The Optical Colors of Giant Elliptical Galaxies and their Metal-Rich Globular Clusters Indicate a Bottom-Heavy Initial Mass Function

TL;DR

This study finds that massive elliptical galaxies have redder optical colors than their metal-rich globular clusters, suggesting a bottom-heavy initial mass function in these galaxies, which affects their stellar populations.

Contribution

It provides evidence that the initial mass function in massive ellipticals is steeper at low masses, explaining color differences without age or metallicity variations.

Findings

Galaxies are redder than their GCs at the same galactocentric distance.

Spectroscopic indices show similar ages and metallicities between galaxies and GCs.

Color differences are best explained by a bottom-heavy IMF in galaxies.

Abstract

We report a systematic and statistically significant offset between the optical (g-z or B-I) colors of seven massive elliptical galaxies and the mean colors of their associated massive metal-rich globular clusters (GCs) in the sense that the parent galaxies are redder by 0.12-0.20 mag at a given galactocentric distance. However, spectroscopic indices in the blue indicate that the luminosity-weighted ages and metallicities of such galaxies are equal to that of their averaged massive metal-rich GCs at a given galactocentric distance, to within small uncertainties. The observed color differences between the red GC systems and their parent galaxies cannot be explained by the presence of multiple stellar generations in massive metal-rich GCs, as the impact of the latter to the populations' integrated g-z or B-I colors is found to be negligible. However, we show that this paradox can be explained if the stellar initial mass function (IMF) in these massive elliptical galaxies was significantly steeper at subsolar masses than canonical IMFs derived from star counts in the solar neighborhood, with the GC colors having become bluer due to dynamical evolution, causing a significant flattening of the stellar MF of the average surviving GC.