Star Formation Histories of z ∼ 1 Galaxies in LEGA-C

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 10 10 M 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 (σ * ) and ongoing star-formation (SF) activity, with the stellar content in higher-σ * 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 ∼ 1 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 ∼ 1 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.

Rejuvenation in z ∼ 0.8 Quiescent Galaxies in LEGA-C

“…Both the rejuvenated and non-rejuvenated quiescent populations have S/N ∼ 20, characteristic of LEGA-C galaxies. We have shown in Chauke et al (2018), using noisy synthetic spectra, that our fitting algorithm converges for this S/N. In Figure 2, we compare the average normalised spectrum (LEGA-C as well as best-fit model) of quiescent galaxies that are identified as rejuvenated to stellar mass and H δ matched quiescent galaxies (i.e.…”

“…The stellar masses derived using our method, M * ,spec , are in good agreement with photometry-based stellar masses derived with FAST (Kriek et al 2009). See Chauke et al (2018) for further details and results of the fitting algorithm.…”

“…The average rejuvenated galaxy has stronger H δ and H γ lines, which are characteristic of young stellar populations; however, its G-band (absorption lines of the CH molecule around 4300Å), which is characteristic of older stellar populations, is not as strong as that of quiescent galaxies without rejuvenation episodes. See Figure A.8 in the appendix for individual best-fit spectra of rejuvenated galaxies in our sample obtained from MCMC fullspectrum fitting (Chauke et al 2018). Compared to PSB or E+A galaxies, our sample of rejuvenated galaxies have weaker Balmer lines and redder V-J colors.…”

“…The reconstructed star formation histories of rejuvenated galaxies obtained from MCMC full-spectrum fitting (the walkers are shown in gray,Chauke et al 2018). The fraction of stellar mass formed from the rejuvenation episode and the redshift of the peak SFR of the event are shown in black.…”

Chemical evolution of elliptical galaxies with a variable IMF

“…Later star formation after the primary star burst is likely (Chauke et al 2018;Morishita et al 2018). A SFH with a long tail of low-SFR after the starburst, or a later second smaller starburst that contributes a decent portion (10%, e.g., Tojeiro et al 2013) of stars formed from metal-enriched gas may not influence the stellar mass-weighted metallicity but can alter the luminosityweighted metallicity significantly as is show in Appendix A.…”