Radii of Young Star Clusters in Nearby Galaxies

Abstract

We measure the projected half-light radii of young star clusters in 31 galaxies from the Legacy Extragalactic UV Survey (LEGUS). We implement a custom pipeline specifically designed to be robust against contamination, which allows us to measure radii for 6097 clusters. This is the largest sample of young star cluster radii currently available. We find that most (but not all) galaxies share a common cluster radius distribution, with the peak at around 3 pc. We find a clear mass-radius relation of the form 𝑅 eff ∝ 𝑀 0.24 . This relation is present at all cluster ages younger than 1 Gyr, but with a shallower slope for clusters younger than 10 Myr. We present simple toy models to interpret these age trends, finding that high-mass clusters are more likely to be not tidally limited and expand. We also find that most clusters in LEGUS are gravitationally bound, especially at older ages or higher masses. Lastly, we present the cluster density and surface density distributions, finding a large scatter that appears to decrease with cluster age. The youngest clusters have a typical surface density of 100 𝑀 pc −2 .

Great Balls of FIRE I: The formation of star clusters across cosmic time in a Milky Way-mass galaxy

“…cl , and in agreement with the slope measured from the aggregated LEGUS catalogue of star clusters in nearby star-forming galaxies (Brown & Gnedin 2021).…”

“…Sizes, masses, and environmental properties in different M31 PHAT fields are taken from Johnson et al (2015Johnson et al ( , 2017Johnson et al ( , 2016 respectively. Cluster masses in M51 and NGC628 are taken from the data compilation and density profile fits performed by Brown & Gnedin (2021) on data from the LEGUS survey (Calzetti et al 2015;Adamo et al 2017;Cook et al 2019), and radially-binned Σ gas and Σ SFR from Messa et al (2018b) and Chevance et al (2020) for M51 and NGC628 respectively. Lastly we use data for M82, NGC253, and the Milky Way central molecular zone (CMZ) compiled by Choksi & Kruijssen (2019).…”

“…The initial mass function (hereafter simply "mass function") of young clusters may also vary: various works have reported evidence of a high-mass truncation at a certain characteristic cutoff mass scale (Gieles et al 2006b;Adamo et al 2015;Johnson et al 2017;Messa et al 2018a;Wainer et al 2022), with reported values ranging from ∼ 10 4 − 10 6 𝑀 , generally between regions of low and high star formation intensity Σ SFR 1 . And recently, several works have noted possible variations in the mass-radius relation of young star clusters, with more intensely star-forming environments hosting more compact clusters for a given mass (Krumholz et al 2019;Choksi & Kruijssen 2019;Grudić et al 2021b;Brown & Gnedin 2021). In all, it is clear that there is an intimate connection between star-forming environment and the properties of young star clusters.…”

The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

“…8). For comparison we show the recently published mass-size relation from Brown & Gnedin (2021). It appears that the simulated clusters are either too small for low star formation efficiencies or too large for high star formation efficiencies.…”

“…Table3.1 where we show both the average values at all times as well as from 200 Myr, Summary of simulation properties. 𝜖 ff : star formation efficiency per free-fall time, no-PI does not allow for HII regions; SFR: average star formation rate calculated between 0-500 Myr / 200-500 Myr; 𝜂 average mass loading (OFR/SFR) calculated between 0-500 Myr / 200-500 Myr; 𝑓 SN,A/B : fraction of SN exploding at densities higher (A) and lower (B) than 𝜌 ambient = 10 −2 cm −3 ; 𝛼 is the slope of the power-law cluster mass function; Γ FoF and Γ BC : average cluster formation efficiencies for FoF groups and bound clusters (BC) with ages < 10 Myr; Age Spread: average age dispersion of bound clusters; r 1/2 offset: offset of the half-mass radius from theBrown & Gnedin (2021) relation shown in Fig.13; M 5mm : average mass of the five most massive clusters; 𝑓 bound : fraction of FoF groups identified as bound.…”