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An image of the GOODS-South field as observed by JWST. More than 45,000 galaxies are visible here.

An image of the GOODS-South field as observed by JWST. More than 45,000 galaxies are visible here. In this image, blue, green, and red were assigned to Webb’s NIRCam (Near-Infrared Camera) data at 0.9, 1.15, and 1.5 microns; 2.0, 2.77, and 3.55 microns; and 3.56, 4.1, and 4.44 microns (F090W, F115W, and F150W; F200W, F277W, and F335M; and F356W, F410M, and F444W), respectively.

Credit: NASA, ESA, CSA, B. Robertson (UC Santa Cruz), B. Johnson (Center for Astrophysics, Harvard & Smithsonian), S. Tacchella (Univers

Dr Jacopo Chevallard

Postdoctoral Research Assistant

Research theme

  • Astronomy and astrophysics

Sub department

  • Astrophysics

Research groups

  • Galaxy formation and evolution
jacopo.chevallard@physics.ox.ac.uk
Telephone: 01865 273467
Denys Wilkinson Building, room 463
  • About
  • Publications

Resolving the nature and putative nebular emission of GS9422: an obscured AGN without exotic stars

Monthly Notices of the Royal Astronomical Society Oxford University Press 540:1 (2025) 851-870

Authors:

Sandro Tacchella, William McClymont, Jan Scholtz, Roberto Maiolino, Xihan Ji, Natalia C Villanueva, Stéphane Charlot, Francesco D’Eugenio, Jakob M Helton, Christina C Williams, Joris Witstok, Rachana Bhatawdekar, Stefano Carniani, Jacopo Chevallard, Mirko Curti, Kevin Hainline, Zhiyuan Ji, Benjamin D Johnson, Joel Leja, Yijia Li, Michael V Maseda, Dávid Puskás, Marcia Rieke, Brant Robertson

Abstract:

Understanding the sources that power nebular emission in high-redshift galaxies is fundamentally important not only for shedding light on to the drivers of reionization, but to constrain stellar populations and the growth of black holes. Here, we focus on an individual object, GS9422, a galaxy at with exquisite data from the JWST Advanced Deep Extragalactic Survey (JADES), JWST Extragalactic Medium-band Survey (JEMS), and First Reionization Epoch Spectroscopically Complete Observations (FRESCO) surveys, including 14-band JWST/NIRCam photometry and deep NIRSpec prism and grating spectroscopy. We map the continuum emission and nebular emission lines across the galaxy on 0.2-kpc scales. GS9422 has been claimed to have nebular-dominated continuum and an extreme stellar population with top-heavy initial mass function. We find clear evidence for different morphologies in the emission lines, the rest-ultraviolet and rest-optical continuum emission, demonstrating that the full continuum cannot be dominated by nebular emission. While multiple models reproduce the spectrum reasonably well, our preferred model with a type-2 active galactic nucleus (AGN) and local damped Ly (DLA) clouds can explain both the spectrum and the wavelength-dependent morphology. The AGN powers the off-planar nebular emission, giving rise to the Balmer jump and the emission lines, including Ly, which therefore does not suffer DLA absorption. A central, young stellar component dominates the rest-UV emission and – together with the DLA clouds – leads to a spectral turn over. A disc-like, older stellar component explains the flattened morphology in the rest-optical continuum. We conclude that GS9422 is consistent with being a normal galaxy with an obscured, type-2 AGN – a simple scenario, without the need for exotic stellar populations.
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Zapped then napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic slope at z=8.5

Astronomy & Astrophysics EDP Sciences (2025)

Authors:

William M Baker, Francesco D'Eugenio, Roberto Maiolino, Andrew J Bunker, Charlotte Simmonds, Sandro Tacchella, Joris Witstok, Santiago Arribas, Stefano Carniani, Stéphane Charlot, Jacopo Chevallard, Mirko Curti, Emma Curtis-Lake, Gareth C Jones, Nimisha Kumari, Pierluigi Rinaldi, Brant Robertson, Christina C Williams, Chris Willott, Yongda Zhu
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Witnessing the onset of reionization through Lyman-α emission at redshift 13

Nature Nature Research 639:8056 (2025) 897-901

Authors:

Joris Witstok, Peter Jakobsen, Roberto Maiolino, Jakob M Helton, Benjamin D Johnson, Brant E Robertson, Sandro Tacchella, Alex J Cameron, Renske Smit, Andrew J Bunker, Aayush Saxena, Fengwu Sun, Stacey Alberts, Santiago Arribas, William M Baker, Rachana Bhatawdekar, Kristan Boyett, Phillip A Cargile, Stefano Carniani, Stéphane Charlot, Jacopo Chevallard, Mirko Curti, Emma Curtis-Lake, Francesco D’Eugenio, Gareth C Jones

