Spatially resolved variations of the IMF mass normalisation in early-type galaxies as probed by molecular gas kinematics

TL;DR

This study uses molecular gas kinematics to spatially map the initial mass function (IMF) variations within early-type galaxies, revealing diverse IMF normalizations and confirming internal IMF variation without clear correlation to galaxy properties.

Contribution

First spatially-resolved analysis of IMF variations in external galaxies using molecular gas kinematics, providing new insights into IMF diversity within early-type galaxies.

Findings

IMF normalization varies within galaxies, with some requiring a light IMF and others a heavier one.

No clear correlation found between IMF normalization and galaxy properties.

IMF variation is confirmed within massive early-type galaxies.

Abstract

We here present the first spatially-resolved study of the IMF in external galaxies derived using a dynamical tracer of the mass-to-light ratio. We use the kinematics of relaxed molecular gas discs in seven early-type galaxies (ETGs) selected from the ATLAS3D survey to dynamically determine mass-to-light ratio (M/L) gradients. These M/L gradients are not very strong in the inner parts of these objects, and galaxies that do show variations are those with the highest specific star formation rates. Stellar population parameters derived from star formation histories are then used in order to estimate the stellar initial mass function function (IMF) mismatch parameter, and shed light on its variation within ETGs. Some of our target objects require a light IMF, otherwise their stellar population masses would be greater than their dynamical masses. In contrast, other systems seem to require heavier IMFs to explain their gas kinematics. Our analysis again confirms that IMF variation seems to be occurring within massive ETGs. We find good agreement between our IMF normalisations derived using molecular gas kinematics and those derived using other techniques. Despite this, we do not see find any correlation between the IMF normalisation and galaxy dynamical properties or stellar population parameters, either locally or globally. In the future larger studies which use molecules as tracers of galaxy dynamics can be used to help us disentangle the root cause of IMF variation.