Evidence of a Bottom-heavy Initial Mass Function in Massive Early-type
  Galaxies from Near-infrared Metal Lines
  We present new evidence for a variable stellar initial mass function (IMF) in
massive early-type galaxies, using high-resolution, near-infrared spectroscopy
from the Folded-port InfraRed Echellette spectrograph (FIRE) on the Magellan
Baade Telescope at Las Campanas Observatory. In this pilot study, we observe
several gravity-sensitive metal lines between 1.1 $\mu$m and 1.3 $\mu$m in
eight highly-luminous ($L \sim 10 L_*$) nearby galaxies. Thanks to the broad
wavelength coverage of FIRE, we are also able to observe the Ca II triplet
feature, which helps with our analysis. After measuring the equivalent widths
(EWs) of these lines, we notice mild to moderate trends between EW and central
velocity dispersion ($\sigma$), with some species (K I, Na I, Mn I) showing a
positive EW-$\sigma$ correlation and others (Mg I, Ca II, Fe I) a negative one.
To minimize the effects of metallicity, we measure the ratio $R$ = [EW(K I) /
EW(Mg I)], finding a significant systematic increase in this ratio with respect
to $\sigma$. We then probe for variations in the IMF by comparing the measured
line ratios to the values expected in several IMF models. Overall, we find that
low-mass galaxies ($\sigma \sim 100$ km s$^{-1}$) favor a Chabrier IMF, while
high-mass galaxies ($\sigma \sim 350$ km s$^{-1}$) are better described with a
steeper (dwarf-rich) IMF slope. While we note that our galaxy sample is small
and may suffer from selection effects, these initial results are still
promising. A larger sample of galaxies will therefore provide an even clearer
picture of IMF trends in this regime.
