correlates with SF levels at least $3$Gyrs prior: SFHs retain `memory' on a large fraction of the Hubble time. Our results illustrate a novel approach to resolve the formation phase of galaxies, and in identifying their individual evolutionary paths, connects progenitors and descendants across cosmic time. This is uniquely enabled by the high-quality continuum spectroscopy provided by the LEGA-C survey.