The cluster initial mass function of the M82 disk Super Star Clusters

TL;DR

This study reconstructs the initial mass distribution of super star clusters in M82's disk, showing a power-law form and attributing the observed mass function features to observational biases rather than intrinsic properties.

Contribution

It demonstrates that the observed cluster mass function's shape results from observational incompleteness, favoring a power-law initial mass function over a log-normal one.

Findings

The best fit CIMF is a power-law with index 1.8.

The observed turn-over is due to observational incompleteness.

Simulations reproduce the observed mass-radius relation.

Abstract

The presence of a population of a large number ($\sim$400) of almost coeval (100--300 Myr) super star clusters (SSCs) in the disk of M82 offers an opportunity to construct the Cluster Initial Mass Function (CIMF) from the observed present-day Cluster Mass Function (CMF). We carry out the dynamical and photometric evolution of the CMF assuming the clusters move in circular orbits under the gravitational potential of the host galaxy using the semi-analytical simulation code EMACSS. We explore power-law and log-normal functions for the CIMFs, and populate the clusters in the disk assuming uniform, power-law, and exponential radial distribution functions. We find that the observed CMF is best produced by a CIMF that is power-law in form with an index of 1.8, for a power-law radial distribution function. More importantly, we establish that the observed turn-over in the present-day CMF is the result of observational incompleteness rather than due to dynamically induced effects, or an intrinsically log-normal CIMF, as was proposed for the fossil starburst region B of this galaxy. Our simulations naturally reproduce the mass-radius relation observed for a sub-sample of M82 SSCs.