The near infrared SED of young star clusters in the FEAST galaxies: Missing ingredients at 1– 5 µm

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 and Duarte-Cabral, Ana ORCID: https://orcid.org/0000-0002-5259-4774 2025. The near infrared SED of young star clusters in the FEAST galaxies: Missing ingredients at 1– 5 µm. The Astrophysical Journal

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Abstract

We present a combined HST and JWST 0.2 - to - 5 m analysis of the spectral energy distributions (SEDs) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Pa and 3.3 m polycyclic aromatic hydrocarbon (PAH) emission, are still associated to 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 m that is not accounted for by current stellar population models; 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 m. The near-infrared (NIR) excess is most prominent in low-mass ( M) and young ( Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages Myr to a fraction of strong Pa 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.

Item Type:	Article
Status:	In Press
Schools:	Schools > Physics and Astronomy
Additional Information:	Additional authors: Bruce G. Elmegreen, Helena Faustino Vieira, John S. Gallagher, Kathryn Grasha, Kelsey E. Johnson, Mark R. Krumholz, Drew Lapeer, Thomas S. -Y. Lai, Matteo Messa, Göran Östlin, Linn Roos, Linda J. Smith, Monica Tosi.
Publisher:	American Astronomical Society
ISSN:	0004-637X
Date of First Compliant Deposit:	19 September 2025
Date of Acceptance:	29 August 2025
Last Modified:	24 Sep 2025 13:45
URI:	https://orca.cardiff.ac.uk/id/eprint/181205

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