The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization. (arXiv:2103.16610v2 [astro-ph.CO] UPDATED) <a href="http://arxiv.org/find/astro-ph/1/au:+Becker_G/0/1/0/all/0/1">George D. Becker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+DAloisio_A/0/1/0/all/0/1">Anson D'Aloisio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Christenson_H/0/1/0/all/0/1">Holly M. Christenson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhu_Y/0/1/0/all/0/1">Yongda Zhu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Worseck_G/0/1/0/all/0/1">Gábor Worseck</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bolton_J/0/1/0/all/0/1">James S. Bolton</a>
The mean free path of ionizing photons, $lambda_{rm mfp}$, is a key factor
in the photoionization of the intergalactic medium (IGM). At $z gtrsim 5$,
however, $lambda_{rm mfp}$ may be short enough that measurements towards QSOs
are biased by the QSO proximity effect. We present new direct measurements of
$lambda_{rm mfp}$ that address this bias and extend up to $z sim 6$ for the
first time. Our measurements at $z sim 5$ are based on data from the Giant
Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At $z
sim 6$ we use QSO spectra from Keck ESI and VLT X-Shooter. We measure
$lambda_{rm mfp} = 9.09^{+1.62}_{-1.28}$ 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, 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 $lambda_{rm mfp}$ over $5 < z < 6$. This evolution disfavors
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% neutral at $z=6$, although our measurement at $z = 6.0$ is
even lower than these models prefer.
The mean free path of ionizing photons, $lambda_{rm mfp}$, is a key factor
in the photoionization of the intergalactic medium (IGM). At $z gtrsim 5$,
however, $lambda_{rm mfp}$ may be short enough that measurements towards QSOs
are biased by the QSO proximity effect. We present new direct measurements of
$lambda_{rm mfp}$ that address this bias and extend up to $z sim 6$ for the
first time. Our measurements at $z sim 5$ are based on data from the Giant
Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At $z
sim 6$ we use QSO spectra from Keck ESI and VLT X-Shooter. We measure
$lambda_{rm mfp} = 9.09^{+1.62}_{-1.28}$ 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, 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 $lambda_{rm mfp}$ over $5 < z < 6$. This evolution disfavors
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% neutral at $z=6$, although our measurement at $z = 6.0$ is
even lower than these models prefer.
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