Karwin Christopher M, Murgia Simona, Campbell Sheldon, Moskalenko Igor V
Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
Hansen Experimental Physics Laboratory and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA.
Astrophys J. 2019 Aug 1;880(2). doi: 10.3847/1538-4357/ab2880. Epub 2019 Jul 30.
The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to ~600 kpc across and comprises ~90% of the galaxy's total mass. This halo size translates into a large diameter of 42° on the sky, for an M31-Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with -ray telescopes. Using 7.6 yr of Large Area Telescope (-LAT) observations, we make a detailed study of the -ray emission between 1-100 GeV toward M31's outer halo, with a total field radius of 60° centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic-ray propagation code GALPROP to construct specialized interstellar emission models to characterize the foreground -ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upward of ~120-200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertainties in the MW foreground-and in particular, modeling of the H i-related components-have not been fully explored and may impact the results.
仙女座星系是离我们最近的旋涡星系，一直是众多研究的对象。它拥有一个巨大的暗物质晕，其直径可能跨越约600千秒差距，占星系总质量的约90%。对于仙女座星系与银河系之间785千秒差距的距离，这个晕的大小在天空中对应的视角直径为42°，但其表面亮度可能较低，这使得用伽马射线望远镜探测它具有挑战性。利用大面积望远镜（LAT）7.6年的观测数据，我们对朝向仙女座星系外晕、以仙女座星系为中心总视场半径为60°的1 - 100 GeV之间的伽马射线发射进行了详细研究，并对与观测相关的系统不确定性进行了深入分析。我们使用宇宙射线传播代码GALPROP构建专门的星际发射模型，以表征来自银河系的前景伽马射线发射，包括对各向同性成分的自洽确定。我们发现有证据表明存在一个扩展的过剩信号，它似乎与传统的银河系前景不同，从仙女座星系中心向上的总径向扩展约为120 - 200千秒差距。我们讨论了过剩发射的可能解释，但强调银河系前景的不确定性，特别是与中性氢相关成分的建模，尚未得到充分探索，可能会影响结果。
