Top-heavy integrated galactic stellar initial mass functions (IGIMFs) in starbursts

TL;DR

This paper investigates how extreme starburst conditions lead to top-heavy stellar initial mass functions (IMFs) within galaxies, affecting the galaxy-wide integrated IMF (IGIMF) and aligning with observed IMF variations across cosmic time.

Contribution

It introduces a model linking starburst-driven dense star clusters to top-heavy IGIMFs, highlighting the dependence on cluster mass functions and star formation rates.

Findings

IGIMFs can have slopes between 1.5 and 2.3 for massive stars.

Top-heavy IGIMFs occur in galaxies with intense starbursts and specific cluster mass function slopes.

The model explains observed IMF variations with redshift.

Abstract

Star formation rates (SFR) larger than 1000 Msun/ yr are observed in extreme star bursts. This leads to the formation of star clusters with masses > 10^6 Msun in which crowding of the pre-stellar cores may lead to a change of the stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of ultra-compact dwarf galaxies and recent results on globular clusters suggest the IMF to become top-heavy with increasing star-forming density. We explore the implications of top-heavy IMFs in these very massive and compact systems for the integrated galactic initial mass function (IGIMF), which is the galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The resulting IGIMFs can have slopes, alpha_3, for stars more massive than about 1 Msun between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass function (ECMF) slope (beta) of 2.0, but only if the ECMF has no low-mass clusters in galaxies with major starbursts. Alternatively, beta would have to decrease with increasing SFR >10 Msun/ yr such that galaxies with major starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of observationally deduced IMF variations with redshift.