Radial variations in the stellar initial mass function of early-type galaxies

TL;DR

This study investigates how the stellar initial mass function varies within early-type galaxies, revealing that in massive galaxies, the IMF's low-mass star fraction is higher in the core, implying local rather than universal IMF characteristics.

Contribution

It provides the first direct evidence that the IMF varies radially within early-type galaxies, challenging the notion of a universal IMF and highlighting local formation processes.

Findings

IMF varies significantly with galactocentric distance in massive ETGs

Low-mass star fraction is concentrated in galaxy centers of massive ETGs

No significant IMF gradient in low-{ ho} galaxies

Abstract

The hypothesis of a universal initial mass function (IMF) -- motivated by observations in nearby stellar systems -- has been recently challenged by the discovery of a systematic variation of the IMF with the central velocity dispersion, {\sigma}, of early-type galaxies (ETGs), towards an excess of low-mass stars in high-{\sigma} galaxies. This trend has been derived so far from integrated spectra, and remains unexplained at present. To test whether such trend depends on the local properties within a galaxy, we have obtained new, extremely deep, spectroscopic data, for three nearby ETGs, two galaxies with high {\sigma} (~300 km/s), and one lower mass system, with {\sigma} ~ 100 km/s. From the analysis of IMF-sensitive spectral features, we find that the IMF depends significantly on galactocentric distance in the massive ETGs, with the enhanced fraction of low-mass stars f mostly confined to their central regions. In contrast, the low-{\sigma} galaxy does not show any significant radial gradient in the IMF, well described by a shallower distribution, relative to the innermost regions of massive galaxies, at all radii. Such a result indicates that the IMF should be regarded as a local (rather than global) property, and suggests a significant difference between the formation process of the core and the outer regions of massive ETGs.