[论文解读] The glow of annihilating dark matter in Omega Centauri

A. M. Brown, R. Massey+4 more|arXiv (Cornell University)|Jul 19, 2019

Astrophysics and Cosmic Phenomena参考文献 2被引用 8

一句话总结

本文提出，银河系最大球状星团——半人马座ω星团的伽马射线辐射源自质量为31 ± 4 GeV的暗物质粒子湮灭。通过恒星运动学与伽马射线能谱拟合，作者发现J因子值较高，且与观测谱形拟合良好，排除了毫秒脉冲星等天体物理源的可能性，使半人马座ω星团成为间接探测暗物质的首选目标。

ABSTRACT

Dark matter (DM) is the most abundant material in the Universe, but has so far been detected only via its gravitational effects. Several theories suggest that pairs of DM particles can annihilate into a flash of light at gamma-ray wavelengths. While gamma-ray emission has been observed from environments where DM is expected to accumulate, such as the centre of our Galaxy, other high energy sources can create a contaminating astrophysical gamma-ray background, thus making DM detection difficult. In principle, dwarf galaxies around the Milky Way are a better place to look -- they contain a greater fraction of DM with no astrophysical gamma-ray background -- but they are too distant for gamma-rays to have been seen. A range of observational evidence suggests that Omega Centauri (omega Cen or NGC 5139), usually classified as the Milky Way's largest globular cluster, is really the core of a captured and stripped dwarf galaxy. Importantly, Omega Cen is ten times closer to us than known dwarfs. Here we show that not only does Omega Cen contain DM with density as high as compact dwarf galaxies, but also that it emits gamma-rays with an energy spectrum matching that expected from the annihilation of DM particles with mass 31$\pm$4 GeV (68\% confidence limit). No astrophysical sources have been found that would otherwise explain Omega Cen's gamma-ray emission, despite deep multi-wavelength searches. We anticipate our results to be the starting point for even deeper radio observations of Omega Cen. If multi-wavelength searches continue to find no astrophysical explanations, this pristine, nearby clump of DM will become the best place to study DM interactions through forces other than gravity.

研究动机与目标

• 确定半人马座ω星团观测到的伽马射线辐射是否可归因于暗物质湮灭。
• 利用恒星速度弥散数据评估半人马座ω星团中暗物质的存在与分布。
• 排除毫秒脉冲星等天体物理源作为伽马射线信号起源的可能性。
• 评估半人马座ω星团因距离近且暗物质密度高，作为间接探测暗物质高潜力目标的优越性。

提出的方法

• 利用恒星视线速度弥散数据，假设动力学平衡，建模半人马座ω星团的引力势场。
• 采用三组分质量模型拟合数据：发光恒星、中心黑洞（质量上限M_BH < 5×10⁴ M☉）及扩展的暗物质组分。
• 使用Navarro-Frenk-White（NFW）剖面描述暗物质分布，并计算J因子，该量是间接探测的关键参数。
• 将观测到的伽马射线能谱分布（SED）拟合至暗物质湮灭模型，自由参数包括暗物质粒子质量与湮灭截面。
• 采用贝叶斯框架传播J因子的不确定性，并评估统计显著性。
• 将伽马射线SED与已知毫秒脉冲星谱进行比较，并利用多波段数据（X射线、射电）排除脉冲星污染的可能性。

实验结果

研究问题

• RQ1 半人马座ω星团观测到的伽马射线辐射能否由暗物质湮灭解释？
• RQ2 基于恒星运动学，半人马座ω星团内部暗物质密度与分布的推断结果如何？
• RQ3 伽马射线谱是否与特定质量与湮灭截面的暗物质粒子一致？
• RQ4 毫秒脉冲星或其他天体物理源是否可能解释观测到的伽马射线通量？
• RQ5 半人马座ω星团因距离近且暗物质含量高，相比遥远的矮椭圆星系，为何更适合作为间接探测暗物质的优越目标？

主要发现

• 数据强烈表明半人马座ω星团中存在暗物质，最佳拟合暗物质质量为10⁵.³⁺⁰.⁵ M☉，位于光学半光半径7 pc内。
• 几何J因子为log₁₀(J) = 22.1⁺¹.³₋₀.⁹ GeV² cm⁻⁵（95%置信区间），高于大多数矮椭圆星系，包括Segue I。
• 最佳拟合暗物质粒子质量为31 ± 4 GeV，湮灭截面与热退耦合丰度一致（⟨σv⟩ ≈ 3×10⁻²⁶ cm³ s⁻¹）。
• 伽马射线能谱拟合的卡方值为χ² = 6.8（自由度为8），表明与数据具有良好的统计拟合度。
• 毫秒脉冲星被排除为辐射源：谱形不匹配，且所需脉冲星数量（19±9）在射电巡天中应已被探测到。
• 未发现伽马射线源的X射线或射电对应体，且即使在保守假设下，ω星团中脉冲星数量也过少，不足以解释该通量。