[{"dataavailabilitystatement":"The specific observations analysed in this work can be accessed via https://doi.org/10.5281/zenodo.13871850","file_date_updated":"2026-07-20T13:41:19Z","scopus_import":"1","status":"public","acknowledgement":"We would like to thank the anonymous referee for their positive and constructive report and all the observers world-wide for their substantial effort in securing all the public JWST data that were essential for this work. The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant DNRF140. KEH acknowledges support from the Independent Research Fund Denmark (DFF) under grant 5251-00009B and co-funding by the European Union (ERC, HEAVYMETAL, 101071865). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. We used the following software for this work: Python, and the scientific Python ecosystem, in particular NumPy (Harris et al. 2020), SciPy (including cKDTree) (Virtanen et al. 2021), Matplotlib (Hunter 2007), and Astropy (Astropy Collaboration 2013, 2018, 2022).","language":[{"iso":"eng"}],"article_number":"A290","month":"06","abstract":[{"text":"The high-redshift progenitors of present-day galaxy clusters are believed to substantially contribute to the global star formation rate density and drive the large-scale reionisation of the Universe. Here we present a blind and unbiased search for and characterisation of galaxy overdensities during the reionisation epoch at redshifts z ∼ 5.5 − 7 based on rest-frame optical JWST/NIRCam grism spectroscopy of the Abell 2744 lensing field as part of the JWST All the Little Things (ALT) survey. Using a physically motivated, cosmological inference friends-of-friends (FoF) algorithm, we identified six galaxy overdensities, including five robust systems at z = 5.66–6.77. They are all characterised by total halo masses of Mhalo ≳ 1011 M⊙, inferred from a range of proxies. We find that the galaxy members in these overdense environments are on average less massive though equally metal-rich, and generally comprised of younger stellar populations, as indicated by their bluer spectral slopes and less prominent Balmer breaks compared to field galaxies at similar redshifts. Further, we use this novel rest-frame optical selection of galaxy proto-clusters to infer the fraction and 3D distribution of strong Lyman-α emitters (LAEs) and damped Lyman-α absorbers (DLAs) in the overdensity environments. We find that two out of the six galaxy overdensities have excess H I absorption compared to the field average, while the other four are consistent within their large scatter in density. These results present the first direct observational constraints on the tomography of the dense, neutral gas reservoirs in large-scale galaxy overdensities at z > 5 and highlight the limitations of pre-JWST searches for reionisation-era galaxy overdensities relying on the detection of strong LAEs alone.","lang":"eng"}],"OA_place":"publisher","_id":"22369","has_accepted_license":"1","type":"journal_article","article_type":"original","publication_identifier":{"eissn":["1432-0746"],"issn":["0004-6361"]},"volume":710,"citation":{"short":"C. Terp, K.E. Heintz, J.J. Matthee, R.P. Naidu, P.A. Oesch, C. Witten, D. Kashino, C.L. Pollock, C. Di Cesare, A. Torralba Torregrosa, Astronomy &#38; Astrophysics 710 (2026).","mla":"Terp, Chamilla, et al. “All the Massive Galaxy Overdensities during Reionisation: JWST Rest-Frame Optical Selection Reveals Young, Chemically Evolved Galaxies Embedded in Dense, Neutral Gas at z &#62; 5.” <i>Astronomy &#38; Astrophysics</i>, vol. 710, A290, EDP Sciences, 2026, doi:<a href=\"https://doi.org/10.1051/0004-6361/202659436\">10.1051/0004-6361/202659436</a>.","ieee":"C. Terp <i>et al.