21厘米森林的迅速兴起I：对5.38 < z < 5.84期间星系际介质光学深度的限制
The burgeoning rise of the 21-cm forest I: constraints on the optical depth of the intergalactic medium at $5.38 < z < 5.84$
AI总结：
研究通过对高红移射电源的红移21厘米吸收，分析观测数据并结合档案数据，用幂律连续谱拟合类星体光谱，考虑岛屿和全局两种模型，限制了再电离末期IGM光学深度，证实IGM被加热，不支持极冷HI区域存在，为相关研究开辟道路。
AI中文摘要：
红移的21厘米线是再电离时代很有前景的探测器，那时第一代恒星使星系际介质（IGM）电离。本文旨在通过对高红移射电源的红移21厘米吸收来限制IGM中性氢分数和自旋温度。分析了对射电噪类星体PSO J352.4034 - 15.3373（z = 5.84）在203 - 222.5 MHz频段18小时的观测。得到连续图像和光谱，用幂律连续谱拟合类星体光谱并考虑两种模型。结合档案数据和测量确定了类星体流量密度和光谱指数。未发现5.38 < z < 5.84期间中性氢的21厘米吸收证据。假设岛屿模型得出IGM自旋温度下限，假设全局模型限制了21厘米光学深度。结果对再电离末期21厘米光学深度提供了限制，证实IGM被加热到绝热冷却极限以上，与理论预测和高红移21厘米功率谱测量一致，也不支持z < 5.84存在极冷HI区域，为未来高红移源的21厘米吸收研究开辟了道路。
英文摘要：
The redshifted 21-cm line is a promising probe of the Epoch of Reionization, during which the first generation of stars ionized the InterGalactic Medium (IGM). We aimed to constrain the IGM neutral Hydrogen fraction and spin temperature via redshifted 21-cm absorption against high-redshift radio sources. We analysed an 18-hour observation of the radio-loud quasar PSO J352.4034-15.3373 ($z = 5.84$) in the 203-222.5 MHz band. We obtained a continuum image with a $0.66$ mJy/beam rms noise and a spectrum with $3.6$ mJy/beam rms noise per 390 kHz-wide channel. We fit the quasar spectrum with a power-law continuum modified by an intervening 21-cm absorption, testing two scenarios: an island model, a residual cold neutral Hydrogen patch surviving at the end of reionization, and a global model, approximating the overall decline of the IGM HI fraction and spin temperature with redshift. Combining archival data spanning 150 MHz to 3.0 GHz with our measurements, we determined a quasar flux density of $86.8 \pm 1.1$ mJy at 200 MHz and a spectral index of $-0.88 \pm 0.02$ across the full frequency range. We found no evidence for the 21-cm absorption from intervening neutral Hydrogen at $5.38<z<5.84$. Assuming the island model, we set a 95% confidence lower limit on the IGM spin temperature of $1.73$ K in residual HI regions. Assuming the global model, we constrained the 21-cm optical depth to $τ_{21}< 0.02$ (95% C.L.). These results provide constraints on the 21-cm optical depth near the end of reionization, over the $5.38 < z < 5.84$ range, and confirm that the IGM was heated above the adiabatic cooling limit ($\sim 0.8$ K at $z = 5.68$), consistent with theoretical predictions and with 21-cm power spectrum measurements at higher redshifts. Our results also disfavour the presence of extremely cold HI regions at $z < 5.84$ and open the way to future 21-cm absorption from high-redshift sources.
