Probing Ultralate Reionization: Direct Measurements of the Mean Free Path over 5 < z < 6

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jats:titleAbstract</jats:title> jats:pThe mean free path of ionizing photons, jats:italicλ</jats:italic> jats:submfp</jats:sub>, is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of jats:italicλ</jats:italic> jats:submfp</jats:sub> from QSO spectra over the redshift range 5 < jats:italicz</jats:italic> < 6, including the first measurements at jats:italicz</jats:italic> ≃ 5.3 and 5.6. Our sample includes data from the XQR-30 VLT large program, as well as new Keck/ESI observations of QSOs near jats:italicz</jats:italic> ∼ 5.5, for which we also acquire new [C jats:scii</jats:sc>] 158 jats:italicμ</jats:italic>m redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="" overflow="scroll"> mml:msub mml:mrow mml:miλ</mml:mi> </mml:mrow> mml:mrow mml:mimfp</mml:mi> </mml:mrow> </mml:msub> mml:mo=</mml:mo> mml:msubsup mml:mrow mml:mn9.33</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn1.80</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn2.06</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="" xlink:href="apjaceef4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="" overflow="scroll"> mml:msubsup mml:mrow mml:mn5.40</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn1.40</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn1.47</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="" xlink:href="apjaceef4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="" overflow="scroll"> mml:msubsup mml:mrow mml:mn3.31</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn1.34</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn2.74</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="" xlink:href="apjaceef4ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, and jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="" overflow="scroll"> mml:msubsup mml:mrow mml:mn0.81</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn0.48</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn0.73</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="" xlink:href="apjaceef4ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> pMpc at jats:italicz</jats:italic> = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that jats:italicλ</jats:italic> jats:submfp</jats:sub> increases steadily and rapidly with time over 5 < jats:italicz</jats:italic> < 6. Notably, we find that jats:italicλ</jats:italic> jats:submfp</jats:sub> deviates significantly from predictions based on a fully ionized and relaxed IGM as late as jats:italicz</jats:italic> = 5.3. By comparing our results to model predictions and indirect jats:italicλ</jats:italic> jats:submfp</jats:sub> constraints based on IGM Lyjats:italicα</jats:italic> opacity, we find that the evolution of jats:italicλ</jats:italic> jats:submfp</jats:sub> is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift 6.</jats:p>

5101 Astronomical Sciences, 51 Physical Sciences
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American Astronomical Society