PhD Defense by Gonzalo Juan Prieto-Lyon

PhD Thesis: Link for the PhD thesis: Gonzalo Prieto-Lyon – Niels Bohr Institute - University of Copenhagen

Title: The emergent properties of the first ionizers. Lyman-Alpha and ionizing photon production at the edge of reionization

Abstract: The Epoch of Reionization was the last major phase change of the Universe. The newly formed galaxies emitted heavy ionizing radiation causing a shift from a neutral to an ionized
intergalactic medium. Even though the community has reached some consensus on the endpoint of the Epoch of Reionization (EoR), the early stages and main contributing galaxies remain a mystery. One of the best tools to investigate the state of the EoR, is the Lyman-Alpha emission, its hydrogen resonant properties give it the advantage of being absorbed by neutral hydrogen, making it a great tracer for the state of the IGM. Nonetheless, if we want to use Lyman-Alpha to infer the timeline of reionization we first need to understand its emergent properties, that is the typical strength and profile of the line as it leaves the interstellar and circumgalactic mediums. The launch of JWST has given us a new opportunity to face these challenges, with its infrared capabilities we can now observe the rest-frame optical lines. Hα is particularly interesting allowing measurements of: The non-escaping ionizing photon production; the velocity shift of Lyman-Alpha due to resonant scattering; the fraction of Lyman-Alpha photons that escape into the IGM. In this thesis I present a study on galaxies at the end of the EoR (z ∼ 3 − 7). These galaxies are not strongly affected by the neutral hydrogen of the IGM as those at higher redshift, making them useful for understanding the emergent properties of the Lyman-Alpha line profile and their ionizing photon production through Hα.

In the first project (Prieto-Lyon et al., 2023a), we present for the first time in history demographics study of the ionizing photon production of UV-faint galaxies. We study a sample of 370 galaxies (with 102 Lyman-Alpha galaxies) at z∼ 3−7, reaching as low as MUV ∼ −15.5. With multiband photometry from JWST/NIRCam imaging we inferred Hα flux measurements and the non escaping ionizing fraction. We found that these analogs of the galaxies in the EoR tend to have a big diversity of ionizing photon production, with an intrinsic scatter of half an order of magnitude. We explore trends with other observables and find a preference for strong ionizers to be UV-faint; young, dust-free galaxies; have assumed low metallicities; as well as having Lyman-Alpha emission. We conclude that the first ionizers, similar to our UV-faint galaxies, may have a rich distribution of ionizing photon production that needs to be considered when assessing the drivers of reionization.

In the second part (Prieto-Lyon et al., 2023b), we present new, never seen, Lyman-Alpha velocity offsets of UV-faint galaxies at z ∼ 3−6, down to MUV ∼ −16. Due to the degeneracy between the effect of neutral hydrogen from the nearby IGM and ISM/CGM being degenerate with each other, it is necessary to observe the Lyman-Alpha profile at the end of EoR to have a baseline idea of the typical emergent emission. We find through VLT/MUSE and JWST/NIRSpec observations that UV-faint galaxies typically have a median Lyman-Alpha velocity offset lower than their brighter counterparts outside EoR, but also lower than most galaxies found deep in this epoch. We find that the typical UV-faint galaxy needs at least 1pMpc of ionized hydrogen in the line-of-sight to transmit 30% of its original Lyman-Alpha emission, from which follows that detection of low velocity offsets at EoR suggest the presence of large ionized bubbles.

In the third and final part (Prieto-Lyon in progress), we obtained restframe UV spectra of 236 galaxies, including 65 Lyman-Alpha detections. We added ancilliary data from JWST / FRESCO, which allowed us to obtain systemic redshifts and Hα fluxes for 51 sources. We present new results on Lyman-Alpha shape and strength, including the first high-resolution results of the Lyman-Alpha FWHM distribution at z∼ 5 − 6, new Lyman-Alpha velocity offset results, and a new empirical model for Lyman-Alpha equivalent width and escape fraction. Our results show that Lyman-Alpha is narrower and more offset than what can be inferred from lower redshift samples, leading to a boosted Lyman-Alpha transmission of up to 5 times. This suggests that the IGM might be more neutral at z>6 than previously predicted. Our line strength models 45 − 62% of MUV = −19.5 galaxies have a Lyman-Alpha equivalent width > 25 Å and escape fraction > 0.2, respectively.

Past z∼8, Lyman-Alpha will be our best tool to constraint the timeline of reionization, this and similar works that measure emergent Lyman-Alpha properties will be key to break the ISM/CGM and IGM degeneracy. The growing samples of Lyman-Alpha deep into the EoR will need of such models to be able to accurately infer the neutral gas fraction at the earlier stages of the EoR.

Supervisor:

  • Charlotte Mason, Niels Bohr Institute, University of Copenhagen

Evaluation Committee:

  • Anne Verhamme, University of Geneva
  • Hakim Atek, Sorbonne University
  • Johan Fynbo, the Niels Bohr Institute, University of Copenhagen

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There will be a following reception at NBB in the DAWN Section on the 2nd floor (Niels Bohr Building, Rådmandsgade 62-64, 2200 Copenhagen N)