The apparent Large Magellanic Cloud star cluster age gap

TL;DR

This study models the star cluster age distribution in the Large Magellanic Cloud to explain the observed age gap, linking star formation history, cluster mass, and observational limits.

Contribution

It introduces a model connecting star formation history and cluster initial mass to reproduce the LMC's age gap and explains its origin.

Findings

The model reproduces the observed lack of clusters aged 4-11 Gyr.

A maximum initial cluster mass of at least 2x10^5 M_sun is needed to match ancient clusters.

The age gap results from lower star formation rates and observational detection limits.

Abstract

In the Large Magellanic Cloud (LMC), there have been very few clusters observed with ages between 4 and 11 Gyr. This phenomenon is sometimes referred to as the `LMC age gap'. We constructed a model of the cluster age distribution aimed at reproducing this scenario. We linked the star formation history to the cluster initial mass function via a power-law relation between maximum initial cluster mass and global star formation rate. Using a constant cluster-forming efficiency of 5%, we obtained the cluster formation history. Applying a model of cluster mass loss calibrated using N-body simulations and an observational completeness limit, we computed the observable fraction of initially formed clusters. We were then able to model the cluster age distribution. For a maximum initial cluster mass below $10^5$M$_\odot$ at a star formation rate of 1 M$_\odot$pc$^{-2}$Gyr$^{-1}$, our model reproduced the observed lack of clusters with ages between 4 and 11 Gyr. However, our model required a maximum initial mass at 1 M$_\odot$pc$^{-2}$Gyr$^{-1}$ of at least $2\cdot 10^5$M$_\odot$ in order to reproduce the population of ancient globular clusters. A linear change between maximum initial cluster mass relations from 8 to 12 Gyr reproduced the age gap to a satisfactory extent. In our model, the age gap is a consequence of the star-forming history and current observational limits. The age gap corresponds to a period characterised by a lower star formation rate, whereby no clusters with an initial mass above approximately 2 to 5$\cdot 10^5$M$_\odot$ were formed. In the present day, these clusters have become so faint that only few of them have been detected. The pattern of both young-and-bright and old-and-massive clusters being more easily detectable than clusters of intermediate ages might reflect a more general phenomenon and not necessarily one specific to the LMC.