Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT , we study , for the first time , radial gradients of optical and Near-Infrared IMF-sensitive features in a representative sample of galaxies at the very high-mass end of the galaxy population .
The sample consists of seven early-type galaxies ( ETGs ) at z \sim 0.05 , with central velocity dispersion in the range 300 \lesssim \sigma \lesssim 350 km s ^ { -1 } .
Using state-of-art stellar population synthesis models , we fit a number of spectral indices , from different chemical species ( including TiO ’ s and Na indices ) , to constrain the IMF slope ( i.e .
the fraction of low-mass stars ) , as a function of galactocentric distance , over a radial range out to \sim 4 kpc .
ETGs in our sample show a significant correlation of IMF slope and surface mass density .
The bottom-heavy population ( i.e .
an excess of low-mass stars in the IMF ) is confined to central galaxy regions with surface mass density above \sim 10 ^ { 10 } M _ { \odot } kpc ^ { -2 } , or , alternatively , within a characteristic radius of \sim 2 kpc .
Radial distance , in physical units , and surface mass density , are the best correlators to IMF variations , with respect to other dynamical ( e.g .
velocity dispersion ) and stellar population ( e.g .
metallicity ) properties .
Our results for the most massive galaxies suggest that there is no single parameter that fully explains variations in the stellar IMF , but IMF radial profiles at z \sim 0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions .