Nowhere Left to Hide: Uncovering All of the Massive Young Embedded Star Clusters in the Antennae with JWST*

Abstract The Antennae galaxies merger produces the brightest infrared emission of any galaxy within ≈20 Mpc, mostly from intense star formation taking place in supergiant molecular cloud complexes in the overlap region. Here, we present new, high-resolution NIRCam and MIRI images of the Antennae galaxies taken with the F150W, F187N, F335M, F360M, F410M, and F770W filters on JWST to search for the predicted but as-yet-undiscovered population of deeply embedded, optically obscured star clusters. We identify a population of 45 sources, 40 previously unknown, with high Br α /H α and Pa α /H α flux ratios, which are likely very young clusters still embedded or just emerging from their natal cocoons, and estimate their age, extinction ( A V ), and mass. We find that all are extremely young (≲2.5 Myr), have A V between 2 and 10 mag, and masses between ≈10 4 and several ×10 6 M ⊙ . We believe we have now uncovered all clusters with M ≳ 3 × 10 4 M ⊙ and A V ≳ 2 mag in the Antennae. While our sample represents a small fraction (≈15%) of clusters younger than 3 Myr by number, it dominates the ionizing photon luminosity across the galaxy pair (≈60%). We find elevated H 2 /polycyclic aromatic hydrocarbon ratios of the interstellar medium surrounding the most massive pair of embedded clusters, supporting the idea that merger-induced shock-heated gas plays an important role in the formation of extremely massive clusters.