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ABSTRACT An important characteristic of cosmic hydrogen reionization is the growth of ionized gas bubbles surrounding early luminous objects. Ionized bubble sizes are beginning to be probed using Lyman α emission from high-redshift galaxies, and will also be probed by upcoming 21 cm maps. We present results from a study of bubble sizes using the state-of-the-art thesan radiation-hydrodynamics simulation suite, which self-consistently models radiation transport and realistic galaxy formation. We employ the mean free path method and track the evolution of the effective ionized bubble size at each point (Reff) throughout the Epoch of Reionization. We show that there is a slow growth period for regions ionized early, but a rapid ‘flash ionization’ process for regions ionized later as they immediately enter a large, pre-existing bubble. We also find that bright sources are preferentially in larger bubbles, and find consistency with recent observational constraints at z ≳ 9, but tension with idealized Lyman α damping-wing models at z ≈ 7. We find that high-overdensity regions have larger characteristic bubble sizes, but the correlation decreases as reionization progresses, likely due to runaway formation of large percolated bubbles. Finally, we compare the redshift at which a region transitions from neutral to ionized (zreion) with the time it takes to reach a given bubble size and conclude that zreion is a reasonable local probe of small-scale bubble size statistics ($$R_\text{eff} \lesssim 1\, \rm {cMpc}$$). However, for larger bubbles, the correspondence between zreion and size statistics weakens due to the time delay between the onset of reionization and the expansion of large bubbles, particularly at high redshifts.
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The thesan project: tracking the expansion and merger histories of ionized bubbles during the Epoch of Reionization
ABSTRACT The growth of ionized hydrogen bubbles in the intergalactic medium around early luminous objects is a fundamental process during the Epoch of Reionization (EoR). Observations using Ly $$\alpha$$ emission from high-redshift galaxies and forthcoming 21 cm maps are beginning to constrain the sizes of these ionized regions. In this study, we analyse bubble sizes and their evolution using the state-of-the-art thesan radiation-hydrodynamics simulation suite, which self-consistently models radiation transport and realistic galaxy formation throughout a large $$(95.5\, \text{cMpc})^3$$ volume of the universe. Analogous to the accretion and merger tree histories employed in galaxy formation simulations, we characterize the growth and merger rates of ionized bubbles by focusing on the spatially resolved redshift of reionization. By tracing the chronological expansion of bubbles, we partition the simulation volume and construct a natural ionization history. We identify three distinct stages of ionized growth: (1) initial slow expansion around the earliest ionizing sources, (2) accelerated growth through percolation, and (3) rapid expansion dominated by the largest bubble. Notably, we find that the largest bubble emerges by $$z \approx 9\!-\!10$$, well before the midpoint of reionization. This bubble becomes dominant during the second growth stage, and defines the third stage by rapidly expanding to encompass the remainder of the simulation volume. Additionally, we observe a sharp decline in the number of bubbles with radii around $$\sim 10$$ cMpc, indicating a characteristic scale in the final segmented size distribution. Overall, these chronologically sequenced spatial reconstructions offer crucial insights into the physical mechanisms driving ionized bubble growth during the EoR, providing a framework for interpreting reionization itself.
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- Award ID(s):
- 2348872
- PAR ID:
- 10677339
- Publisher / Repository:
- Monthly Notices of the Royal Astronomical Society
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 541
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 1088 to 1105
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/mnras/staf996
