The Stellar Initial Mass Function in Early-Type Galaxies from Absorption Line Spectroscopy. III. Radial Gradients

Pieter van Dokkum, Charlie Conroy, Alexa Villaume, Jean Brodie, Aaron Romanowsky

Published 2016-11-29Version 1

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.