Stellar clusters in M83: formation, evolution, disruption and the influence of the environment

Abstract

We study the stellar cluster population in two adjacent fields in the nearby, face‐on spiral galaxy M83 using multiwavelength Wide Field Camera 3/Hubble Space Telescope imaging. After automatic detection procedures, the clusters are selected through visual inspection to be centrally concentrated, symmetric, and resolved on the images, which allows us to differentiate between clusters and likely unbound associations. We compare our sample with previous studies and show that the differences between the catalogues are largely due to the inclusion of a large numbers of diffuse associations within previous catalogues as well as the inclusion of the central starburst region, where the completeness limit is significantly worse than in the surrounding regions. We derive the size distribution of the clusters, which is well described by a lognormal distribution with a peak at ∼2.5 pc, and find evidence for an expansion in the half‐light radius of clusters with age. The luminosity function of the clusters is well approximated by a power law with an index of −2 over most of the observed range; however, a steepening is seen at MV=−9.3 and −8.8 in the inner and outer fields, respectively. Additionally, we show that the cluster population is inconsistent with a pure power‐law mass distribution, but instead exhibits a truncation at the high‐mass end. If described as a Schechter function, the characteristic mass is 1.6 × 105 and 0.5 × 105 M⊙ for the inner and outer fields, respectively, in agreement with previous estimates of other cluster populations in spiral galaxies. Comparing the predictions of the mass‐independent disruption (MID) and mass‐dependent disruption (MDD) scenarios with the observed distributions, we find that both models can accurately fit the data. However, for the MID case, the fraction of clusters destroyed (or mass lost) per decade in age is dependent on the environment; hence, the age and mass distributions of clusters are not universal. In the MDD case, the disruption time‐scale scales with galactocentric distance (being longer in the outer regions of the galaxy) in agreement with analytic and numerical predictions. Finally, we discuss the implications of our results on other extragalactic surveys, focusing on the fraction of stars that form in clusters and the need (or lack thereof) for infant mortality.

Properties of the cluster population of NGC 1566 and their implications

“…This was also found by Chandar et al (2014) (radially dependent index of the age distribution) who needed to invoke radially dependent differences in the cluster formation history in order to bring the age distributions in the inner and outer regions of the galaxy into agreement. Bastian et al (2012) found that both the MID and MDD scenarios could provide good fits to the observed age distributions of clusters in M83, however, disruption needed to be strongly dependent on environment. Additionally, these authors found that the mass function for the clusters was truncated, and the truncation value depended on environment.…”

“…The power law section of the mass distribution, where dN/dM ∝ M −β , has been found to be best fit in most cases with β ≈ 2 (e.g. Bastian et al 2012).…”

“…Similar to the work done by and again by Bastian et al (2012) and Silva-Villa et al (2014) for M83, we used concentration indices (CIs) to refine the sample further. A CI is a measurement of how centrally concentrated emission is for a source.…”

“…Bastian et al 2012). Without knowledge of the stellar dynamics within clusters/groups, it is impossible to unambiguously determine whether the collection is bound or not.…”

“…If approximated as a single power-law, log(dN/dt) ∝ t ζ , where studies to date have found ζ to vary between 0 and -1 (e.g. Fall et al 2005, Bastian et al 2012, Ryon et al 2014.…”