EWOCS-IX: JWST/NIRCam observations of Westerlund 2
Year: 2026
Type: article
Abstract: Context . Investigating substellar populations in young massive clusters offers crucial insights into the formation of brown dwarfs (BDs) and the role of environmental conditions in shaping their properties. Westerlund 2 (Wd2), as one of the nearest dense and massive clusters in the Milky Way, represents an ideal laboratory for studying the effects of high stellar density and ionizing radiation from massive stars on BD formation. Aims . This paper presents deep JWST/NIRCam observations of Wd2 in a large number of filters between 1.15 and 4.1 µm. Our analysis focuses on identifying and characterizing the BD population within the cluster. Methods . We carried out point spread function (PSF) photometry on deep JWST/NIRCam data obtained in four wide and six medium-band filters, using DOLPHOT. The resulting catalog was used to identify BD candidates in Wd2 through spectral energy distribution (SED) fitting with atmospheric models. The methodology was validated using synthetic JWST photometry of spectroscopically confirmed young BDs together with simulated populations of typical contaminants, allowing us to define an empirical selection criteria. Results . The 50% detection limit of our NIRCam catalog is ∼0.015–0.02 M ⊙ , at the distance, age, and extinction of Wd2, providing the deepest view of a supermassive star cluster to date. The validation demonstrates that the adopted SED-fitting methodology reliably recovers young brown dwarfs while remaining robust against contamination from reddened evolved stars. Applying this procedure to Wd2, we identify 353 substellar candidates. Most candidates (301) lie above the 10 Myr isochrone in the Hertzsprung-Russell diagram consistent with cluster membership and are defined as strong candidates. Comparison with a control field indicates a contamination level of ≲5% for the strong candidate sample down to masses of ∼0.01–0.015 M ⊙ . We identify 73 candidates exhibiting an infrared excess indicative of circumstellar disks. These objects occupy the expected infrared-excess locus in de-reddened color diagrams and represent 27.7 +3.4 −3.2 % of the candidates detected in F410M. Conclusions . These results demonstrate the capability of JWST/NIRCam medium-band photometry to identify robust samples of candidate brown dwarfs in distant, embedded, and crowded massive star clusters using imaging observations alone. The resulting catalog provides a well-characterized sample for future spectroscopic confirmation and subsequent studies of the substellar population of Wd2.
Source: Astronomy and Astrophysics
Authors: V. Almendros-Abad, M. G. Guarcello, K. Mužić, A. Scholz, M. Andersen, A. Bayo, W. Best, D. Capela, M. Gennaro, A. Ginsburg, J. B. Lovell, K. Monsch, E. Moraux, L. Prisinzano, T. Rom, E. Sabbi, P. Zeidler, C. Argiroffi, R. Bonito, D. Calzetti, V. Cusimano, F. Damiani, J. J. Drake, T. J. Haworth, N. D. Richardson, M. Robberto, S. Sciortino, N. J. Wright, T. Ziliotto
Institutions: Osservatorio Astronomico di Palermo, National Institute for Astrophysics, University of Lisbon, Institute of Astrophysics and Space Sciences, University of St Andrews, European Southern Observatory, European Southern Observatory, The University of Texas at Austin, Space Telescope Science Institute, University of Florida, Center for Astrophysics Harvard & Smithsonian, Centre National de la Recherche Scientifique, Institut de Planétologie et d'Astrophysique de Grenoble, Université Grenoble Alpes, University of Split, University of Arizona, NSF NOIRLab, International Gemini Observatory, European Space Agency, University of Palermo, University of Massachusetts Amherst, Lockheed (United States), Queen Mary University of London, Embry–Riddle Aeronautical University, Johns Hopkins University, Keele University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, UK Research and Innovation, Deutsche Forschungsgemeinschaft, European Southern Observatory, Istituto Nazionale di Astrofisica, HORIZON EUROPE Framework Programme, Science and Technology Facilities Council, Fundação para a Ciência e a Tecnologia
Awards: G103270039, G1118123141, G1304755368, G1897041155, G2121682310, Exploring free-floating planetary-mass objects with the James Webb Space Telescope, G256731684, G2816612011, G3993362825, G4066435496, G4251865906, EXC 2094: ORIGINS: Vom Ursprung des Universums bis zu den ersten Bausteinen des Lebens, G4479138923, G5568641063, CAREER: Mass Measurements in the Galaxy, G7004349483, G748231991, Astronomy at Queen Mary 2023-2026
OpenAlex ID: W7207714779