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Abstract Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium (IGM) at z = 6. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman-limit opacity by tracking the column-density distributions in our simulations. Within Δ t = 10 Myr of I-front passage, we find that self-shielding systems ( N H I > 10 17.2 cm −2 ) are comprised of two distinct populations: (1) overdensity Δ ∼ 50 structures in photoionization equilibrium with the ionizing background, and (2) Δ ≳ 100 density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in Δ t ∼ 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Δ t = 300 Myr, they contribute ≲10% to the opacity in an average 1 Mpc 3 patch of the universe. The percentage can be a factor of a few larger in overdense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find M = 10 4 –10 8 M ⊙ and effective diameters d eff = 1–20 ckpc h −1 . These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power.
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Small-scale clumping of dark matter and the mean free path of ionizing photons at z = 6
Abstract Recently, the mean free path of ionizing photons in the z = 6 intergalactic medium (IGM) was measured to be very short, presenting a challenge to existing reionization models. At face value, the measurement can be interpreted as evidence that the IGM clumps on scales M ≲ 10 8 M ⊙ , a key but largely untested prediction of the cold dark matter (CDM) paradigm. Motivated by this possibility, we study the role that the underlying dark matter cosmology plays in setting the z > 5 mean free path. We use two classes of models to contrast against the standard CDM prediction: (1) thermal relic warm dark matter (WDM), representing models with suppressed small-scale power; (2) an ultralight axion exhibiting a white noise-like power enhancement. Differences in the mean free path between the WDM and CDM models are subdued by pressure smoothing and the possible contribution of neutral islands to the IGM opacity. For example, comparing late reionization scenarios with a fixed volume-weighted mean neutral fraction of 20% at z = 6, the mean free path is 19 (45)% longer in a WDM model with m x = 3 (1) keV. The enhanced power in the axion-like model produces better agreement with the short mean free path measured at z = 6. However, drawing robust conclusions about cosmology is hampered by large uncertainties in the reionization process, extragalactic ionizing background, and thermal history of the Universe. This work highlights some key open questions about the IGM opacity during reionization.
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- Award ID(s):
- 2045600
- PAR ID:
- 10429296
- Date Published:
- Journal Name:
- Journal of Cosmology and Astroparticle Physics
- Volume:
- 2023
- Issue:
- 01
- ISSN:
- 1475-7516
- Page Range / eLocation ID:
- 002
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1475-7516/2023/01/002
