KINETyS: Constraining spatial variations of the stellar initial mass function in early-type galaxies ★

Abstract

The heavyweight stellar initial mass function (IMF) observed in the cores of massive early-type galaxies (ETGs) has been linked to formation of their cores in an initial swiftlyquenched rapid starburst. However, the outskirts of ETGs are thought to be assembled via the slow accumulation of smaller systems in which the star formation is less extreme; this suggests the form of the IMF should exhibit a radial trend in ETGs. Here we report radial stellar population gradients out to the half-light radii of a sample of eight nearby ETGs. Spatially resolved spectroscopy at 0.8-1.35µm from the VLT's KMOS instrument was used to measure radial trends in the strengths of a variety of IMF-sensitive absorption features (including some which are previously unexplored). We find weak or no radial variation in some of these which, given a radial IMF trend, ought to vary measurably, e.g. for the Wing-Ford band we measure a gradient of +0.06 ± 0.04 per decade in radius.Using stellar population models to fit stacked and individual spectra, we infer that the measured radial changes in absorption feature strengths are primarily accounted for by abundance gradients which are fairly consistent across our sample (e.g. we derive an average [Na/H] gradient of -0.53±0.07). The inferred contribution of dwarf stars to the total light typically corresponds to a bottom heavy IMF, but we find no evidence for radial IMF variations in the majority of our sample galaxies.

SDSS-IV MaNGA: stellar initial mass function variation inferred from Bayesian analysis of the integral field spectroscopy of early-type galaxies

“…A number of earlier investigations have reported the presence of IMF gradients indicating dwarf-richer stellar populations towards the centers of massive ETGs (e.g. Martín-Navarro et al 2015a;La Barbera et al 2017;van Dokkum et al 2017;Vaughan et al 2018b;Sarzi et al 2018;Parikh et al 2018), with the exceptions in some other investigations (Vaughan et al 2018a;Zieleniewski et al 2015Zieleniewski et al , 2017McConnell et al 2016;Alton et al 2017).…”

“…However, other investigations question the results, suggesting that the radial variation in the stellar population is generated by abundance gradients of individual elements, rather than by a change in the IMF (e.g. Zieleniewski et al 2015Zieleniewski et al , 2017McConnell et al 2016;Alton et al 2017;Vaughan et al 2018a).…”

Calibrated, cosmological hydrodynamical simulations with variable IMFs III: spatially resolved properties and evolution

“…An alternative definition of the dwarf-to-giant ratio is the fraction of the luminosity that is contributed by lowmass (m < 0.5 M ) dwarf stars, f dwarf . It has been shown by Alton et al (2017) that this quantity correlates very strongly with the equivalent widths of IMF-sensitive stellar absorption features, and thus may be a better diagnostic of the low-mass regime of the IMF than F0.5,1. However, since f dwarf by itself is also dependent on age, we normalize f dwarf for our galaxies by f dwarf,Salp , the value that would have been obtained had the stars evolved instead with a Salpeter IMF, given the same ages and metallicities.…”

“…Milky-Way-like) IMF from ≈ 0.1 to 1 times the half-light radius, re (Boroson & Thompson 1991;Martín-Navarro et al 2015b; La Barbera et al 2016;van Dokkum et al 2017;Sarzi et al 2018;Oldham & Auger 2018a). The presence of such gradients is still energetically debated, however, with some studies finding no gradients or arguing that it is currently too difficult to disentangle the IMF from radial abundance gradients (McConnell et al 2016;Zieleniewski et al 2017;Davis & McDermid 2017;Alton et al 2017Alton et al , 2018Vaughan et al 2018a,b).…”

Combining stellar populations with orbit-superposition dynamical modelling: the formation history of the lenticular galaxy NGC 3115

“…Independently, stellar-population models of galaxies have been utilised for their own range of applications, covering similar scales in sample size. These models have been applied to constrain the IMF shape, normalisation, and low-mass cut-off (for instance, see Conroy & van Dokkum 2012;Smith et al 2012;Martín-Navarro et al 2015;Alton et al 2017;Rosani et al 2018;Dries et al 2018;Vaughan et al 2018); the measurement and interpretation of αenhancement (as in Thomas et al 2005;Conroy et al 2014;Greene et al 2015;McDermid et al 2015); the measurement of spatial gradients in stellar properties (for instance, see Mehlert et al 2003;Sánchez-Blázquez et al 2007;Kuntschner et al 2010;Li et al 2018); assembly timescales for galaxy formation (Martin-Navarro et al 2018). Similarly to dynamical models, stellar population models have been used to uncover new relationships between galaxy structural parameters through a variety of scaling relations (see Gallazzi et al 2008;Graves et al 2009a,b;McDermid et al 2015;Walcher et al 2015, for some such relations).…”