Abstract:

Cosmic reionization began when ultraviolet (UV) radiation produced in the first galaxies began illuminating the cold, neutral gas that filled the primordial Universe1, 2. Recent James Webb Space Telescope (JWST) observations have shown that surprisingly UV-bright galaxies were in place beyond redshift z = 14, when the Universe was less than 300 Myr old3, 4–5. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-α (Ly-α), the principal hydrogen transition6, 7, 8–9. However, spectral signatures encoding crucial properties of these sources, such as their emergent radiation field, largely remain elusive. Here we report spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES10) of a galaxy at redshift z = 13.0 that reveals a singular, bright emission line unambiguously identified as Ly-α, as well as a smooth turnover. We observe an equivalent width of EWLy-α > 40 Å (rest frame), previously only seen at z < 9 where the intervening intergalactic medium becomes increasingly ionized11. Together with an extremely blue UV continuum, the unexpected Ly-α emission indicates that the galaxy is a prolific producer and leaker of ionizing photons. This suggests that massive, hot stars or an active galactic nucleus have created an early reionized region to prevent complete extinction of Ly-α, thus shedding new light on the nature of the earliest galaxies and the onset of reionization only 330 Myr after the Big Bang.
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Photometric detection at 7.7 μm of a galaxy beyond redshift 14 with JWST/MIRI

Nature Astronomy Nature Research 9:5 (2025) 729-740

Authors:

Jakob M Helton, George H Rieke, Stacey Alberts, Zihao Wu, Daniel J Eisenstein, Kevin N Hainline, Stefano Carniani, Zhiyuan Ji, William M Baker, Rachana Bhatawdekar, Andrew J Bunker, Phillip A Cargile, Stéphane Charlot, Jacopo Chevallard, Francesco D’Eugenio, Eiichi Egami, Benjamin D Johnson, Gareth C Jones, Jianwei Lyu, Roberto Maiolino, Pablo G Pérez-González, Marcia J Rieke, Brant Robertson, Aayush Saxena

Abstract:

The James Webb Space Telescope (JWST) has spectroscopically confirmed numerous galaxies at z > 10. While weak rest-frame ultraviolet emission lines have only been seen in a handful of sources, the stronger rest-frame optical emission lines are highly diagnostic and accessible at mid-infrared wavelengths with the Mid-Infrared Instrument (MIRI) of JWST. We report the photometric detection of the distant spectroscopically confirmed galaxy JADES-GS-z14-0 at z=14.32−0.20+0.08 with MIRI at 7.7 μm. The most plausible solution for the stellar-population properties is that this galaxy contains half a billion solar masses in stars with a strong burst of star formation in the most recent few million years. For this model, at least one-third of the flux at 7.7 μm originates from the rest-frame optical emission lines Hβ and/or [O iii]λλ4959, 5007. The inferred properties of JADES-GS-z14-0 suggest rapid mass assembly and metal enrichment during the earliest phases of galaxy formation. This work demonstrates the unique power of mid-infrared observations in understanding galaxies at the redshift frontier.
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JADES Data Release 3: NIRSpec/Microshutter Assembly Spectroscopy for 4000 Galaxies in the GOODS Fields

The Astrophysical Journal Supplement Series 277:1 (2025)

Authors:

Francesco D’Eugenio, Alex J Cameron, Jan Scholtz, Stefano Carniani, Chris J Willott, Emma Curtis-Lake, Andrew J Bunker, Eleonora Parlanti, Roberto Maiolino, Christopher NA Willmer, Peter Jakobsen, Brant E Robertson, Benjamin D Johnson, Sandro Tacchella, Phillip A Cargile, Tim Rawle, Santiago Arribas, Jacopo Chevallard, Mirko Curti, Eiichi Egami, Daniel J Eisenstein, Nimisha Kumari, Tobias J Looser, Marcia J Rieke, Bruno Rodríguez Del Pino, Aayush Saxena, Hannah Übler, Giacomo Venturi, Joris Witstok, William M Baker, Rachana Bhatawdekar, Nina Bonaventura, Kristan Boyett, Stephane Charlot, A Lola Danhaive, Kevin N Hainline, Ryan Hausen, Jakob M Helton, Xihan Ji, Zhiyuan Ji, Gareth C Jones, Ignas Juodžbalis, Michael V Maseda, Pablo G Pérez-González, Michele Perna, Dávid Puskás, Irene Shivaei, Maddie S Silcock, Charlotte Simmonds, Renske Smit, Fengwu Sun, Natalia C Villanueva, Christina C Williams, Yongda Zhu
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