# The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

**Authors:** George D. Becker, Anson D'Aloisio, Holly M. Christenson, Yongda Zhu, Gábor Worseck, James S. Bolton  
**arXiv:** [2103.16610](https://arxiv.org/abs/2103.16610)  
**Date:** 2021-03-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2103.16610v1/

## Key points

- George D. Becker and colleagues present new direct measurements of the mean free path of ionizing photons, $\lambda_{\rm mfp}$.
- The study extends measurements up to $z \sim 6$ for the first time using QSO spectra from Keck ESI and VLT X-Shooter.
- The authors measure $\lambda_{\rm mfp} = 9.09^{+1.28}_{-1.60}$ proper Mpc and $0.75^{+0.65}_{-0.45}$ proper Mpc (68% confidence) at $z = 5.1$ and 6.0, respectively.
- Neglecting the proximity effect biases the result high by a factor of two or more at $z = 6$, suggesting rapid evolution in $\lambda_{\rm mfp}$ over $5 < z < 6$.
- The findings disfavor models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by $z = 6$.
- The authors' mean free path results are most consistent with late reionization models wherein the IGM is still 20% neutral at $z = 6$.

## Summary

In the study titled "The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization," George D. Becker and colleagues present new direct measurements of the mean free path of ionizing photons, $\lambda_{\rm mfp}$, which is a crucial factor in the photoionization of the intergalactic medium (IGM). The authors address potential biases that arise from the proximity effect when measuring $\lambda_{\rm mfp}$ towards quasars at redshifts $z \gtrsim 5$. The study extends measurements up to $z \sim 6$ for the first time using QSO spectra from Keck ESI and VLT X-Shooter. At $z \sim 5$, data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs are used. The authors measure $\lambda_{\rm mfp} = 9.09^{+1.28}_{-1.60}$ proper Mpc and $0.75^{+0.65}_{-0.45}$ proper Mpc (68% confidence) at $z = 5.1$ and 6.0, respectively. The results at $z = 5.1$ are consistent with existing measurements, indicating that bias from the proximity effect is minor at this redshift. However, neglecting the proximity effect biases the result high by a factor of two or more at $z = 6$. This finding suggests rapid evolution in $\lambda_{\rm mfp}$ over $5 < z < 6$, as our measurement falls well below extrapolations from lower redshifts. The study's findings disfavor models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by $z = 6$. Instead, the results are qualitatively consistent with models wherein reionization completed at $z = 6$ or even significantly later. The authors' mean free path results are most consistent with late reionization models wherein the IGM is still 20% neutral at $z = 6$, although their measurement at $z = 6.0$ is even lower than these models prefer. Overall, this study provides new direct measurements of $\lambda_{\rm mfp}$ that address potential biases and extend up to $z \sim 6$. The findings suggest rapid evolution in $\lambda_{\rm mfp}$ over $5 < z < 6$ and disfavor early reionization models.