The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization

Stephenson, H. M.O.; Stott, J. P.; Pirie, C. A.; Duncan, K. J.; Mcleod, D. J.; Best, P. N.; Brinch, M.; Clausen, M.; Cochrane, R. K.; Dunlop, J. S.; Flury, S. R.; Geach, J. E.; Hale, C. L.; Ibar, E.; Li, Zefeng; Matthee, Jorryt J; Mclure, R. J.; Ossa-Fuentes, L.; Patrick, A. L.; Sobral, D.; Swinbank, A. M.

The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization

Stephenson HMO, Stott JP, Pirie CA, Duncan KJ, Mcleod DJ, Best PN, Brinch M, Clausen M, Cochrane RK, Dunlop JS, Flury SR, Geach JE, Hale CL, Ibar E, Li Z, Matthee JJ, Mclure RJ, Ossa-Fuentes L, Patrick AL, Sobral D, Swinbank AM. 2025. The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization. Monthly Notices of the Royal Astronomical Society. 544(2), 1412–1431.

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DOI

10.1093/mnras/staf1725

Journal Article | Published | English

Scopus indexed

Author

Stephenson, H. M.O.; Stott, J. P.; Pirie, C. A.; Duncan, K. J.; Mcleod, D. J.; Best, P. N.; Brinch, M.; Clausen, M.; Cochrane, R. K.; Dunlop, J. S.; Flury, S. R.; Geach, J. E.
All

Department

Matthee Group

Abstract

We used observations from the JWST Emission Line Survey (JELS) to measure the half-light radii (re) of 23 Hα-emitting starforming (SF) galaxies at z = 6.1 in the PRIMER/COSMOS field. Galaxy sizes were measured in JWST near-infrared camera observations in rest-frame Hα (tracing recent star formation) with the F466N and F470N narrow-band filters from JELS, and compared against rest-R- and V -band (tracing established stellar populations) and near-ultraviolet sizes. We find a size–stellar mass(re − M∗) relationship with a slope that is consistent with literature values at lower redshifts, though offset to lowersizes. We observe a large scatter in re at low stellar mass (M∗ < 10^8.4 Mo) which we believe is the result of bursty star formation histories (SFHs) of SF galaxies at the Epoch of Reionization (EoR). We find that the stellar and ionized gas components are similar in size at z = 6.1. The evidence of already-established stellar components in these Hα emitters (HAEs) indicates previous episodes of star formation have occurred. As such, following other JELS studies finding our HAEs are undergoing a current burst of star formation, we believe our results indicate that SF galaxies at the end of the EoR have already experienced a bursty SFH. From our re − M∗ relationship, we find re,F444W = 0.76 ± 0.46 kpc for fixed stellar mass M∗ = 10^9.25 M, which is in agreement with other observations and simulations of SF galaxies in the literature. We find a close-pair (major) merger fraction of (fmaj. merger =0.44 ± 0.22) fmerger = 0.43 ± 0.11 for galaxy separations d <~ 25 kpc, which is in agreement with other z ≈ 6 studies.

Publishing Year

2025

Date Published

2025-12-01

Journal Title

Monthly Notices of the Royal Astronomical Society

Publisher

Oxford University Press

Acknowledgement

This work makes use of ASTROPY, 7 a community-developed core PYTHON package for Astronomy (Astropy Collaboration 2013, 2018, 2022), as well as the NUMPY (C. R. Harris et al. 2020) and SCIPY (P.Virtanen et al. 2020) packages(see also T. E. Oliphant 2007). All plots were created using the MATPLOTLIB 2D graphics PYTHON package (J. D. Hunter 2007). Conversions between redshift and lookback time in our selected cosmological model were done using the Javascript cosmological calculator from E. L. Wright (2006).8 The authors would like to thank the anonymous referee for their constructive comments and suggestions which have strengthened the analysis of this work and improved the paper. The authors also gratefully acknowledge Ian Smail for providing valuable feedback and helping to guide the science of this paper. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes9 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. These observations are associated with programs GO no. 2321 (JELS) and GO no. 1837 (PRIMER). The authors acknowledge the PRIMER team for developing their observing program with a zero-exclusive-access period. HMOS acknowledges support from an STFC PhD studentship and the Faculty of Science and Technology at Lancaster University. PNB is grateful for support from the UK STFC via grants ST/V000594/1 and ST/Y000951/1. JSD acknowledges the support of the Royal Society via a Royal Society Research Professorship. LOF acknowledges funding by ANID BECAS/DOCTORADO NACIONAL 21220499.CLH acknowledges support from the Oxford Hintze Centre for Astrophysical Surveys which is funded through generous support from the Hintze family charity foundation. EI gratefully acknowledge financial support from ANID – MILENIO –NCN2024 112 and ANID FONDECYT Regular 1221846. For the purpose of open access, the authors have applied a Creative Commons attribution (CC BY) licence to any author accepted manuscript version arising.

Volume

544

Issue

Page

1412-1431

ISSN

0035-8711

eISSN

1365-2966

IST-REx-ID

20660

Cite this

Stephenson HMO, Stott JP, Pirie CA, et al. The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization. Monthly Notices of the Royal Astronomical Society. 2025;544(2):1412-1431. doi:10.1093/mnras/staf1725

Stephenson, H. M. O., Stott, J. P., Pirie, C. A., Duncan, K. J., Mcleod, D. J., Best, P. N., … Swinbank, A. M. (2025). The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization. Monthly Notices of the Royal Astronomical Society. Oxford University Press. https://doi.org/10.1093/mnras/staf1725

Stephenson, H. M.O., J. P. Stott, C. A. Pirie, K. J. Duncan, D. J. Mcleod, P. N. Best, M. Brinch, et al. “The JWST Emission Line Survey (JELS): The Sizes and Merger Fraction of Star-Forming Galaxies during the Epoch of Reionization.” Monthly Notices of the Royal Astronomical Society. Oxford University Press, 2025. https://doi.org/10.1093/mnras/staf1725.

H. M. O. Stephenson et al., “The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization,” Monthly Notices of the Royal Astronomical Society, vol. 544, no. 2. Oxford University Press, pp. 1412–1431, 2025.

Stephenson, H. M. O., et al. “The JWST Emission Line Survey (JELS): The Sizes and Merger Fraction of Star-Forming Galaxies during the Epoch of Reionization.” Monthly Notices of the Royal Astronomical Society, vol. 544, no. 2, Oxford University Press, 2025, pp. 1412–31, doi:10.1093/mnras/staf1725.

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