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Peer-reviewedCC BYAstronomy & Astrophysics

EWOCS-IX: JWST/NIRCam observations of Westerlund 2

  • Almendros-Abad V.1
  • Guarcello M. G.1
  • Mužić K.2
  • Scholz A.3
  • Andersen M.4
  • Bayo A.4
  • Best W.5
  • Capela D.3
  • Gennaro M.6
  • Ginsburg A.7
  • Lovell J. B.8
  • Monsch K.8
17 more authors
  • Moraux E.9
  • Prisinzano L.1
  • Rom T.10
  • Sabbi E.11
  • Zeidler P.12
  • Argiroffi C.13
  • Bonito R.1
  • Calzetti D.14
  • Cusimano V.13
  • Damiani F.1
  • Drake J. J.15
  • Haworth T. J.16
  • Richardson N. D.17
  • Robberto M.7
  • Sciortino S.1
  • Wright N. J.18
  • Ziliotto T.19
19 affiliations
  1. 1Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo
  2. 2Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa
  3. 3SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh
  4. 4European Southern Observatory
  5. 5The University of Texas at Austin, Department of Astronomy
  6. 6Space Telescope Science Institute
  7. 7Department of Astronomy, University of Florida
  8. 8Center for Astrophysics | Harvard & Smithsonian
  9. 9Univ. Grenoble Alpes, CNRS, IPAG
  10. 10University of Split, Faculty of Science, Department of Physics
  11. 11Gemini Observatory/NSF NOIRLab
  12. 12AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute
  13. 13Department of Physics and Chemistry, University of Palermo
  14. 14Department of Astronomy, University of Massachusetts Amherst
  15. 15Lockheed Martin Solar and Astrophysics Laboratory
  16. 16Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London
  17. 17Department of Physics and Astronomy, Embry-Riddle Aeronautical University
  18. 18Astrophysics Research Centre, Keele University
  19. 19NSF’s National Optical-Infrared Astronomy Research Laboratory
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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.

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Cite

BibTeXRIS
BibTeX
@article{almendrosabad2026ewocsix,
  title = {{EWOCS-IX: JWST/NIRCam observations of Westerlund 2}},
  author = {Almendros-Abad V. and Guarcello M. G. and Mužić K. and Scholz A. and Andersen M. and Bayo A. and Best W. and Capela D. and Gennaro M. and Ginsburg A. and Lovell J. B. and Monsch K. and Moraux E. and Prisinzano L. and Rom T. and Sabbi E. and Zeidler P. and Argiroffi C. and Bonito R. and Calzetti D. and Cusimano V. and Damiani F. and Drake J. J. and Haworth T. J. and Richardson N. D. and Robberto M. and Sciortino S. and Wright N. J. and Ziliotto T.},
  journal = {Astronomy \& Astrophysics},
  year = {2026},
  month = oct,
  doi = {10.1051/0004-6361/202557062},
  url = {https://doi.org/10.1051/0004-6361/202557062},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - EWOCS-IX: JWST/NIRCam observations of Westerlund 2
AU  - Almendros-Abad V.
AU  - Guarcello M. G.
AU  - Mužić K.
AU  - Scholz A.
AU  - Andersen M.
AU  - Bayo A.
AU  - Best W.
AU  - Capela D.
AU  - Gennaro M.
AU  - Ginsburg A.
AU  - Lovell J. B.
AU  - Monsch K.
AU  - Moraux E.
AU  - Prisinzano L.
AU  - Rom T.
AU  - Sabbi E.
AU  - Zeidler P.
AU  - Argiroffi C.
AU  - Bonito R.
AU  - Calzetti D.
AU  - Cusimano V.
AU  - Damiani F.
AU  - Drake J. J.
AU  - Haworth T. J.
AU  - Richardson N. D.
AU  - Robberto M.
AU  - Sciortino S.
AU  - Wright N. J.
AU  - Ziliotto T.
PY  - 2026
DA  - 2026/10/23
JO  - Astronomy & Astrophysics
DO  - 10.1051/0004-6361/202557062
UR  - https://doi.org/10.1051/0004-6361/202557062
AB  - 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.
SN  - 0004-6361
SN  - 1432-0746
ER  -