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Black Hole

Lensing of space time around a black hole. At Oxford we study black holes observationally and theoretically on all size and time scales - it is some of our core work.

Credit: ALAIN RIAZUELO, IAP/UPMC/CNRS. CLICK HERE TO VIEW MORE IMAGES.

Dr Aayush Saxena

Postdoctoral Research Assistant

Research theme

  • Astronomy and astrophysics

Sub department

  • Astrophysics

Research groups

  • Cosmology
  • Galaxy formation and evolution
aayush.saxena@physics.ox.ac.uk
Denys Wilkinson Building, room 558
Aayush Saxena's website
  • About
  • Publications

Efficient Ionizers with Low H β + [O iii ] Equivalent Widths: JADES Spectroscopy of a Peculiar High-redshift Population

The Astrophysical Journal American Astronomical Society 988:1 (2025) 73

Authors:

Isaac H Laseter, Michael V Maseda, Charlotte Simmonds, Ryan Endsley, Daniel Stark, Andrew J Bunker, Rachana Bhatawdekar, Kristan Boyett, Alex J Cameron, Stefano Carniani, Mirko Curti, Zhiyuan Ji, Pierluigi Rinaldi, Aayush Saxena, Sandro Tacchella, Chris Willott, Joris Witstok, Yongda Zhu

Abstract:

Early JWST photometric studies discovered a population of UV-faint ( 700 Å) exclude the most metal-poor efficient ionizers and favor (1) more chemically enriched systems with comparable extreme radiation fields and (2) older starbursting systems. In contrast, metallicity degeneracies are reduced in Hα space, enabling the identification of these metal-poor efficient ionizers by their specific star formation rate.
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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 American Astronomical Society 277:1 (2025) 4

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, Aayush Saxena, Gareth C Jones

Abstract:

We present the third data release of the JWST Advanced Deep Extragalactic Survey (JADES), providing both imaging and spectroscopy in the two GOODS fields. Spectroscopy consists of medium-depth and deep NIRSpec/microshutter assembly spectra of 4000 targets, covering the spectral range 0.6–5.3 μm and observed with both the low-dispersion prism (R = 30–300) and all three medium-resolution gratings (R = 500–1500). We describe the observations, data reduction, sample selection, and target allocation. We measured 2375 redshifts (2053 from multiple emission lines); our targets span the range from z = 0.5 up to z = 13, including 404 at z > 5. The data release includes 2D and 1D fully reduced spectra, with slit-loss corrections and background subtraction optimized for point sources. We also provide redshifts and signal-to-noise ratio > 5 emission-line flux catalogs for the prism and grating spectra, and concise guidelines on how to use these data products. Alongside spectroscopy, we are also publishing fully calibrated NIRCam imaging, which enables studying the JADES sample with the combined power of imaging and spectroscopy. Together, these data provide the largest statistical sample to date to characterize the properties of galaxy populations in the first billion years after the Big Bang.
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Monster Radio Jet (>66 kpc) Observed in Quasar at z ∼​​​​​ 5

The Astrophysical Journal Letters American Astronomical Society 980:1 (2025) L8

Authors:

Anniek J Gloudemans, Frits Sweijen, Leah K Morabito, Emanuele Paolo Farina, Kenneth J Duncan, Yuichi Harikane, Huub JA Röttgering, Aayush Saxena, Jan-Torge Schindler

Abstract:

We present the discovery of a large extended radio jet associated with the extremely radio-loud quasar J1601+3102 at z ∼​​​​​ 5 from subarcsecond resolution imaging at 144 MHz with the International LOFAR Telescope. These large radio lobes have been argued to remain elusive at z > 4 due to energy losses in the synchrotron emitting plasma as a result of scattering of the strong cosmic microwave background at these high redshifts. Nonetheless, the 0 .′′ 3 resolution radio image of J1601+3102 reveals a northern and a southern radio lobe located at 9 and 57 kpc from the optical quasar, respectively. The measured jet size of 66 kpc makes J1601+3102 the largest extended radio jet at z > 4 to date. However, it is expected to have an even larger physical size in reality due to projection effects brought about by the viewing angle. Furthermore, we observe the rest-frame UV spectrum of J1601+3102 with Gemini/GNIRS to examine its black hole properties, which results in a mass of 4.5 × 108 M⊙ with an Eddington luminosity ratio of 0.45. The black hole mass is relatively low compared to the known high-z quasar population, which suggests that a high black hole mass is not strictly necessary to generate a powerful jet. This discovery of the first ∼​​​​​100 kpc radio jet at z > 4 shows that these objects exist despite energy losses from inverse Compton scattering and can put invaluable constraints on the formation of the first radio-loud sources in the early Universe.
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