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巨型双子座GMOS巡天观测z>4.4类星体- I.测量穿越宇宙时间的平均自由程

The Giant Gemini GMOS survey of z>4.4 quasars - I. Measuring the mean free path across cosmic time

Gábor Worseck, J. Xavier Prochaska, John M. O'Meara, George D. Becker, Sara Ellison, Sebastian Lopez, Avery Meiksin, Brice Ménard, Michael T. Murphy, Michele Fumagalli

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16 pages, 11 figures, plus 20 pages of online material; submitted to MNRAS
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摘要：

我们已经获得了163个类星体的光谱在$z_\mathrm{em}>4.4$与双子座多目标光谱仪的双子座南北望远镜，最大的公开样品的高质量，低分辨率的光谱在这些红移。从这个均匀的数据集，我们产生了堆叠类星体光谱在三个红移间隔$z\sim 5$。我们模拟了低于静止坐标系莱曼极限（$λ_\mathrm{r}<912$极限）的通量，以评估星系际介质对HI电离辐射的平均自由程$λ_\mathrm{mfp}^{912}$。在平均红移$z_\mathrm{q}=4.56$，4.86和5.16，我们测量$λ_\mathrm{mfp}^{912}=(22.2\pm 2.3, 15.1\pm 1.8, 10.3\pm 1.6)h_{70}^{-1}$适当的Mpc与样本方差占主导地位的不确定性。结合我们的结果与低红移的$λ_\mathrm{mfp}^{912}$测量，数据很好地模拟了一个简单的幂律$λ_\mathrm{mfp}^{912}=A[(1+z)/5]^η$与$A=(37\pm 2)h_{70}^{-1}$ Mpc和$η= -5.4\pm 0.4$之间的$z=2.3$和$z=5.5$。这种快速的演化需要一种物理机制--超越宇宙膨胀--来降低宇宙的有效莱曼极限不透明度。我们推测，大多数的HI莱曼极限不透明度表现在气体外的星系暗物质晕，跟踪大规模的结构（如灯丝），其平均密度（因此中性分数）随宇宙时间的推移而减少。我们的测量强烈红移依赖的平均自由程后不久完成HI再电离作为一个有价值的边界条件的数值模型。在$z=5.2$测量了$λ_\mathrm{mfp}^{912}\approx 10$ Mpc后，我们确认星系际介质在那个时期是高度电离的，并且平均自由程的红移演化没有显示出一个中断，这表明HI再电离最近结束。

英文摘要：

We have obtained spectra of 163 quasars at $z_\mathrm{em}>4.4$ with the Gemini Multi Object Spectrometers on the Gemini North and South telescopes, the largest publicly available sample of high-quality, low-resolution spectra at these redshifts. From this homogeneous data set, we generated stacked quasar spectra in three redshift intervals at $z\sim 5$. We have modelled the flux below the rest-frame Lyman limit ($λ_\mathrm{r}<912$Å) to assess the mean free path $λ_\mathrm{mfp}^{912}$ of the intergalactic medium to HI-ionizing radiation. At mean redshifts $z_\mathrm{q}=4.56$, 4.86 and 5.16, we measure $λ_\mathrm{mfp}^{912}=(22.2\pm 2.3, 15.1\pm 1.8, 10.3\pm 1.6)h_{70}^{-1}$ proper Mpc with uncertainties dominated by sample variance. Combining our results with $λ_\mathrm{mfp}^{912}$ measurements from lower redshifts, the data are well modelled by a simple power-law $λ_\mathrm{mfp}^{912}=A[(1+z)/5]^η$ with $A=(37\pm 2)h_{70}^{-1}$ Mpc and $η= -5.4\pm 0.4$ between $z=2.3$ and $z=5.5$. This rapid evolution requires a physical mechanism -- beyond cosmological expansion -- which reduces the cosmic effective Lyman limit opacity. We speculate that the majority of HI Lyman limit opacity manifests in gas outside galactic dark matter haloes, tracing large-scale structures (e.g. filaments) whose average density (and consequently neutral fraction) decreases with cosmic time. Our measurements of the strongly redshift-dependent mean free path shortly after the completion of HI reionization serve as a valuable boundary condition for numerical models thereof. Having measured $λ_\mathrm{mfp}^{912}\approx 10$ Mpc at $z=5.2$, we confirm that the intergalactic medium is highly ionized by that epoch and that the redshift evolution of the mean free path does not show a break that would indicate a recent end to HI reionization.

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