Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type
  Galaxies: Evidence for Abundance-Driven Gradients
  Samples of early-type galaxies show a correlation between stellar velocity
dispersion and the stellar initial mass function (IMF) as inferred from
gravity-sensitive absorption lines in the galaxies' central regions. To search
for spatial variations in the IMF, we have observed two early-type galaxies
with Keck/LRIS and measured radial gradients in the strengths of absorption
features from 4000-5500 \AA $\,$ and 8000-10,000 \AA. We present spatially
resolved measurements of the dwarf-sensitive spectral indices NaI (8190 \AA)
and Wing-Ford FeH (9915 \AA), as well as indices for species of H, C$_2$, CN,
Mg, Ca, TiO, and Fe. Our measurements show a metallicity gradient in both
objects, and Mg/Fe consistent with a shallow gradient in \alpha-enhancement,
matching widely observed trends for massive early-type galaxies. The NaI index
and the CN$_1$ index at 4160 \AA $\,$ exhibit significantly steeper gradients,
with a break at $r \sim 0.1 r_{\rm eff}$ ($r \sim 300$ pc). Inside this radius
NaI strength increases sharply toward the galaxy center, consistent with a
rapid central rise in [Na/Fe]. In contrast, the ratio of FeH to Fe index
strength decreases toward the galaxy center. This behavior cannot be reproduced
by a steepening IMF inside $0.1 r_{\rm eff}$ if the IMF is a single power law.
While gradients in the mass function above $\sim 0.4 M_\odot$ may occur,
exceptional care is required to disentangle these IMF variations from the
extreme variations in individual element abundances near the galaxies' centers.
