[论文解读] MeerKAT discovery of a double radio relic and odd radio circle: connecting cluster and galaxy merger shocks

B. Koribalski, A. Veronica+12 more|arXiv (Cornell University)|Apr 24, 2023

Galaxies: Formation, Evolution, Phenomena被引用 4

一句话总结

本文报告了MeerKAT在合并星系团PSZ2 G277.93+12.34中发现了一个罕见的双射电遗迹系统和一个新的奇异性射电环（ORC），揭示了星系团和星系尺度并合所产生的激波结构。双遗迹相距约2.6 Mpc，而直径达400 kpc的ORC表明其形成机制可能与并合驱动的激波有关。

ABSTRACT

We present the serendipitous discovery of (1) a large double radio relic associated with the galaxy cluster PSZ2 G277.93+12.34 and (2) a new odd radio circle, ORC J1027-4422, both found in the same deep MeerKAT 1.3 GHz wide-band radio continuum image. The angular separation of the two arc-shaped cluster relics is ~16 arcmin or ~2.6 Mpc for a cluster redshift of z ~ 0.158. The thin southern relic, which shows several ridges/shocks including one possibly moving inwards, has a linear extent of ~1.64 Mpc. In contrast, the northern relic is about twice as wide, twice as bright, but only has a largest linear size of ~0.66 Mpc. Complementary SRG/eROSITA X-ray images reveal extended emission from hot intracluster gas between the two relics and around the narrow-angle tail (NAT) radio galaxy PMN J1033-4335 (z ~ 0.153) located just east of the northern relic. The radio morphologies of the NAT galaxy and the northern relic, which are also detected with the Australian Square Kilometer Array Pathfinder (ASKAP) at 888 MHz, suggest both are moving in the same outward direction. The discovery of ORC J1027-4422 in a different part of the same MeerKAT image makes it the 4th known single ORC. It has a diameter of ~90 arcsec corresponding to 400 kpc at a tentative redshift of z ~ 0.3 and remains undetected in X-ray emission. Supported by simulations, we discuss similarities between outward moving galaxy and cluster merger shocks as the formation mechanisms for ORCs and radio relics, respectively.

研究动机与目标

• 研究合并星系团中弥漫射电发射的起源，特别是双射电遗迹与奇异性射电环（ORCs）的成因。
• 确定ORCs与射电遗迹是否具有相同的形成机制，且与并合激波相关。
• 利用射电与X射线数据约束星系团并合中激波的物理特性，特别是谱指数与马赫数。
• 探讨相对论性粒子加速与大尺度激波结构中磁场的作用。

提出的方法

• 利用MeerKAT射电望远镜进行深度1.3 GHz宽频带观测，以探测PSZ2 G277.93+12.34场中的弥漫射电源。
• 将MeerKAT数据与SRG/eROSITA X射线图像结合，识别星系际介质的高温发射并定位激波区域。
• 利用高灵敏度、高分辨率成像分析遗迹与ORC的射电谱指数与形态。
• 将射电结构与光学及红外数据（VISTA半球巡天）交叉匹配，识别相关星系与星系群。
• 利用低频ASKAP RACS数据（887.5 MHz）确认射电发射并改善谱指数测量。
• 通过比较北部遗迹与窄角尾（NAT）射电星系的形态与动力学，推断共动激波的演化。

Figure 1: MeerKAT 1.3 GHz radio continuum image of a newly discovered double relic associated with the galaxy cluster PSZ2 G277.93+12.34. The image has a resolution of 7.7″ and is primary beam corrected. The pointing center lies $\sim$ 43′ south-west of the cluster centre.

实验结果

研究问题

• RQ1 PSZ2 G277.93+12.34中的双射电遗迹形态起源为何？其与星系团并合几何结构有何关联？
• RQ2 新发现的ORC J1027–4422是否可用与星系团中射电遗迹相同的激波驱动机制解释？
• RQ3 NAT射电星系与北部射电遗迹的运动关系如何？这对激波特性的传播有何启示？
• RQ4 尽管存在双遗迹，为何在PSZ2 G277.93+12.34中未探测到射电 halo？这对并合动力学有何启示？
• RQ5 ORC的物理特性（如环直径与无X射线辐射）与已知射电遗迹及并合模拟结果相比如何？

主要发现

• 在PSZ2 G277.93+12.34中发现双射电遗迹系统，南部遗迹延伸约1.64 Mpc，北部遗迹约0.66 Mpc，两者相距约2.6 Mpc，红移z ≈ 0.158。
• 南部遗迹表现出多个脊状结构及可能向内运动的激波特征，而北部遗迹宽度与亮度均为其两倍，但更紧凑。
• eROSITA X射线数据揭示遗迹之间及NAT射电星系PMN J1033–4335周围的高温星系际介质，证实了激波的存在。
• 未在星系团中探测到射电 halo，表明当前并合阶段粒子再加速尚未占主导地位。
• ORC J1027–4422直径约90″（z ≈ 0.3时对应400 kpc），呈部分环状，中心呈双峰发射，无X射线对应体。
• ORC J1027–4422的形态与缩小版的双遗迹及射电 halo 非常相似，支持其与射电遗迹相同的并合激波起源。

Figure 2: MeerKAT 1.3 GHz radio continuum image of the cluster double relic (as in Fig. 1 ) overlaid with eRASS:3 X-ray (0.3 – 2 keV) contours in blue. The eROSITA image was smoothed with a 30″ Gaussian; the contour levels are 0.001, 0.002, 0.004, 0.008 and 0.016. The overlaid circles indicate the a