SDSS-IV MaNGA: the spatially resolved stellar initial mass function in ∼400 early-type galaxies

Abstract

MaNGA provides the opportunity to make precise spatially resolved\nmeasurements of the IMF slope in galaxies owing to its unique combination of\nspatial resolution, wavelength coverage and sample size. We derive radial\ngradients in age, element abundances and IMF slope analysing optical and\nnear-infrared absorption features from stacked spectra out to the half-light\nradius of 366 early-type galaxies with masses $9.9 - 10.8\\;\\log M/M_{\\odot}$.\nWe find flat gradients in age and [$\\alpha$/Fe] ratio, as well as negative\ngradients in metallicity, consistent with the literature. We further derive\nsignificant negative gradients in the [Na/Fe] ratio with galaxy centres being\nwell enhanced in Na abundance by up to 0.5 dex. Finally, we find a gradient in\nIMF slope with a bottom-heavy IMF in the centre (typical mass excess factor of\n1.5) and a Milky Way-type IMF at the half-light radius. This pattern is\nmass-dependent with the lowest mass galaxies in our sample featuring only a\nshallow gradient around a Milky Way IMF. Our results imply the local\nIMF-$\\sigma$ relation within galaxies to be even steeper than the global\nrelation and hint towards the local metallicity being the dominating factor\nbehind the IMF variations. We also employ different stellar population models\nin our analysis and show that a radial IMF gradient is found independently of\nthe stellar population model used. A similar analysis of the Wing-Ford band\nprovides inconsistent results and further evidence of the difficulty in\nmeasuring and modelling this particular feature.\n