Skip to main content
Home
Department Of Physics text logo
  • Research
    • Our research
    • Our research groups
    • Our research in action
    • Research funding support
    • Summer internships for undergraduates
  • Study
    • Undergraduates
    • Postgraduates
  • Engage
    • For alumni
    • For business
    • For schools
    • For the public
Menu
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

Detection of the 2175 Å UV bump at z > 7: evidence for rapid dust evolution in a merging reionization-era galaxy

Monthly Notices of the Royal Astronomical Society Oxford University Press 542:2 (2025) 1136-1154

Authors:

Katherine Ormerod, Joris Witstok, Renske Smit, Anna de Graaff, Jakob M Helton, Michael V Maseda, Irene Shivaei, Andrew J Bunker, Stefano Carniani, Francesco D’Eugenio, Rachana Bhatawdekar, Jacopo Chevallard, Marijn Franx, Nimisha Kumari, Roberto Maiolino, Pierluigi Rinaldi, Brant Robertson, Sandro Tacchella

Abstract:

Dust is a fundamental component of the interstellar medium within galaxies, as dust grains are highly efficient absorbers of ultraviolet (UV) and optical photons. Accurately quantifying this obscuration is crucial for interpreting galaxy spectral energy distributions (SEDs). The extinction curves in the Milky Way (MW) and Large Magellanic Cloud exhibit a strong feature known as the 2175 Å UV bump, most often attributed to small carbonaceous dust grains. This feature was recently detected in faint galaxies out to , suggesting rapid formation channels. Here, we report the detection of a strong UV bump in a luminous Lyman-break galaxy at , GNWY-7379420231, through observations taken as part of the NIRSpec Wide GTO survey. We fit a dust attenuation curve that is consistent with the MW extinction curve within , in a galaxy just Myr after the big bang. From the integrated spectrum, we infer a young mass-weighted age ( Myr) for this galaxy, however spatially resolved SED fitting unveils the presence of an older stellar population ( Myr). Furthermore, morphological analysis provides evidence for a potential merger. The underlying older stellar population suggests the merging system could be pre-enriched, with the dust illuminated by a merger-induced starburst. Moreover, turbulence driven by stellar feedback in this bursty region may be driving polycyclic aromatic hydrocarbon formation through top-down shattering. The presence of a UV bump in GNWY-7379420231 solidifies growing evidence for the rapid evolution of dust properties within the first billion years of cosmic time.
More details from the publisher
Details from ORA
More details

JADES – the small blue bump in GN-z11: insights into the nuclear region of a galaxy at z = 10.6

Monthly Notices of the Royal Astronomical Society Oxford University Press 541:3 (2025) 2134-2161

Authors:

Xihan Ji, Roberto Maiolino, Gary Ferland, Francesco D’Eugenio, Rachana Bhatawdekar, Stéphane Charlot, Jacopo Chevallard, Mirko Curti, Emma Curtis-Lake, Kevin Hainline, Zhiyuan Ji, Brant Robertson, Bruno Rodríguez Del Pino, Jan Scholtz, Sandro Tacchella, Christina C Williams, Joris Witstok

Abstract:

We report the detection of continuum excess in the rest-frame ultraviolet (UV) between 3000 and 3550 Å in the JWST/Near Infrared Spectrograph (NIRSpec) spectrum of GN-z11, a luminous galaxy . The shape of the continuum excess resembles a Balmer continuum, but has a break around 3546 Å. The fitting result of this excess depends on the assumed origin of the continuum. If the continuum of GN-z11 is dominated by a stellar population with a small Balmer break, the apparent blueshift of the Balmer continuum is not significant and the best-fitting Balmer continuum model indicates a temperature of K. In contrast, if the continuum is dominated by active galactic nucleus emission, a nebular continuum model cannot fit the spectrum properly. The absence of the Balmer jump indicates an electron temperature of K, significantly higher than the temperature of K inferred from [O iii] and [O iii]. The temperature difference can result from mixing of different ionized regions: the Balmer emission mainly arises from dense and hot clouds in the broad-line region, whereas the forbidden lines originate from less dense and colder gas. An alternative explanation for the observed continuum excess is the Fe ii emission, which shows a characteristic jump blueward of the Balmer limit as previously seen in the spectra of many lower redshift quasars. Through comparisons with cloudy models, we show an Fe abundance above solar is likely needed, which could be achieved via enrichment from Type-Ia supernovae, hypernovae, or pair-instability supernovae.
More details from the publisher
Details from ORA
More details

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.
More details from the publisher
Details from ORA
More details

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
More details from the publisher
More details

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.
More details from the publisher
Details from ORA
More details
More details

Pagination

  • Current page 1
  • Page 2
  • Page 3
  • Page 4
  • Page 5
  • Page 6
  • Page 7
  • Page 8
  • Page 9
  • …
  • Next page Next
  • Last page Last

Footer Menu

  • Contact us
  • Giving to the Dept of Physics
  • Work with us
  • Media

User account menu

  • Log in

Follow us

FIND US

Clarendon Laboratory,

Parks Road,

Oxford,

OX1 3PU

CONTACT US

Tel: +44(0)1865272200

University of Oxfrod logo Department Of Physics text logo
IOP Juno Champion logo Athena Swan Silver Award logo

© University of Oxford - Department of Physics

Cookies | Privacy policy | Accessibility statement

Built by: Versantus

  • Home
  • Research
  • Study
  • Engage
  • Our people
  • News & Comment
  • Events
  • Our facilities & services
  • About us
  • Current students
  • Staff intranet