The Stellar Initial Mass Function in Early-Type Galaxies From Absorption
  Line Spectroscopy. IV. A Super-Salpeter IMF in the center of NGC 1407 from
  Non-Parametric Models
  It is now well-established that the stellar initial mass function (IMF) can
be determined from the absorption line spectra of old stellar systems, and this
has been used to measure the IMF and its variation across the early-type galaxy
population. Previous work focused on measuring the slope of the IMF over one or
more stellar mass intervals, implicitly assuming that this is a good
description of the IMF and that the IMF has a universal low-mass cutoff. In
this work we consider more flexible IMFs, including two-component power-laws
with a variable low-mass cutoff and a general non-parametric model. We
demonstrate with mock spectra that the detailed shape of the IMF can be
accurately recovered as long as the data quality are high (S/N$\gtrsim300$) and
cover a wide wavelength range (0.4um-1.0um). We apply these flexible IMF models
to a high S/N spectrum of the center of the massive elliptical galaxy NGC 1407.
Fitting the spectrum with non-parametric IMFs, we find that the IMF in the
center shows a continuous rise extending toward the hydrogen-burning limit,
with a behavior that is well-approximated by a power-law with an index of -2.7.
These results provide strong evidence for the existence of extreme
(super-Salpeter) IMFs in the cores of massive galaxies.
