The Near Infrared Spectral Energy Distribution of Young Star Clusters in the FEAST Galaxies: Missing Ingredients at 1–5 μm

October 2025 • 2025ApJ...992...96P

Authors • Pedrini, Alex • Adamo, Angela • Bik, Arjan • Calzetti, Daniela • Linden, Sean T. • Gregg, Benjamin • Bajaj, Varun • Ryon, Jenna E. • Buckner, Anne S. M. • Bortolini, Giacomo • Cignoni, Michele • Correnti, Matteo • Duarte-Cabral, Ana • Elmegreen, Bruce G. • Faustino Vieira, Helena • Gallagher, John S. • Grasha, Kathryn • Johnson, Kelsey E. • Krumholz, Mark R. • Lapeer, Drew • Lai, Thomas S.-Y. • Messa, Matteo • Östlin, Göran • Roos, Linn • Smith, Linda J. • Tosi, Monica

Abstract • We present a combined Hubble Space Telescope (HST) and JWST 0.2–to–5 μm analysis of the spectral energy distributions (SED) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging Extrgalactic Star Clusters survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Paα and 3.3 μm polycyclic aromatic hydrocarbon emission, are still associated with their natal gas cloud and have been largely missed in previous HST optical campaigns. We modeled their SEDs using the CIGALE fitting code and identified (i) a systematic flux excess at 1.5–2.5 μm that is not accounted for by current stellar population models and (ii) the preference for a set of dust model parameters that is not aligned with expectations from self-consistent analyses of star-forming regions, suggesting model shortcomings also in the 3–5 μm. The near-infrared excess is most prominent in low-mass (≤3000 M_⊙) and young (≤6 Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages ≥6 Myr to a fraction of strong Paα emitters with equivalent widths suggestive of significantly younger ages. A parallel analysis with the slug code suggests that stochastic initial mass function (IMF) sampling of pre-main-sequence stars combined with extinction might partially reduce the gap. We conclude that the inclusion of young stellar object SEDs, along with more realistic sampling of the cluster IMF, might be needed to fully account for the stellar population and dust properties of eYSCs.

Links

• PDF https://iopscience.iop.org/article/10.3847/1538-4357/ae0182/pdf
• PREPRINT http://arxiv.org/abs/2509.01670
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2025ApJ...992...96P
• DATA https://doi.org/10.17909/6dc1-9h53
• DATA https://doi.org/10.17909/f4vm-c771
• ELECTR https://doi.org/10.3847/1538-4357/ae0182