首次利用分子吸收线确定红移z=0.68处的宇宙微波背景辐射温度
First Determination of the Cosmic Microwave Background Radiation Temperature at $z\!=\!0.68$ Using Molecular Absorption Lines
AI总结：
通过分析类星体B0218+357的毫米波数据，首次精确测定红移z=0.68处的CMB温度为4.50±0.17K。
摘要：
我们分析了利用阿塔卡马大型毫米波/亚毫米波阵列观测的向类星体B0218+357的毫米波数据，获得了在宇宙红移z=0.68处的HCN、HCO+、HNC、H13CN和H13CO+的J=2-1和J=3-2的旋转跃迁吸收光谱。对于HCN、HCO+和HNC，我们识别出两种不同的吸收成分，这两种成分在两种跃迁中都存在，而同位素光谱则只检测到一种成分。在本文中，我们通过这些成分的吸收强度准确评估了激发温度及其不确定性，并利用这些来确定CMB温度。通过蒙特卡洛采样将连续体覆盖因子的不确定性传播到激发温度中。我们进一步通过假设对数正态列密度分布来校正观测到的光学深度，以抵消由于列密度非均匀性引起的偏差。在假设旋转能级与宇宙微波背景（CMB）处于辐射平衡的情况下，我们推导出了在光学薄区域的激发温度分布。由于HCO+的激发受到额外速度分量和部分碰撞激发的影响，这种物质被排除在最终确定CMB温度的决定之外。通过HCN和HNC获得的激发温度的加权平均，我们确定了在z=0.68处的CMB温度为4.50±0.17K。这构成了基于类星体吸收线系统的首次测量CMB温度的z=0.68处的测量，并代表了在此红移处最精确的确定，与标准大爆炸宇宙学模型高度一致。
英文摘要：
We analyzed millimeter-wave data toward the quasar B0218+357 observed with the Atacama Large Millimeter/submillimeter Array and obtained absorption spectra of the $J$=2-1 and $J$=3-2 rotational transitions of HCN, HCO$^{+}$, HNC, H$^{13}$CN, and H$^{13}$CO$^{+}$ at the cosmological redshift of $z\!=\!0.68$. For HCN, HCO$^{+}$, and HNC, we identified two distinct absorption components that are common to both transitions, whereas a single component was detected in the isotopologue spectra. In this paper, we accurately evaluate the excitation temperatures and their uncertainties from the absorption strengths of these components, and use them to determine the CMB temperature. Uncertainties in the continuum covering factor were propagated into the excitation temperature via Monte Carlo sampling. We further corrected the observed optical depths for biases due to column-density nonuniformity by assuming a lognormal column-density distribution. Under the assumption that the rotational levels are in radiative equilibrium with the cosmic microwave background (CMB), we derived excitation temperature profiles in the optically thin regime. Because the excitation of HCO$^+$ is biased by an additional velocity component and partial collisional excitation, this species was excluded from the final determination of the CMB temperature. From a weighted mean of the excitation temperatures obtained from HCN and HNC, we determined the CMB temperature at $z\!=\!0.68$ to be ${4.50\pm0.17\,\mathrm{K}}$. This constitutes the first measurement of the CMB temperature at $z\!=\!0.68$ based on a quasar absorption line system and represents the most precise determination at this redshift, highly consistent with the standard Big Bang cosmological model.
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