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