SILVERRUSH. XI. Constraints on the Lyα Luminosity Function and Cosmic Reionization at z = 7.3 with Subaru/Hyper Suprime-Cam
Hinako Goto, Kazuhiro Shimasaku, S. Yamanaka, Rieko Momose, Makoto Ando, Yuichi Harikane, Takuya Hashimoto, Akio Inoue, Masami Ouchi
Abstract
Abstract The Ly α luminosity function (LF) of Ly α emitters (LAEs) has been used to constrain the neutral hydrogen fraction in the intergalactic medium (IGM) and thus the timeline of cosmic reionization. Here we present the results of a new narrowband imaging survey for z = 7.3 LAEs in a large area of ∼3 deg 2 with Subaru/Hyper Suprime-Cam. No LAEs are detected down to L Ly α ≃ 10 43.2 erg s −1 in an effective cosmic volume of ∼2 × 10 6 Mpc 3 , placing an upper limit on the bright part of the z = 7.3 Ly α LF for the first time and confirming a decrease in bright LAEs from z = 7.0. By comparing this upper limit with the Ly α LF in the case of fully ionized IGM, which is predicted using an observed z = 5.7 Ly α LF on the assumption that the intrinsic Ly α LF evolves in the same way as the UV LF, we obtain the relative IGM transmission <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Ly</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>IGM</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:mn>7.3</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Ly</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>IGM</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:mn>5.7</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo><</mml:mo> <mml:mn>0.77</mml:mn> </mml:math> and then the volume-averaged neutral fraction x H I (7.3) > 0.28. Cosmic reionization is thus still ongoing at z = 7.3, consistent with results from other x H I estimation methods. A similar analysis using literature Ly α LFs finds that at z = 6.6 and 7.0, the observed Ly α LF agrees with the predicted one, consistent with full ionization.