The Spatial Evolution of Star Clusters in NGC 628 with JWST

TL;DR

The paper investigates how galactic environment governs the spatial distribution of star clusters in the face-on spiral galaxy NGC 628 by applying the INDICATE local clustering statistic to a comprehensive 6890-cluster catalog that combines JWST FEAST-derived embedded clusters with optical HST clusters. By examining dependencies on galactocentric radius, evolutionary stage, and mass, the study reveals that embedded clusters are highly clustered, while emerging clusters become progressively looser in their associations; the concentrated population narrows to 2–6 kpc as clusters age. Inner regions near the nucleus and co-rotation radius show distinct radial dynamical effects that disrupt tight concentrations, whereas spiral arms host the strongest clustering and host Type 2 mass segregation, where YMCs tend to reside in concentrated regions with abundant lower-mass neighbors. The results highlight how spatial structure traces hierarchical star formation, gas density, and galactic dynamics, offering a framework to interpret cluster demographics and their evolution in spiral galaxies, with future work planned to compare cluster spatial distributions to molecular gas maps. All figures rely on robust statistics, including biases corrections, and showcase INDICATE as a powerful tool for per-cluster spatial analysis in heterogeneous astrophysical datasets.

Abstract

We examine the spatial distribution of star clusters in NGC 628 using the statistical tool INDICATE to quantify clustering tendencies. Our sample, based on HST and JWST observations, is the most complete to date, spanning ages from 1 Myr to >100 Myr. We find cluster spatial behaviour varies with galactic position, age, and mass. Most emerging young clusters are tightly spatially associated with each other, while fully emerged clusters are in \sim1.5 times looser spatial associations, irrespective of age. Young Massive Clusters (YMCs \ge 10^4 M_{\odot}) tend to associate with lower-mass clusters but not strongly with other YMCs, implying that intense star formation regions produce a few YMCs alongside many lower-mass clusters rather than multiple YMCs together. Young concentrated clusters show a wide radial distribution in the galactic disk, which narrows with age; with concentrated clusters >100 Myr mostly residing between 2-6 kpc. This pattern may reflect either faster dispersal of isolated tight cluster spatial "structure" in a lower gas density outer disk or gradual inside-out growth, with the formation of this structure shifting outward over time. We also detect distinct spatial behaviours for clusters within 2 kpc, linked to the inner Lindblad resonance (\le1 kpc), nuclear ring (\sim0.5-1 kpc), and the start of spiral arms (\sim1.25-2 kpc), suggesting these regions exhibit strong radial motions that could hinder clusters from forming and remaining in tight concentrations. Our results highlight how spatially-resolved studies of clusters can reveal the influence of galactic dynamics on star formation and cluster evolution.

Figures (13)

• Figure 1: Final cluster sample as described in Sect. \ref{['sect_final_sample']}, overlaid on the Spitzer Infrared Nearby Galaxies Survey $8\mu$m image of NGC 628. Coloured markers denote which catalogue the cluster originates from, as specified by the legend. The grey and red-dashed panels denote the position of the HST and JWST images respectively.
• Figure 2: Plot of age against mass for our cluster catalogue. Green stars represent optical YSCs, orange crosses eYSCs that are optically visible, and blue triangles eYSCs that are not optically visible. See Sect. \ref{['sect_cat_selection']} for details.
• Figure 3: Plots of index values, $I_5$ , calculated by INDICATE for NGC 628, with the environmental mask by 2021AA...656A.133Q overlaid showing the positions of Spiral Arms A-G (marked by various shades of grey), and Galactic Center (dark grey circle, 'GC'). Clusters with an index value above the significance threshold ($I_5 > 2.4$) are represented by circle markers, colour-coded by their $I_5$ index, where higher values denote greater degrees of spatial concentration. Purple triangles are clusters with an index value below the significance threshold.
• Figure 4: Index values, $I_5$, plotted as a function of galactic position (left) and as a histogram (right), for the (Top:) eYSCs and (Bottom:) recently emerged clusters (RECs), defined as the optical YSCs $<10$ Myr, discussed in Sect. \ref{['sect_results_eYSCs']}. The index values represent the degree of association clusters have with their counterparts of similar age. On the left panels, clusters with an index value above their sample's significance threshold are represented by circle markers, colour-coded by their $I_5$ value, as per the respective colour bars. Clusters with index values below the sample's significance threshold are marked by Purple triangles on the left panels, and shaded areas on the histograms. On the left panels, the environmental mask by 2021AA...656A.133Q is overlaid to show the location of Spiral Arms A-G (marked by various shades of grey), and Galactic Center (dark grey circle, labelled 'GC'). The grey, red-dashed and black-dotted boxes denote the position of the HST mosaics, the JWST coverage, and their overlap region respectively. Line colour and style in the histograms on the right correspond to that of clusters within the respective coverage and overlap areas, as denoted in the position plots.
• Figure 5: Index values, $I_5$, for the Young (top), Intermediate (middle) and Old clusters (bottom), as discussed in Sect. \ref{['sect_results_evo']}. The index values represent the degree of association clusters have with their counterparts of similar age. Left panels show $I_5$ as a function of galactic position, and right panels as a histogram. The grey boxes on the left panel denote the position of the HST coverage, with the environmental mask by 2021AA...656A.133Q overlaid. The teal cross and ellipses mark the galactic center, a radius of 1.25 kpc, 2 kpc and the galactic co-rotation radius (6.3 kpc; 2021MNRAS.508..912S), respectively. Clusters with an index value above their sample's significance threshold are represented by circle markers, colour-coded by their index value. Clusters with index values below the sample's significance threshold are marked by Purple triangles and shaded on the position and histogram plots.