Dynamical masses of brightest cluster galaxies II: constraints on the stellar IMF

Abstract

We use stellar and dynamical mass profiles, combined with a stellar population analysis, of 32 brightest cluster galaxies (BCGs) at redshifts of 0.05 $\le z\le$ 0.30, to place constraints on their stellar Initial Mass Function (IMF). We measure the spatially-resolved stellar population properties of the BCGs, and use it to derive their stellar mass-to-light ratios (${{\rm Y}}_{\star \mathrm{POP}}$). We find young stellar populations ($<$200 Myr) in the centres of 22 per cent of the sample, and constant ${{\rm Y}}_{\star \mathrm{POP}}$ within 15 kpc for 60 per cent of the sample. We further use the stellar mass-to-light ratio from the dynamical mass profiles of the BCGs (${{\rm Y}}_{\star \mathrm{DYN}}$), modelled using a Multi-Gaussian Expansion (MGE) and Jeans Anisotropic Method (JAM), with the dark matter contribution explicitly constrained from weak gravitational lensing measurements. We directly compare the stellar mass-to-light ratios derived from the two independent methods, ${{\rm Y}}_{\star \mathrm{POP}}$ (assuming some IMF) to ${{\rm Y}}_{\star \mathrm{DYN}}$ for the subsample of BCGs with no young stellar populations and constant ${{\rm Y}}_{\star \mathrm{POP}}$. We find that for the majority of these BCGs, a Salpeter (or even more bottom-heavy) IMF is needed to reconcile the stellar population and dynamical modelling results although for a small number of BCGs, a Kroupa (or even lighter) IMF is preferred. For those BCGs better fit with a Salpeter IMF, we find that the mass-excess factor against velocity dispersion falls on an extrapolation (towards higher masses) of known literature correlations. We conclude that there is substantial scatter in the IMF amongst the highest-mass galaxies.

15 pages, 11 figures, accepted for publication in MNRAS

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