射电星系NGC 1265揭开了英仙座星系团的吸积激波
Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster
摘要：
我们提出了一个一致的头-尾射电星系NGC 1265的三维模型，解释了复杂的无线电形态和频谱的星系和射电泡通过冲击波过去的通道。使用解析解的完整的黎曼问题和流体动力学模拟，我们研究了如何通过转换成一个环形涡环的等离子体气泡。老化的电子群体的绝热压缩使其被激励并发出低表面亮度和陡谱无线电发射。NGC 1265的大的下落速度和低的法拉第旋转测量值和喷流的方差强烈地表明，这种转变是由于英仙座上的吸积激波大约位于R_200。通过估算射电泡的体积变化，可以推断出激波的马赫数为M = 4.2 - 1.2 ^{+0.8}，密度跃变为3.4 - 0.4 ^{+0.2}，温度跃变为6.3 - 2.7 ^{+2.5}，以及21.5 +/-10.5的压力跃变，同时考虑到无线电等离子体的状态方程和环面的体积的不确定性。通过外推X射线剖面，我们得到了在kT < 0.4 keV和n < 5e-5 / cm^3的下落的温热星系际介质中气体温度和密度的上限。椭圆形的射电环面的方向与星系头部和尾部在天空平面上的方向相结合，不可能与投影效应相一致。相反，这表明激波后剪切流是由激波表面曲率引起的，特征半径为850 kpc。剪切流的能量密度对应于14%的热能密度。激波注入的涡旋可能在产生和放大星系团中的磁场方面起重要作用。未来对头尾星系的LOFAR观测可以作为对星系团吸积冲击的补充探测。
英文摘要：
We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that explains the complex radio morphology and spectrum by a past passage of the galaxy and radio bubble through a shock wave. Using analytical solutions to the full Riemann problem and hydrodynamical simulations, we study how this passage transformed the plasma bubble into a toroidal vortex ring. Adiabatic compression of the aged electron population causes it to be energized and to emit low-surface brightness and steep-spectrum radio emission. The large infall velocity of NGC 1265 and the low Faraday rotation measure values and variance of the jet strongly argue that this transformation was due to the accretion shock onto Perseus situated roughly at R_200. Estimating the volume change of the radio bubble enables inferring a shock Mach number of M = 4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of 6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for uncertainties in the equation of state of the radio plasma and volume of the torus. Extrapolating X-ray profiles, we obtain upper limits on the gas temperature and density in the infalling warm-hot intergalactic medium of kT < 0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio torus in combination with the direction of the galaxy's head and tail in the plane of the sky is impossible to reconcile with projection effects. Instead, this argues for post-shock shear flows that have been caused by curvature in the shock surface with a characteristic radius of 850 kpc. The energy density of the shear flow corresponds to a turbulent-to-thermal energy density of 14%. The shock-injected vorticity might be important in generating and amplifying magnetic fields in galaxy clusters. Future LOFAR observations of head-tail galaxies can be complementary probes of accretion shocks onto galaxy clusters.
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