第一次探测到超亮红外星系的GeV发射：费米大区域望远镜看到的Arp 220
First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope
摘要：
星暴星系中的宇宙射线（CR）与致密的星际介质（ISM）碰撞产生高能γ射线。Arp 220是最近的超亮红外星系（ULIRG），其恒星形成处于极端水平，因此长期以来一直被预测会发射高能伽马射线。然而，尽管进行了密集的搜索，但没有发现伽马射线发射的证据。在这里，我们报告的发现高能伽马射线发射超过200兆电子伏的阿普220在$\sim 6.3 σ$的置信水平使用7.5年的\textsl {费米}大面积望远镜的观测。伽马射线辐射在观测期间没有表现出明显的变化，这与恒星形成星系的伽马射线光度和总红外光度之间的准线性标度关系是一致的，这表明这些伽马射线来自CR相互作用。由于Arp 220的高密度介质使其成为理想的CR量热计，因此在已知Arp 220中的超新星速率的情况下，可以使用伽马射线光度来测量超新星（SN）遗迹为CR提供动力的效率。我们发现，这个效率约为$4.2\pm2.6\%$的CR超过1 GeV。
英文摘要：
Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by colliding with the dense interstellar medium (ISM). Arp 220 is the nearest ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme levels, so it has long been predicted to emit high-energy gamma-rays. However, no evidence of gamma-ray emission was found despite intense efforts of search. Here we report the discovery of high-energy gamma-ray emission above 200 MeV from Arp 220 at a confidence level of $\sim 6.3 σ$ using 7.5 years of \textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows no significant variability over the observation period and it is consistent with the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, suggesting that these gamma-rays arise from CR interactions. As the high density medium of Arp 220 makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to measure the efficiency of powering CRs by supernova (SN) remnants given a known supernova rate in Arp 220. We find that this efficiency is about $4.2\pm2.6\%$ for CRs above 1 GeV.
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