Low-frequency Radio Recombination Lines Away from the Inner Galactic Plane
Low-frequency Radio Recombination Lines Away from the Inner Galactic Plane
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DOI：
10.3847/1538-3881/ad08ba
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发表时间：
2023-02
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通讯作者：
Akshatha K. Vydula;J. Bowman;David Lewis;K. Crawford;M. Kolopanis;A. Rogers;S. Murray;N. Mahesh-N.
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作者：
Akshatha K. Vydula;J. Bowman;David Lewis;K. Crawford;M. Kolopanis;A. Rogers;S. Murray;N. Mahesh-N.
银河系中的漫射射电复合线（RRL）是红移21厘米实验的可能前景。我们使用中心在−26.°7 decl.来描述南天弥漫的RRL我们发现在大天线波束（72° × 110°）上平均的RRL达到R. A.在这个区域，Cα吸收幅度为33 ± 11 mK（1σ），在50-87 MHz范围内平均（21 cm谱线为27 μ z × 15），并随频率降低而强烈增加。Cβ和Hα线与未检测到一致，振幅分别为13 ± 14和12 ± 10 mK（1σ）。在108- 124.5MHz（z ~（11）），在3.4mK（1σ）的噪声水平下，没有发现碳或氢谱线的证据。保守地假设，观察到的线来自广泛的弥漫的星际介质，而不是一些紧凑的区域，这些振幅提供了上限的内在扩散线。观测结果支持了这样的预期，即在红移21厘米的实验，特别是那些针对宇宙黎明的实验，超越探测阶段之前，银河系RRL可以被忽略为大面积天空的重要前景。我们拟合了在大的边缘光束上平均的谱线的光谱依赖性模型，其中可能包含多个线源，并且可能存在线混合，并且发现对于Cα和Hα线，包括预期的平滑的、与频率相关的偏离局部热力学平衡（LTE）的自由度优于简单的LTE假设。对于Cα，我们估计出沿着内银道面的偏离系数为0.79 < B n β n < 4.5，远离内银道面的偏离系数为0 < B n β n < 2.3。
Diffuse radio recombination lines (RRLs) in the Galaxy are possible foregrounds for redshifted 21 cm experiments. We use EDGES drift scans centered at −26.°7 decl. to characterize diffuse RRLs across the southern sky. We find that RRLs averaged over the large antenna beam (72° × 110°) reach minimum amplitudes of R.A. = 2–6 hr. In this region, the Cα absorption amplitude is 33 ± 11 mK (1σ) averaged over 50–87 MHz (27 ≳ z ≳ 15 for the 21 cm line) and increases strongly as frequency decreases. Cβ and Hα lines are consistent with no detection with amplitudes of 13 ± 14 and 12 ± 10 mK (1σ), respectively. At 108–124.5 MHz (z ≈ 11) in the same region, we find no evidence for carbon or hydrogen lines at the noise level of 3.4 mK (1σ). Conservatively assuming that observed lines come broadly from the diffuse interstellar medium, as opposed to a few compact regions, these amplitudes provide upper limits on the intrinsic diffuse lines. The observations support expectations that Galactic RRLs can be neglected as significant foregrounds for a large region of sky until redshifted 21 cm experiments, particularly those targeting cosmic dawn, move beyond the detection phase. We fit models of the spectral dependence of the lines averaged over the large beam of EDGES, which may contain multiple line sources with possible line blending, and find that including degrees of freedom for expected smooth, frequency-dependent deviations from local thermodynamic equilibrium (LTE) is preferred over simple LTE assumptions for Cα and Hα lines. For Cα we estimate departure coefficients 0.79 < b n β n < 4.5 along the inner Galactic plane and 0 < b n β n < 2.3 away from the inner Galactic plane.