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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2409.11467 |
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Table of Contents:
- The reduced speed of light approximation (RSLA) has been employed to speed up radiative transfer simulations of reionization by a factor of $\gtrsim 5-10$. However, it has been shown to cause significant errors in the HI-ionizing background near reionization's end in simulations of representative cosmological volumes. We show that using the RSLA is, to a good approximation, equivalent to re-scaling the global ionizing emissivity in a redshift-dependent way. We derive this re-scaling and show that it can be used to ``correct'' the emissivity in RSLA simulations. This method requires the emissivity to be re-scaled after the simulation has been run, which limits its applicability to situations where the emissivity is set ``by hand'' or determined by free parameters. We test our method by running full speed of light simulations using these re-scaled emissivities and comparing them with their RSLA counterparts. We find that for reduced speeds of light $\tilde{c} \geq 0.2$, the 21 cm power spectrum at $0.1 \leq k /[h{\rm Mpc}^{-1}] \leq 0.2$ and key Ly$α$ forest observables agree to within $20\%$, and often within $10\%$, throughout reionization. Position-dependent time-delay effects cause inaccuracies in reionization's morphology on large scales at the factor of $2$ level for $\tilde{c} \leq 0.1$. Our method allows for up to a factor of $5$ speedup in studies that express the emissivity in terms of free parameters, including efforts to constrain the emissivity using observations. This is a crucial step towards constraining the ionizing properties of high-redshift galaxies using efficient radiative transfer simulations.