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