</i>, “All the massive galaxy overdensities during reionisation: JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z &#62; 5,” <i>Astronomy &#38; Astrophysics</i>, vol. 710. EDP Sciences, 2026.","ama":"Terp C, Heintz KE, Matthee JJ, et al. All the massive galaxy overdensities during reionisation: JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z &#62; 5. <i>Astronomy &#38; Astrophysics</i>. 2026;710. doi:<a href=\"https://doi.org/10.1051/0004-6361/202659436\">10.1051/0004-6361/202659436</a>","apa":"Terp, C., Heintz, K. E., Matthee, J. J., Naidu, R. P., Oesch, P. A., Witten, C., … Torralba Torregrosa, A. (2026). All the massive galaxy overdensities during reionisation: JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z &#62; 5. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href=\"https://doi.org/10.1051/0004-6361/202659436\">https://doi.org/10.1051/0004-6361/202659436</a>","chicago":"Terp, Chamilla, Kasper E. Heintz, Jorryt J Matthee, Rohan P. Naidu, Pascal A. Oesch, Callum Witten, Daichi Kashino, Clara L. Pollock, Claudia Di Cesare, and Alberto Torralba Torregrosa. “All the Massive Galaxy Overdensities during Reionisation: JWST Rest-Frame Optical Selection Reveals Young, Chemically Evolved Galaxies Embedded in Dense, Neutral Gas at z &#62; 5.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences, 2026. <a href=\"https://doi.org/10.1051/0004-6361/202659436\">https://doi.org/10.1051/0004-6361/202659436</a>.","ista":"Terp C, Heintz KE, Matthee JJ, Naidu RP, Oesch PA, Witten C, Kashino D, Pollock CL, Di Cesare C, Torralba Torregrosa A. 2026. All the massive galaxy overdensities during reionisation: JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z &#62; 5. Astronomy &#38; Astrophysics. 710, A290."},"intvolume":"       710","date_created":"2026-07-19T22:01:47Z","ddc":["520"],"OA_type":"diamond","publisher":"EDP Sciences","publication":"Astronomy & Astrophysics","supplementarymaterial":"yes","oa_version":"Published Version","researchdata_availability":"yes","oa":1,"date_updated":"2026-07-20T13:42:20Z","file":[{"file_name":"2026_AstronomyAstrophysics_Terp.pdf","access_level":"open_access","file_id":"22378","date_updated":"2026-07-20T13:41:19Z","creator":"dernst","success":1,"file_size":6156048,"relation":"main_file","date_created":"2026-07-20T13:41:19Z","content_type":"application/pdf","checksum":"0a63db666cfac8e4e6271b75419a26f2"}],"department":[{"_id":"JoMa"}],"title":"All the massive galaxy overdensities during reionisation: JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z > 5","quality_controlled":"1","doi":"10.1051/0004-6361/202659436","article_processing_charge":"No","author":[{"last_name":"Terp","first_name":"Chamilla","full_name":"Terp, Chamilla"},{"last_name":"Heintz","first_name":"Kasper E.","full_name":"Heintz, Kasper E."},{"first_name":"Jorryt J","full_name":"Matthee, Jorryt J","last_name":"Matthee","orcid":"0000-0003-2871-127X","id":"7439a258-f3c0-11ec-9501-9df22fe06720"},{"first_name":"Rohan P.","full_name":"Naidu, Rohan P.","last_name":"Naidu"},{"last_name":"Oesch","full_name":"Oesch, Pascal A.","first_name":"Pascal A."},{"first_name":"Callum","full_name":"Witten, Callum","last_name":"Witten"},{"last_name":"Kashino","full_name":"Kashino, Daichi","first_name":"Daichi"},{"last_name":"Pollock","first_name":"Clara L.","full_name":"Pollock, Clara L."},{"full_name":"Di Cesare, Claudia","first_name":"Claudia","id":"2d002343-372f-11ef-98ec-a164d20427cb","last_name":"Di Cesare"},{"first_name":"Alberto","full_name":"Torralba Torregrosa, Alberto","last_name":"Torralba Torregrosa","orcid":"0000-0001-5586-6950","id":"018f0249-0e87-11f0-b167-cbce08fbd541"}],"das_tickbox":"1","day":"01","tmp":{"image":"/images/cc_by.png","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)","short":"CC BY (4.0)","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode"},"date_published":"2026-06-01T00:00:00Z","PlanS_conform":"1","year":"2026","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","publication_status":"published"}]