The stellar initial mass function in red-sequence galaxies: 1-micron
  spectroscopy of Coma Cluster galaxies with Subaru/FMOS
  To investigate possible variations in the stellar initial mass function (IMF)
in red-sequence galaxies, we have obtained infrared spectroscopy with
Subaru/FMOS for a sample of 92 red-sequence galaxies in the Coma cluster.
Velocity dispersions, ages and element abundances for these galaxies have been
previously determined from optical data. By stacking the FMOS spectra in the
rest frame, removing sky-subtraction residuals and other artefacts fixed in the
observed frame, we derive composite spectra in the 9600-10500 Angstrom range
for galaxies grouped according to their velocity dispersion or Mg/Fe ratio. We
measure the Wing-Ford band of FeH and a new index centred on a CaI line at
10345 Angstrom; these features are strong in cool dwarf stars, and hence
reflect the form of the IMF at low mass (<0.5M_sun) The CaI line, unlike the
Wing-Ford band and other `classical' IMF indicators (NaI doublet, CaII
triplet), is unaffected by the abundance of sodium. We compare the measured
indices against predictions from spectral synthesis models matched to the
element abundances estimated from the optical data. Binning galaxies by
velocity dispersion, we find that both the Wing-Ford and CaI index measurements
are best reproduced by models with the Salpeter IMF. There is no clear evidence
for an increase in dwarf-star content with velocity dispersion. Binning the
observed galaxies instead by Mg/Fe ratio, the behaviour of both indices implies
a trend of IMF from Chabrier-like, at abundance ratios close to solar, to
Salpeter or heavier for highly alpha-enhanced populations. At face value, this
suggests that the IMF depends on the mode of star formation, with intense rapid
star-bursts generating a larger population of low-mass stars.
