The mean free path of hydrogen ionizing photons during the epoch of reionization

Abstract

We use the Aurora radiation-hydrodynamical simulations to study the mean free path (MFP) for hydrogen ionizing photons during the epoch of reionization. We directly measure the MFP by averaging the distance 1 Ry photons travel before reaching an optical depth of unity along random lines-of-sight. During reionization the free paths tend to end in neutral gas with densities near the cosmic mean, while after reionizaton the end points tend to be overdense but highly ionized. Despite the increasing importance of discrete, over-dense systems, the cumulative contribution of systems with $N_{\mathrm{HI}}\lesssim 10^{16.5}\mathrm{c}{\mathrm{m}}^{-2}$ suffices to drive the MFP at $z\approx 6$, while at earlier times higher column densities are more important. After reionization the typical size of HI systems is close to the local Jeans length, but during reionization it is much larger. The mean free path for photons originating close to galaxies, $\mathrm{MF}{\mathrm{P}}_{\mathrm{gal}}$, is much smaller than the cosmic MFP. After reionization this enhancement can remain significant up to starting distances of $\sim 1$ comoving Mpc. During reionization, however, $\mathrm{MF}{\mathrm{P}}_{\mathrm{gal}}$ for distances $\sim 10^2-10^3$ comoving kpc typically exceeds the cosmic MFP. These findings have important consequences for models that interpret the intergalactic MFP as the distance escaped ionizing photons can travel from galaxies before being absorbed and may cause them to under-estimate the required escape fraction from galaxies, and/or the required emissivity of ionizing photons after reionization.

13 pages, 9 figures, 1 table; submitted to MNRAS

References in corpus (11)

• The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
• Efficient Simulations of Early Structure Formation and Reionization
• Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$ trough below redshift six
• The morphology of HII regions during reionization
• Cosmic Reionization On Computers I. Design and Calibration of Simulations
• Metal-line absorption around $z\approx$2.4 star-forming galaxies in the Keck Baryonic Structure Survey
• TRAPHIC - Radiative Transfer for Smoothed Particle Hydrodynamics Simulations
• Effect of the intergalactic environment on the observability of Ly-alpha emitters during reionization
• Bubble size statistics during reionization from 21-cm tomography
• Multi-frequency, thermally coupled radiative transfer with TRAPHIC: Method and tests
• Self-shielding of hydrogen in the IGM during the epoch of reionization

Cited by in corpus (17)

• The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization
• The THESAN project: properties of the intergalactic medium and its connection to Reionization-era galaxies
• The THESAN project: Lyman-alpha emission and transmission during the Epoch of Reionization
• The Role of Galaxies and AGN in Reionising the IGM - I: Keck Spectroscopy of 5 < z < 7 Galaxies in the QSO Field J1148+5251
• Hydrodynamic Response of the Intergalactic Medium to Reionization
• Public Release of A-SLOTH: Ancient Stars and Local Observables by Tracing Halos
• The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization
• Lyman-alpha transmission properties of the intergalactic medium in the CoDaII simulation
• Impact of the reduced speed of light approximation on the post-overlap neutral hydrogen fraction in numerical simulations of the epoch of reionization
• The Lyman continuum escape fraction and the Mean Free Path of hydrogen ionizing photons for bright z~4 QSOs from SDSS DR14
• Hydrodynamic Response of the Intergalactic Medium to Reionization II: Physical Characteristics and Dynamics of Ionizing Photon Sinks
• Hybrid-chimes: A model for radiative cooling and the abundances of ions and molecules in simulations of galaxy formation
• Probing Large-scale UV Background Inhomogeneity Associated with Quasars using Metal Absorption
• The Lyman-limit photon mean free path at the end of late reionisation in the Sherwood-Relics simulations
• Effects of Small-Scale Absorption Systems on Neutral Islands during the Late Epoch of Reionization
• Reionizing islands with inhomogeneous recombinations
• Cosmic Dawn II (CoDa II): a new radiation-hydrodynamics simulation of the self-consistent coupling of galaxy formation and reionization