发表机构
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo; The University of Tokyo Institutes for Advanced Study, The University of Tokyo; Department of Astronomy, Graduate School of Science, The University of Tokyo; Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo; Kavli Institute for Astronomy and Astrophysics, Peking University; Department of Astronomy, School of Physics, Peking University; Astronomical Science Program, Graduate Institute for Advanced Studies, SOKENDAI; National Astronomical Observatory of Japan; Institute for Cosmic Ray Research, The University of Tokyo（东京大学宇宙物质前沿研究世界顶级学术中心; 东京大学高等研究院; 东京大学理学研究科天文学系; 东京大学UTIAS数据驱动发现中心; 北京大学科维理天文与天体物理研究所; 北京大学物理学院天文学系; 综合研究大学院大学天文学研究项目; 日本国立天文台; 东京大学宇宙射线研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文提出利用自由-自由射电发射作为独立探针，通过光谱转折和斜率约束小红点周围电离气体的电子柱密度、尺寸及径向分布，并预测ALMA和ngVLA可观测的信号。
AI 中文摘要
近期研究假设小红点（LRDs）是快速吸积的黑洞，周围环绕着致密的气体环境。中性和电离气体在解释LRDs许多令人困惑的光谱特征（包括光学-红外波段的发射线轮廓）中均发挥关键作用，尽管这些气体组分所需的物理条件仍鲜有约束。我们提出自由-自由发射为LRDs中的电离气体提供了一种稳健且独立的探针。自由-自由自吸收产生特征性的光谱转折，其临界频率和光度取决于电子柱密度和电离区域的特征尺寸，因此射电观测可同时约束这两个量。转折频率以下的光谱斜率进一步探测径向密度分布，其空间尺度远小于当前及未来光学-红外天文台所能达到的分辨率。我们还展示了本地及高红移宇宙中两个代表性LRDs的预测自由-自由光谱能量分布，并表明未来ALMA和ngVLA的射电至毫米波观测能够探测当前观测所推断的电离气体参数范围。因此，自由-自由发射为探测LRDs周围电离气体的几何结构和物理条件提供了一种独特的方法。
英文摘要
Recent studies have hypothesized that little red dots (LRDs) are rapidly accreting black holes surrounded by dense gaseous environments. Both neutral and ionized gas play a key role in explaining many of the puzzling spectral features of LRDs, including the emission line profiles in the optical-infrared bands, although the physical conditions required for these gas components remain poorly constrained. We propose that free-free emission provides a robust, independent probe of ionized gas in LRDs. Free-free self-absorption produces a characteristic spectral turnover whose critical frequency and luminosity depend on the electron column density and the characteristic size of the ionized region, such that radio observations simultaneously constrain both quantities. The spectral slope at frequencies below the turnover further probes the radial density distribution on spatial scales far below those achievable by current and future optical-infrared observatories. We also present predicted free-free spectral energy distributions for two representative LRDs in the local and high-redshift universe and show that future radio-to-millimeter observations with ALMA and ngVLA can probe the range of ionized-gas parameters inferred from current observations. Free-free emission therefore offers a unique way to probe the geometry and physical conditions of the ionized gas surrounding LRDs.
Comments9 pages, 5 figures
