The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s supermassive black hole. Here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the Sagittarius dwarf spheroidal galaxy (dSph). This large Milky Way satellite is viewed through the Fermi bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sagittarius dSph has no ongoing star formation, but we nevertheless demonstrate that the dwarf’s millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron–positron pairs injected by millisecond pulsars belonging to the Sagittarius dSph, combined with these objects’ magnetospheric emission. This finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the Galactic Centre, the Andromeda galaxy and other massive Milky Way dSphs.

费米气泡是从银河系核心发出的巨大 γ 射线发射叶，在费米伽马射线太空望远镜上的大面积望远镜收集的约 1-100 GeV 数据中发现。以前的工作揭示了费米气泡内的子结构，这被解释为银河超大质量黑洞准直流出的标志。在这里，我们通过空间模板分析表明，与子结构最亮区域（即所谓的茧）相关的大部分 γ 射线发射可能是由于人马座矮球状星系 (dSph)。从太阳系的位置通过费米气泡观察这颗大型银河系卫星。作为潮汐和冲压压力剥离的残余物，人马座 dSph 没有持续的恒星形成，但是我们仍然证明了矮星的毫秒脉冲星族可以合理地提供我们的分析与其恒星模板相关的γ射线信号。测得的光谱自然可以通过属于人马座 dSph 的毫秒脉冲星注入的高能电子-正电子对对宇宙微波背景光子的逆康普顿散射进行解释，并结合这些物体的磁层发射。这一发现似是而非地表明毫秒脉冲星会在老恒星群中产生显着的 γ 射线发射，这可能会混淆在银河系中心、仙女座星系和其他大质量银河系 dSphs 等区域的间接暗物质搜索。测得的光谱自然可以通过属于人马座 dSph 的毫秒脉冲星注入的高能电子-正电子对对宇宙微波背景光子的逆康普顿散射进行解释，并结合这些物体的磁层发射。这一发现似是而非地表明毫秒脉冲星会在老恒星群中产生显着的 γ 射线发射，这可能会混淆在银河系中心、仙女座星系和其他大质量银河系 dSphs 等区域的间接暗物质搜索。测得的光谱自然可以通过属于人马座 dSph 的毫秒脉冲星注入的高能电子-正电子对对宇宙微波背景光子的逆康普顿散射进行解释，并结合这些物体的磁层发射。这一发现似是而非地表明毫秒脉冲星会在老恒星群中产生显着的 γ 射线发射，这可能会混淆在银河系中心、仙女座星系和其他大质量银河系 dSphs 等区域的间接暗物质搜索。