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ABSTRACT The thermal history and structure of the intergalactic medium (IGM) at $$z \ge 4$$ is an important boundary condition for reionization, and a key input for studies using the Ly $$\alpha$$ forest to constrain the masses of alternative dark matter candidates. Most such inferences rely on simulations that lack the spatial resolution to fully resolve the hydrodynamic response of IGM filaments and minihaloes to H i reionization heating. In this letter, we use high-resolution hydrodynamic + radiative transfer simulations to study how these affect the IGM thermal structure. We find that the adiabatic heating and cooling driven by the expansion of initially cold gas filaments and minihaloes sources significant small-scale temperature fluctuations. These likely persist in much of the IGM until $$z \le 4$$. Capturing this effect requires resolving the clumping scale of cold, pre-ionized gas, demanding spatial resolutions of $${\le} 2$$ $$h^{-1}$$kpc. Pre-heating of the IGM by X-rays can slightly reduce the effect. Our preliminary estimate of the effect on the Ly $$\alpha$$ forest finds that, at $$\log (k /[{\rm km^{-1} s}]) = -1.0$$, the Ly $$\alpha$$ forest flux power (at fixed mean flux) can increase $${\approx} 10~{{\ \rm per\ cent}}$$ going from 8 and 2 $$h^{-1}$$kpc resolution at $$z = 4{\!-\!}5$$ for gas ionized at $$z \ \lt\ 7$$. These findings motivate more careful analyses of how the effects studied here affect the Ly $$\alpha$$ forest.
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Imprints of temperature fluctuations on the z ∼ 5 Lyman-α forest: a view from radiation-hydrodynamic simulations of reionization
Abstract Reionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the $$z$$ ∼ 5 Ly α forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring $${\sim}20\text{--}60{{\ \rm per\ cent}}$$ extra power at k ∼ 0.002 s km−1, with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales (k ≳ 0.1 s km−1), we find that temperature fluctuations suppress power by $${\lesssim}10{{\ \rm per\ cent}}$$. We find that the shape of the power spectrum is mostly sensitive to the reionization mid-point rather than temperature fluctuations from reionization’s patchiness. However, for all of our models with reionization mid-points of $$z$$ ≤ 8 ($$z$$ ≤ 12), the shape differences are $${\lesssim}20{{\ \rm per\ cent}}$$ ($${\lesssim}40{{\ \rm per\ cent}}$$) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at k ≳ 0.01 s km−1. Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization mid-points of $$z$$ < 8 is strongly preferred over another and that all of our models with mid-points of $$z$$ ≥ 8 are excluded at 2.5σ. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower k or, alternatively, if the error bars on the high-k power can be reduced by a factor of 1.5.
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- Award ID(s):
- 1817256
- PAR ID:
- 10161452
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 490
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 3177 to 3195
- 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/stz2807
