Evidence for a constant IMF as a function of radius in two massive ETGs
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.