Hidden in the Halo: New Diffuse Star Clusters in the M82 Starburst with JWST

Diffuse star clusters (DSCs) occupy a poorly explored region of the size-luminosity plane between classical globular clusters (GCs) and dwarf galaxies. Here we report the discovery of four faint DSCs in the halo of the nearby starburst galaxy M82 using archival JWST/NIRCam imaging. We resolve individual stars in all four systems, whose color-magnitude diagrams are consistent with predominantly old ($\sim10$ Gyr) stellar populations, based on comparison with isochrones. DSC 1 hosts a metal-poor population ($[\mathrm{M/H}]\sim-2$), while DSCs 2, 3 and 4 are better represented by $[\mathrm{M/H}]\sim-0.8$ isochrones. To establish their nature, we measure their sizes and integrated magnitudes using curve-of-growth and radial profile fits, which yield consistent results between these methods. The sizes of DSCs range from $\sim$ 5.4 to 10.5 pc, making them more extended than typical GCs (2-3 pc). Their luminosities span M$_{\rm V}\simeq-4.1$ to $-6.4$, reaching $\sim$ 1 mag fainter than the previously known M82 star cluster (SC) population. At these low luminosities, the systems occupy a sparsely sampled region of the size-luminosity plane where the distinction between SCs and dwarf galaxies becomes increasingly ambiguous. Most notably, DSC 3 is the faintest SC candidate identified in M82 to date and approaches the regime occupied by Milky Way ultra-faint compact satellites. These faint DSCs were identified through visual inspection alone, suggesting they are likely underrepresented in existing star cluster catalogs, and that a JWST-based archival search of nearby galaxies would be fruitful.