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arXiv 1206.0772astro-ph.HEastro-ph.CO
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恒星形成星系对宇宙MeV和GeV Gamma-Ray背景的贡献

The Star-Forming Galaxy Contribution to the Cosmic MeV and GeV Gamma-Ray Background

Brian C. Lacki, Shunsaku Horiuchi, John F. Beacom

摘要:

虽然恒星形成星系可能是宇宙GeV $γ$射线背景的主要贡献者,但由于“π介子凸点”在~100 MeV以下下降,它们预计是MeV昏暗的。然而,在MeV范围内的星系观测很少,可能存在其他发射过程。我们调查的MeV背景恒星形成星系运行的宇宙射线种群的单区模型,包括逆康普顿和韧致辐射,以及核线(包括$^{26}$铝),从核心坍缩超新星的发射,和正电子湮没发射,除了介子发射。我们使用银河系和M82作为正常和星暴星系的模板,并比较我们的模型,无线电和GeV-TeV $γ$射线数据。我们发现:(1)在高z正常星系中,较高的气体密度导致强的π介子隆起;(2)如果星暴的磁场强度较低,则星暴可能具有显著的MeV辐射;(3)如果级联主要发射强子$γ$ -射线,则它们可以对星暴的MeV辐射做出贡献。我们的基准模型预测,大多数未解决的GeV背景是来自恒星形成星系,但这一预测是不确定的数量级。在声称的1 MeV背景中,约有2%是来自恒星形成星系的漫射辐射;我们根据背景的光谱形状确定了<10%的上限。恒星形成的贡献被限制在很小的范围内,因为它的光谱是峰值的,而观测到的背景能量在MeV-GeV范围内急剧下降。

英文摘要:

While star-forming galaxies could be major contributors to the cosmic GeV $γ$-ray background, they are expected to be MeV-dim because of the "pion bump" falling off below ~100 MeV. However, there are very few observations of galaxies in the MeV range, and other emission processes could be present. We investigate the MeV background from star-forming galaxies by running one-zone models of cosmic ray populations, including Inverse Compton and bremsstrahlung, as well as nuclear lines (including $^{26}$Al), emission from core-collapse supernovae, and positron annihilation emission, in addition to the pionic emission. We use the Milky Way and M82 as templates of normal and starburst galaxies, and compare our models to radio and GeV--TeV $γ$-ray data. We find that (1) higher gas densities in high-z normal galaxies lead to a strong pion bump, (2) starbursts may have significant MeV emission if their magnetic field strengths are low, and (3) cascades can contribute to the MeV emission of starbursts if they emit mainly hadronic $γ$-rays. Our fiducial model predicts that most of the unresolved GeV background is from star-forming galaxies, but this prediction is uncertain by an order of magnitude. About ~2% of the claimed 1 MeV background is diffuse emission from star-forming galaxies; we place a firm upper limit of <~10% based on the spectral shape of the background. The star-formation contribution is constrained to be small, because its spectrum is peaked, while the observed background is steeply falling with energy through the MeV-GeV range.

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