Galaxy And Mass Assembly (GAMA): The star formation rate dependence of the stellar initial mass function

TL;DR

This study finds that the stellar initial mass function (IMF) varies with galaxy star formation rate, with highly star-forming galaxies producing more massive stars, challenging the long-held assumption of IMF universality.

Contribution

It provides the first direct measurement of IMF slope dependence on star formation rate in low-to-moderate redshift galaxies, revealing a significant variation.

Findings

Highly star-forming galaxies have flatter IMF slopes.

IMF variation impacts galaxy evolution models.

Supports previous evidence of IMF evolution.

Abstract

The stellar initial mass function (IMF) describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the Galaxy And Mass Assembly survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMF-star formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter power-law slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.