A Substantial Population of Low Mass Stars in Luminous Elliptical Galaxies

TL;DR

This study provides direct evidence that low-mass stars are extremely abundant in luminous elliptical galaxies, suggesting a steeper initial mass function than previously assumed, which impacts our understanding of galaxy formation.

Contribution

It offers the first unambiguous detection of low-mass stars in elliptical galaxies through spectral features, indicating a different initial mass function than in the Milky Way.

Findings

Low-mass stars constitute over 80% of stars in elliptical galaxies.

Low-mass stars contribute more than 60% of the stellar mass.

The initial mass function in these galaxies is likely steeper than Salpeter.

Abstract

The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we investigate the stellar mass function in elliptical galaxies by measuring the strength of the Na I doublet and the Wing-Ford molecular FeH band in their spectra. These lines are strong in stars with masses <0.3 Msun and weak or absent in all other types of stars. We unambiguously detect both signatures, consistent with previous studies that were based on data of lower signal-to-noise ratio. The direct detection of the light of low mass stars implies that they are very abundant in elliptical galaxies, making up >80% of the total number of stars and contributing >60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1 - 1 Msun.