用Fermi-LAT抑制发光红外线镓的Gamma-Ray发射Arp 220的初步探测
Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220
摘要：
形成恒星的星系主要通过π介子产生伽马射线，这是由宇宙射线质子与星际介质（ISM）之间的非弹性碰撞产生的。明亮和超亮红外星系（LIRGs和ULIRGs）的密集ISM和高恒星形成率意味着它们应该是强伽马射线发射体，但到目前为止只检测到两个LIRGs。它们发射的理论模型依赖于在相互作用之前逃离这些星系的宇宙射线质子的未知部分。我们分析了82个最亮的IRAS LIRGs和ULIRGs的费米LAT数据。我们分别检查每个系统，并进行叠加分析，以限制它们的伽马射线通量。我们报告了最近的ULIRG Arp 220（~ 4.6 σ）的检测。我们观测到了2.4e-10 phot cm^-2 s^-1的伽马射线通量（0.8- 100 GeV），光子指数为2.23（在77 Mpc时为8.2e10^41 ergs ^-1）。我们还推导出了堆叠LIRG和ULIRG的上限。Arp~220的伽马射线光度和叠加上限与致密恒星形成星系的量热预测一致。随着Arp 220的探测，我们将γ射线-红外光度关联推广到高光度区，其log（L_[0.1-100 GeV]）= 1.25log（L_[8-1000]）+ 26.7，γ射线-射电连续光谱光度关联推广到log（L_[0.1-100 GeV]）= 1.22log（L_[1.4 GHz]）+ 13.3。目前对费米-LAT的调查即将在局部宇宙中探测到更多的LIRGs/ULIRGs，我们预计通过更深的费米-LAT观测或下一代伽马射线探测器会有更多的探测。
英文摘要：
Star-forming galaxies produce gamma-rays primarily via pion production, resulting from inelastic collisions between cosmic ray protons and the interstellar medium (ISM). The dense ISM and high star formation rates of luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that they should be strong gamma-ray emitters, but so far only two LIRGs have been detected. Theoretical models for their emission depend on the unknown fraction of cosmic ray protons that escape these galaxies before interacting. We analyze Fermi-LAT data for 82 of the brightest IRAS LIRGs and ULIRGs. We examine each system individually and carry out a stacking analysis to constrain their gamma-ray fluxes. We report the detection of the nearest ULIRG Arp 220 (~4.6sigma). We observe a gamma-ray flux (0.8--100 GeV) of 2.4e-10 phot cm^-2 s^-1 with photon index of 2.23 (8.2e10^41 ergs s^-1 at 77 Mpc) We also derive upper limits for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of Arp~220 and the stacked upper limits agree with calorimetric predictions for dense star-forming galaxies. With the detection of Arp 220, we extend the gamma-ray--IR luminosity correlation to the high luminosity regime with log(L_[0.1-100 GeV]) = 1.25log(L_[8-1000]) + 26.7 as well as the gamma-ray--radio continuum luminosity correlation with log(L_[0.1-100 GeV]) = 1.22log(L_[1.4 GHz]) + 13.3. The current survey of Fermi-LAT is on the verge of detecting more LIRGs/ULIRGs in the local universe, and we expect even more detections with deeper Fermi-LAT observations or the next generation of gamma-ray detectors.
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