Is there a Size Difference between Red and Blue Globular Clusters?

TL;DR

This study investigates why blue globular clusters appear larger than red ones, concluding that luminosity function differences primarily cause this size discrepancy, while physical size differences are less significant.

Contribution

The paper demonstrates through simulations that luminosity function differences explain the size variation, and physical size differences require black holes, which overestimate observed differences.

Findings

Luminosity functions account for most size differences.

Physical size differences are possible but generally larger than observed.

Half-light radius is not a reliable measure of true cluster size.

Abstract

Blue (metal-poor) globular clusters are observed to have half-light radii that are ~20% larger than their red (metal-rich) counterparts. The origin of this enhancement is not clear and differences in either the luminosity function or in the actual size of the clusters have been proposed. I analyze a set of dynamically self-consistent Monte Carlo globular cluster simulations to determine the origin of this enhancement. I find that my simulated blue clusters have larger half-light radii due to differences in the luminosity functions of metal-poor and metal-rich stars. I find that the blue clusters can also be physically larger, but only if they have a substantial number of black holes heating their central regions. In this case the difference between half-light radii is significantly larger than observed. I conclude that the observed difference in half-light radii between red and blue globular clusters is due to differences in their luminosity functions and that half-light radius is not a reliable proxy for cluster size.