A New Method for Measuring the Upper End of the IMF

TL;DR

This paper introduces a novel method to investigate the upper end of the stellar Initial Mass Function (IMF) by analyzing ionizing photon production in unresolved star clusters, extending the reach of such studies to galaxies up to 10 Mpc away.

Contribution

The paper presents a new technique for probing the upper end of the IMF in distant galaxies using ionizing photon measurements, surpassing previous limitations of star counting.

Findings

Method can probe IMF in galaxies up to 10 Mpc away

No clear dependence of upper IMF on cluster mass in M51a

Applicable to unresolved star clusters in distant galaxies

Abstract

A method is presented here for investigating variations in the upper end of the stellar Initial Mass Function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages<10 Myr and covering a range of masses. We test this method on the young cluster population in the nearby galaxy M51a, for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least 10 Mpc, i.e., a factor 200 further away than possible by counting individual stars in young compact clusters. Our results for this galaxy show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster, down to ~1000 M_sun, although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion.