Journal Article

A substantial population of low-mass stars in luminous elliptical galaxies

van Dokkum, Pieter G., Conroy, Charlie

2010

Overview

The lesser lights of nearby galaxies The bulk of the stellar population is comprised of dwarf stars. This fact is reflected in the stellar initial mass function (IMF), which describes the mass distribution of stars at the time of their formation. The IMF is reasonably well constrained in the disk of the Milky Way, but we have little direct information on the IMF in other galaxies and at earlier cosmic epochs. Pieter van Dokkum and Charlie Conroy have now spectroscopically detected the signature of the many 'invisible' stars in the light of nearby elliptical galaxies by observing the Na I doublet and the Wing–Ford molecular FeH band, lines which are strong in stars with masses of less than a third that of the Sun. The data imply that these smaller stars account for more than 80% of the total number of stars and contribute more than 60% of total stellar mass in elliptical galaxies. The stellar initial mass function describes the mass distribution of stars at the time of their formation. This study reports observations of the Na I doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses <0.3 solar masses and weak or absent in all other types of stars. 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. 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 1 but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na  i doublet 2 , 3 and the Wing–Ford molecular FeH band 4 , 5 in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3 M ⊙ (where M ⊙ is the mass of the Sun) and are weak or absent in all other types of stars 5 , 6 , 7 . We unambiguously detect both signatures, consistent with previous studies 8 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 over 80% of the total number of stars and contributing more than 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 9 in the mass range 0.1 M ⊙ to 1 M ⊙ .

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766/33/34/863 / 639/766/33/34/867 / Astronomy / Astrophysics / Band spectra / Characteristics and properties of external galaxies and extragalactic objects / Disks / Earth, ocean, space / Elliptical galaxies / Exact sciences and technology / Galaxies / Humanities and Social Sciences / letter / Mass distribution / multidisciplinary / Normal galaxies. Extragalactic objects and systems (by type) / Science / Science (multidisciplinary) / Spectra / Stars / Stars & galaxies / Stellar content and populations. Radii. Morphology and overall structure / Stellar systems. Galactic and extragalactic objects and systems. The universe / Universe