The average stellar population age and metallicity of intermediate-redshift quiescent galaxies

Abstract

The HectoMAP spectroscopic survey provides a unique mass-limited sample of more than 35,000 quiescent galaxies ($D_{n}4000>1.5$) covering the redshift range $0.2<z<0.6$. We segregate galaxies in bins of properties based on stellar mass, $D_{n}4000$, and redshift to construct a set of high signal-to-noise spectra representing massive ($M_{\ast }>10^{10}{M}_{\odot }$) quiescent population at intermediate redshift. These high-quality summed spectra enable full spectrum fitting and the related extraction of the average stellar population age and metallicity. The average galaxy age increases with the central D${}_{n}4000$ as expected. The correlation is essentially invariant with stellar mass; thus $D_{n}4000$ is a robust proxy for quiescent galaxy stellar population age. HectoMAP provides the first quiescent sample at intermediate redshift comparable with $z\sim 0$ mass-complete datasets. Scaling relations derived from the HectoMAP summed spectra connect stellar age and metallicity with quiescent galaxy stellar mass up to $z\sim 0.5$. Anti-correlation between the equivalent width of the [O II] emission line and stellar age, together with the mild increase in stellar age with stellar mass, supports a broad range of timescales for the mass assembly of intermediate-redshift quiescent systems. On average, the most massive galaxies ($M_{\ast }>10^{11}{M}_{\odot }$) assemble the bulk of their stars at earlier epochs. A strong increase in the average stellar metallicity with stellar mass, along with the correlation between the [O II] equivalent width and metallicity at $0.2<z<0.4$, suggests that lower-mass galaxies are more likely to have recent star formation episodes; related feedback from massive stars affects the chemical enrichment of these galaxies.

Submitted for publication in the AAS Journals. Comments are welcome