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Recent studies of early-type galaxies have suggested that the initial massfunction (IMF) slope is bottom-heavy, i.e. they contain a larger fraction oflow-mass stars than the Milky Way. However, measurements of the IMF remainchallenging in unresolved galaxies because features in their observed spectraare sensitive to a number of factors including the stellar age, metallicity,and elemental abundances, in addition to the IMF. In this paper, we use newhigh signal-to-noise IMACS (Magellan) spectra to study the elliptical shellgalaxy NGC 3923 at optical (3700-6600 Angstrom), and near-infrared (7900-8500Angstrom) wavelengths, as a function of radius. We have undertaken a number ofindependent approaches to better understand the uncertainties in our results.1) We compare two different stellar population model libraries; 2) we undertakespectral index fitting as well as full spectral fitting; 3) we have performedsimulations for which we a priori know the input IMF, and which closely matchour data; 4) we also investigate the effects of including a two-component,rather than a single stellar population. We show that our results are sensitiveto the assumptions we make and to the methods we use. In addition, we evaluatethe accuracy and precision of our results based on simulated mock data. We findsome indication (although assumption-dependent) for a bottom-heavy IMF in themass-range 0.5-1.0 M_sun, while the IMF in the mass-range 0.08-0.5 M_sunappears to be Milky-Way like and constant. Including near-infrared data to ouranalysis gives consistent results, and improves the precision.

Measuring the stellar population parameters of the early-type galaxy NGC 3923 -- The challenging measurement of the initial mass function