Mass regulates the emerging timescale of young star clusters

Abstract

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in our understanding of the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet they are difficult to constrain due to the complex interplay between stellar feedback and star formation across a vast range of physical scales. Here we present HST and JWST observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628, and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging more rapidly than their lower mass counterparts. This is a critical constraint on simulations of star formation and stellar feedback, which struggle to fully reproduce the formation and emergence of star clusters. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose important limitations to the time available for planet formation in massive cluster environments where disks get exposed to UV irradiation and further gas infall is shut off.

Figures (6)

• Figure 1: A kpc-scale star-forming complex in M51. Left panel: RGB composite image of the complex observed with HST and JWST. The blue channel traces stellar optical emission from the HST ACS/F555W filter, the green channel shows ionized emission from the Br$\alpha$ recombination line (JWST F405N, continuum-subtracted), and the red channel displays 3.3 $\mu$m PAH emission observed with JWST F335M (continuum-subtracted). The white box indicates the region shown in the right panels. Right panels: Monochromatic maps of the zoomed-in region highlighted by the white box. From top to bottom: 3.3 $\mu$m PAH, Pa$\alpha$, and ACS/F555W. Positions of eYSCs and optically detected YSCs are marked with red, orange, and cyan circles for eYSCI, eYSCII, and optical YSCs, respectively. In M51, photometry of these clusters has been performed at 5.8 pc scale. The eYSC and optical YSC classification is described in the Results.
• Figure 2: Top: NIR color–color diagram of the eYSC and oYSCs populations in the four FEAST galaxies. Units are AB mag. Redder colors along the y-axis trace hot dust emission through the F444W filter, while along the x-axis the F187N filter shifts younger objects to redder colors owing to bright Pa$\alpha$ emission. Symbols show the median values for the four galaxies, color-coded by cluster class, with the color scheme matching that in Figure \ref{['fig:rgb']}. Errorbars represent the 16-84 percentiles of each distribution. Histograms display the combined distributions, also color-coded by cluster class. The evolutionary tracks are taken from the yggdrasil single stellar population synthesis code (solid) and CIGALE (dashed), with nebular emission and no dust absorption or emission, as described in the text. The extinction vector is from Fahrion23. Bottom: Same as above, but with a different color combination. Here, the y-axis (F150W–F200W) serves as a proxy for young star cluster age, as indicated by the evolutionary track. The x-axis traces bright emission from the 3.3 $\mu$m PAH feature, displayed as an excess in the F335M filter.
• Figure 3: Emerging timescales as a function of stellar mass (log scale) for the combined population of YSCs in the four FEAST galaxies. Filled circles and open squares indicate the $\tau_{\rm TOT}$ and $\tau_{\rm PDR}$ timescales, respectively. Each bin contains the same number of objects, except for the largest two mass bins, which are split into two additional bins as described in the text; mass binning is further detailed in the Methods. On the y-axis, $\tau/\tau_{\rm MM}$ shows timescales normalized to the $\tau_{\rm TOT}$ of the highest mass bin ($\tau_{\rm MM} = 4.9$ Myr), while the secondary y-axis gives absolute values. Shadowed areas beneath the measurements show the timescales re-estimated by randomly sampling the ages of the oYSCs (see Methods), confirming the observed trends. y-error bars show Poisson uncertainties (set by the number of sources in each bin), and x-error bars indicate the bin widths. Below $10^3$ M$_{\odot}$, results are affected by completeness limits; this region is shaded in grey.
• Figure 4: Emerging timescale as a function of stellar mass (log scale) for YSCs in the four FEAST galaxies individually: M51 (top left), M83 (top right), NGC 628 (bottom left), and NGC 4449 (bottom right). Symbols, binning, and shading are as in Figure \ref{['fig:timescales_all']}. In NGC 4449, masses are affected by completeness limits below 10$^{2.75}$ M$_{\odot}$Pedrini25.
• Figure 5: Emerging timescales as a function of luminosity (absolute AB magnitude) for the combined population of YSCs in the four FEAST galaxies. The four panels display four different NIRCam filters: F115W (top left), F150W (top right), F200W (bottom left), F300M (bottom right) For each filter, we applied a luminosity cut at m$_{\rm abs} > -6$. Symbols and errorbars are as in Figure \ref{['fig:timescales_all']}.