Abstract
We investigate radial gradients in the stellar initial mass function (IMF) in
two early type galaxies using resolved measurements of several far red gravity
sensitive absorption features, including the "Wing-Ford" band (FeH). We use the
Oxford Short Wavelength Integral Field SpecTrogaph (SWIFT) to obtain resolved
spectroscopic observations of NGC~1277 and IC~843, two galaxies with large
central velocity dispersions and evidence for "heavy" IMFs from spectroscopic
and dynamical measurements. Our observations cover the IMF sensitive features
sodium \NaI, calcium triplet CaT as well as FeH, along with MgI 0.88 and TiO
0.89. We also use published optical indices to help untangle the degenerate
parameters of age, metallicity, $\alpha$/Fe abundance and IMF slope. Within
NGC~1277 we measure a flat FeH profile at $\sim$0.38\AA~with a strong Na/Fe
gradient, from which we use stellar population models to infer an old,
$\alpha$-enhanced population with a Chabrier IMF at all radii. IC~843 also
displays a (slightly stronger) flat FeH profile at $\sim$0.4\AA, which together
with optical indices suggests a similarly old, $\alpha$-enhanced population
with a Chabrier IMF at all radii. Our results rule out bottom-heavy single
power law IMFs in these objects and present galaxies which are in conflict with
any simple IMF-$\sigma$ relationship.
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