Evidence of a Bottom-heavy Initial Mass Function in Massive Early-type Galaxies from Near-infrared Metal Lines

TL;DR

This study provides evidence that massive early-type galaxies have a bottom-heavy initial mass function, with more low-mass stars, based on near-infrared spectral line analysis indicating IMF variation with galaxy mass.

Contribution

First direct near-infrared spectroscopic evidence showing IMF variation in massive galaxies, highlighting a trend toward bottom-heavy IMFs in higher-mass systems.

Findings

Low-mass galaxies favor a Chabrier IMF.

High-mass galaxies are better described by a steeper, dwarf-rich IMF.

Line ratios correlate with galaxy velocity dispersion.

Abstract

We present new evidence for a variable stellar initial mass function (IMF) in massive early-type galaxies, using high-resolution, near-infrared spectroscopy from the Folded-port InfraRed Echellette spectrograph (FIRE) on the Magellan Baade Telescope at Las Campanas Observatory. In this pilot study, we observe several gravity-sensitive metal lines between 1.1 $\mu$m and 1.3 $\mu$m in eight highly-luminous ($L \sim 10 L_*$) nearby galaxies. Thanks to the broad wavelength coverage of FIRE, we are also able to observe the Ca II triplet feature, which helps with our analysis. After measuring the equivalent widths (EWs) of these lines, we notice mild to moderate trends between EW and central velocity dispersion ($\sigma$), with some species (K I, Na I, Mn I) showing a positive EW-$\sigma$ correlation and others (Mg I, Ca II, Fe I) a negative one. To minimize the effects of metallicity, we measure the ratio $R$ = [EW(K I) / EW(Mg I)], finding a significant systematic increase in this ratio with respect to $\sigma$. We then probe for variations in the IMF by comparing the measured line ratios to the values expected in several IMF models. Overall, we find that low-mass galaxies ($\sigma \sim 100$ km s$^{-1}$) favor a Chabrier IMF, while high-mass galaxies ($\sigma \sim 350$ km s$^{-1}$) are better described with a steeper (dwarf-rich) IMF slope. While we note that our galaxy sample is small and may suffer from selection effects, these initial results are still promising. A larger sample of galaxies will therefore provide an even clearer picture of IMF trends in this regime.