The Spatial Distribution of the Young Stellar Clusters in the Star-Forming Galaxy NGC 628

Abstract

We present a study of the spatial distribution of the stellar cluster populations in the star forming galaxy NGC 628. Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young ( 100 Myr) stellar clusters within the galaxy, identified from a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent α. We recover a weighted mean index of α ∼ −0.8 for all spatial scales below the break at 3. ′′ 3 (158 pc at a distance of 9.9 Mpc) and an index of α ∼ −0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.

The Hierarchical Distribution of the Young Stellar Clusters in Six Local Star-forming Galaxies

“…The largest galaxy of the M74 galaxy group with an angular size of 10 5 × 9 5, a central and east pointing were obtained. The clustering of the young stellar clusters in this galaxy was investigated in an earlier paper by Grasha et al (2015), where an age dependency for the clustering strength was found with a randomization timescale of ∼40Myr, after which the clusters displayed a flatter, more homogeneous distribution.…”

“…A notable feature is the flat or decreasing distribution toward larger ages, especially visible in the class 3 associations, consistent with cluster disruption on timescales of a few tens of Myr or less; cluster fading can compound this effect as well. The class 3 population does behave distinctly differently compared to class 1 and 2 clusters: in addition to a decrease in their numbers at ages older than a few 10 Myr, their clustering properties in NGC 628 behave quite differently than class 1 and 2 clusters (Grasha et al 2015), and their disruption rate is significantly higher compared to class 1 and 2 clusters (Adamo et al 2017). Thus, we maintain that the class 3 associations represent a distinct type of system and appear to be born with lower densities than class 1 and 2 clusters and are not the remnants of dispersed/disrupted class 1 or 2 clusters.…”

“…The formulation in Equation (2) is optimized to mitigate edge effects and systematic errors on the computed correlation function, necessary to alleviate unrealistic calculations of ω(θ) at large spatial scales due to the deviation from a random distribution which can arise from a limited field of view and edge effects. In Grasha et al (2015), we investigate the influence of the random catalog size and geometry on the outcome of the two-point correlation function. Figure 6 shows the correlation functions for all six galaxies, calculated for the entire sample of star clusters and divided by their cluster classification.…”

“…A comparison of the cluster ages show that there is no difference in the results for ages that are derived with stochastic versus non-stochastic models (Grasha et al 2017). As the clustering results is most heavily influenced by the youngest clusters (<100 Myr; see Section 5.2 as well as Grasha et al 2015), which may lie below the mass limit of 5000 M e (Adamo et al 2017), it is important to examine how mass cuts influence the clustering analysis to ensure that our results are not biased against the detection limitations. Figure 7 shows the clustering correlation for the young (<100 Myr) clusters in NGC 628 with mass cuts applied at both 3000 and 5000 M e .…”

“…The evolution and erasure of the unbound hierarchical structures has been the focus of investigation in recent years, where observations of local galaxies support an age-dependent clustering of the stellar components (e.g., Pellerin et al 2007;Bastian et al 2009;Scheepmaker et al 2009; Pellerin et al 2012;Baumgardt et al 2013;Gouliermis et al 2014aGouliermis et al , 2015Grasha et al 2015), and the clustering becomes progressively weaker for older populations. Hierarchical clustering is expected to dissipate with age (Elmegreen et al 2006;Elmegreen 2010) as the densest regions with the shortest mixing timescales lose their substructures first, whereas the larger, unbound regions will lose their substructure over longer periods of time owing to tidal forces and random velocities (Bate et al 1998), dispersing over time to form the field population.…”