# Optical spectroscopy and initial mass function of $z=0.4$ red galaxies

## TL;DR

This study uses spectral indices at bluer wavelengths to analyze the initial mass function of red galaxies at z=0.4, finding no evidence for evolution since then and emphasizing the impact of sample selection on IMF and age trends.

## Contribution

First application of IMF-sensitive spectral indices at bluer wavelengths to high-redshift galaxies, revealing no IMF evolution and highlighting sample selection effects.

## Key findings

- No evidence for IMF evolution between z=0.4 and now.
- Sample selection significantly affects IMF and age interpretations.
- Red galaxies tend to have bottom-heavy IMFs compared to bluer ones.

## Abstract

Spectral absorption features can be used to constrain the stellar initial mass function (IMF) in the integrated light of galaxies. Spectral indices used at low redshift are in the far red, and therefore increasingly hard to detect at higher and higher redshifts as they pass out of atmospheric transmission and CCD detector wavelength windows. We employ IMF-sensitive indices at bluer wavelengths. We stack spectra of red, quiescent galaxies around $z=0.4$, from the DEEP2 Galaxy Redshift Survey. The $z=0.4$ red galaxies have 2 Gyr average ages so that they cannot be passively evolving precursors of nearby galaxies. They are slightly enhanced in C and Na, and slightly depressed in Ti. Split by luminosity, the fainter half appears to be older, a result that should be checked with larger samples in the future. We uncover no evidence for IMF evolution between $z=0.4$ and now, but we highlight the importance of sample selection, finding that an SDSS sample culled to select archetypal elliptical galaxies at z$\sim$0 is offset toward a more bottom heavy IMF. Other samples, including our DEEP2 sample, show an offset toward a more spiral galaxy-like IMF. All samples confirm that the reddest galaxies look bottom heavy compared with bluer ones. Sample selection also influences age-color trends: red, luminous galaxies always look old and metal-rich, but the bluer ones can be more metal-poor, the same abundance, or more metal-rich, depending on how they are selected.