TY  - JOUR
A1  - Becker, George D.
A1  - D'Aloisio, Anson
A1  - Christenson, Holly M.
A1  - Zhu, Yongda
A1  - Worseck, Gábor
A1  - Bolton, James S.
T1  - The mean free path of ionizing photons at 5 < z < 6
BT  - evidence for rapid evolution near reionization
JF  - Monthly notices of the Royal Astronomical Society
N2  - The mean free path of ionizing photons, lambda(mfp), is a key factor in the photoionization of the intergalactic medium (IGM). At z greater than or similar to 5, however, lambda(mfp) may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of lambda(mfp) that address this bias and extend up to z similar to 6 for the first time. Our measurements at z similar to 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z similar to 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure lambda(mfp) = 9.09(-1.28)(+1.62) proper Mpc and 0.75(-0.45)(+0.65) proper Mpc (68 percent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in lambda(mfp) over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 percent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer.
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - cosmology: observations
KW  - dark ages
KW  - large-scale structure of Universe
KW  - reionization
KW  - first stars
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab2696
SN  - 0035-8711
SN  - 1365-2966
VL  - 508
IS  - 2
SP  - 1853
EP  - 1869
PB  - Oxford Univ. Press
CY  - Oxford
ER  -