Spatial segregation of massive clusters in a simulation of colliding dwarf galaxies

TL;DR

This study uses simulations of colliding dwarf galaxies to analyze star cluster properties, revealing consistent power-law mass functions, spatial segregation of massive clusters, and evidence of cluster merging, independent of star formation rate.

Contribution

It demonstrates that the power-law distribution of cluster masses and their spatial segregation are inherent features established at birth, with cluster merging contributing to mass growth.

Findings

Cluster mass function follows a power law with slope ~ -1 from birth.

Massive clusters are more centrally concentrated than lower mass ones.

Approximately 8% of clusters merge over 300 Myr, with higher merging rates in young clusters.

Abstract

The collective properties of star clusters are investigated using a simulation of the collision between two dwarf galaxies. The characteristic power law of the cluster mass function, N(M), with a logarithmic slope d\log N/d\log M ~ -1, is present from cluster birth and remains throughout the simulation. The maximum mass of a young cluster scales with the star formation rate (SFR). The relative average minimum separation, R(M)= N(M)^{1/p}D_min(M)/D(M_low), for average minimum distance D_min(M) between clusters of mass M, and for lowest mass, M_low, measured in projection (p=2) or three dimensions (p=3), has a negative slope, d log R/d log M ~ -0.2, for all masses and ages. This agrees with observations of R(M) in low-mass galaxies studied previously. Like the slope of N(M), R(M) is apparently a property of cluster birth for dwarf galaxies that does not depend on SFR or time. The negative slope for R(M) implies that massive clusters are more concentrated relative to lower mass clusters throughout the entire mass range. Cluster growth through coalescence is also investigated. The ratio of the kinetic to potential energy of all near-neighbor clusters is generally large, but a tail of low values in the distribution of this ratio suggests that a fraction of the clusters merge, ~8% by number throughout the ~300 Myr of the simulation and up to 60% by mass for young clusters in their first 10 Myr, scaling with the SFR above a certain threshold.