Abstract
There is good evidence that the centers of massive early-type galaxies have a
bottom-heavy stellar initial mass function (IMF) compared to the IMF of the
Milky Way. Here we study the radial variation of the IMF within such galaxies,
using a combination of high quality Keck spectroscopy and a new suite of
stellar population synthesis models that cover a wide range in Z/H. As in the
previous studies in this series, the models are fit directly to the spectra and
treat all elemental abundance ratios as free parameters. Using newly obtained
spectroscopy for six galaxies, including deep data extending to ~1Re for the
galaxies NGC1407, NGC1600, and NGC2695, we find that the IMF strongly varies
with galactocentric radius. For all galaxies the IMF is bottom-heavy in the
central regions, with average "mismatch" parameter a~2.5 at r=0. The IMF
rapidly becomes more bottom-light with increasing radius, flattening off near
the Milky Way value (a~1.1) at R>0.4Re. A consequence is that the
luminosity-weighted average IMF depends on the measurement aperture: within
R=Re we find <a>=1.3-1.5, that is, the IMF of even the most massive galaxies is
only mildly bottom-heavy within the half-light radius. Our results are
consistent with several earlier studies that were based on analyses of radial
gradients of line indices, and support galaxy formation models in which the
central regions of massive galaxies had a different formation history than
their outer parts. Finally, we make use of the high signal-to-noise central
spectra of NGC1407 and NGC2695 to demonstrate that we are indeed measuring IMF
effects, not abundance effects.
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