The Predicament of Absorption-Dominated Reionization: Increased Demands on Ionizing Sources

Abstract

The reionization epoch concludes when ionizing photons reach every corner of the Universe. Reionization has generally been assumed to be limited primarily by the rate at which galaxies produce ionizing photons, but the recent measurement of a surprisingly short ionizing photon mean free path of $0.75_{-0.45}^{+0.65}$ proper Mpc at $z=6$ by Becker et al. (2021) suggests that absorption by residual neutral hydrogen in the otherwise ionized intergalactic medium may play a much larger role than previously expected. Here we show that consistency between this short mean free path and the coeval dark pixel fraction in the Ly$\alpha$ forest requires a cumulative output of $6.1_{-2.4}^{+11}$ ionizing photons per baryon by reionization's end. This represents a dramatic increase in the ionizing photon budget over previous estimates, greatly exacerbating the tension with measurements of the ionizing output from galaxies at later times. Translating this constraint into the instantaneous ionizing production from galaxies in our model, we find ${\log }_{10}{f}_{\mathrm{esc}}{\xi }_{\mathrm{ion}}/{\text{(erg/Hz)}}^{-1}=25.02_{-0.21}^{+0.45}$ at $z\sim 6$. Even with optimistic assumptions about the ionizing production efficiency of early stellar populations, and assuming the galaxy luminosity function extends to extremely faint sources ($M_{\text{UV}}\le -11$), complete reionization requires the escape fraction of ionizing photons to exceed $20\%$ across the galaxy population. This is far larger than observed in any galaxy population at lower redshifts, requiring rapid evolution in galaxy properties after the first billion years of cosmic time. This tension cannot be completely relieved within existing observational constraints on the hydrogen neutral fraction and mean free path.

8 pages, 5 figures, submitted to ApJL

References in corpus (18)

• Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of New Constraints from Planck and Hubble Space Telescope
• Model-independent evidence in favor of an end to reionization by z~6
• Efficient Simulations of Early Structure Formation and Reionization
• New Determinations of the UV Luminosity Functions from z~9 to z~2 show a remarkable consistency with halo growth and a constant star formation efficiency
• Pōniuā'ena: A Luminous $z=7.5$ Quasar Hosting a 1.5 Billion Solar Mass Black Hole
• Inferences on the Timeline of Reionization at z~8 From the KMOS Lens-Amplified Spectroscopic Survey
• A Significantly Neutral Intergalactic Medium Around the Luminous z=7 Quasar J0252-0503
• Constraining the neutral fraction of hydrogen in the IGM at redshift 7.5
• The Global 21-cm Signal in the Context of the High-z Galaxy Luminosity Function
• Texas Spectroscopic Search for Ly$\alpha$ Emission at the End of Reionization III. the Ly$\alpha$ Equivalent-width Distribution and Ionized Structures at $z>7$
• Measurements of the z ~ 6 Intergalactic Medium Optical Depth and Transmission Spikes Using a New z > 6.3 Quasar Sample
• Astraeus I: The interplay between galaxy formation and reionization
• An uncontaminated measurement of the escaping Lyman continuum at $z\sim 3$
• A short mean free path at $z=6$ favors late and rapid reionization by faint galaxies
• Inference of the optical depth to reionization from low multipole temperature and polarisation Planck data
• Are There Enough Ionizing Photons to Reionize the Universe by z=6?
• Prospects for distinguishing galaxy evolution models with surveys at redshifts $z\gtrsim 4$
• Implications of the $z>5$ Lyman-$\alpha$ forest for the 21-cm power spectrum from the epoch of reionization

Cited by in corpus (24)

• Hydrogen reionisation ends by $z=5.3$: Lyman-$\alpha$ optical depth measured by the XQR-30 sample
• EIGER II. first spectroscopic characterisation of the young stars and ionised gas associated with strong H$\beta$ and [OIII] line-emission in galaxies at z=5-7 with JWST
• The THESAN project: properties of the intergalactic medium and its connection to Reionization-era galaxies
• Long Dark Gaps in the Ly$\beta$ Forest at $z<6$: Evidence of Ultra Late Reionization from XQR-30 Spectra
• Reionization after JWST: a photon budget crisis?
• Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Ly$\alpha$ Forest over $5<z<6$
• (Re)Solving Reionization with Lyα: How Bright Lyα Emitters account for the $z\approx 2-8$ Cosmic Ionizing Background
• 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
• Definitive upper bound on the negligible contribution of quasars to cosmic reionization
• The Magellan M2FS Spectroscopic Survey of High-$z$ Galaxies: Lyα Emitters at $z\approx 6.6$ and the Evolution of Lyα Luminosity Function over $z\approx 5.7-6.6$
• Deep Realistic Extragalactic Model (DREaM) Galaxy Catalogs: Predictions for a Roman Ultra-Deep Field
• The space density of ultra-luminous QSOs at the end of reionization epoch by the QUBRICS Survey and the AGN contribution to the hydrogen ionizing background
• Hydrodynamic Response of the Intergalactic Medium to Reionization II: Physical Characteristics and Dynamics of Ionizing Photon Sinks
• Improved treatments of the ionizing photon mean free path in semi-numerical simulations of reionization
• The Morphology of Reionization in a Dynamically Clumpy Universe
• A bottom-up search for Lyman-continuum leakage in the Hubble Ultra Deep Field
• Lyman-alpha Emitters in Ionized Bubbles: Constraining the Environment and Ionized Fraction
• Forecasting constraints on the mean free path of ionizing photons at $z\ge 5.4$ from the Lyman-$\alpha$ forest flux auto-correlation function
• Small-scale clumping of dark matter and the mean free path of ionizing photons at $z=6$
• The Lyman-limit photon mean free path at the end of late reionisation in the Sherwood-Relics simulations
• Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization
• Effects of Small-Scale Absorption Systems on Neutral Islands during the Late Epoch of Reionization
• An emulator for the ionizing photon mean free path in ultra-high resolution simulations: the implications of mean free path measurements for the reionization history
• Lyman Continuum escaping from in-situ formed stars in a tidal bridge at z = 3