Bottom-heavy initial mass functions reveal hidden mass in early galaxies

TL;DR

This study uses JWST data to measure the initial mass function in early galaxies, revealing they have more low-mass stars than the Milky Way, which increases their estimated stellar masses and challenges existing galaxy formation models.

Contribution

First robust measurements of the IMF beyond the local universe showing a bottom-heavy IMF in early galaxies, indicating higher stellar masses than previously inferred.

Findings

Early galaxies have more low-mass stars than the Milky Way.

Bottom-heavy IMFs increase galaxy stellar mass estimates by 3-4 times.

Results challenge current galaxy formation models.

Abstract

JWST observations have revealed that massive galaxies formed and evolved far faster than predicted by galaxy formation models, with many having already assembled a large mass in stars $\sim12$ billion years ago [1-7]. However, masses of distant galaxies are highly uncertain, as they assume a distribution of stellar birth masses (the initial mass function [IMF]) similar to that in the Milky Way (MW). Specifically, the contribution from low-mass stars, which make up the bulk of stellar mass, is not directly observed, but inferred based on an extrapolation of the MW IMF. Here, we provide the first robust measurements of the IMF beyond the local Universe. Using ultra-deep spectra of nine massive, quiescent galaxies at $z\sim0.7$ from the ambitious JWST-IMFERNO program, extended to bluer wavelengths with deep spectra from LEGA-C [8], we find that these distant galaxies have excess low-mass stars. In other words, they have more bottom-heavy IMFs than the MW. For the oldest two galaxies, which are direct descendants of JWST's "impossibly early" galaxies, the bottom-heavy IMFs increase their stellar masses by a factor of $3-4$. These galaxies thus amplify the tension with galaxy formation models.