RadioAstron发现3C 84中重启的秒差距尺度喷流周围存在微型茧状结构
RadioAstron discovery of a mini-cocoon around the restarted parsec-scale jet in 3C 84
AI总结：
本研究利用RadioAstron空间VLBI观测3C 84，首次发现重启喷流周围的微型茧状结构，估算其相关物理参数，揭示喷流能量向星际介质的转移机制并解释喷流轮廓成因。
摘要：
我们展示了RadioAstron天基甚长基线干涉测量（VLBI）对英仙座星系团中心的近邻射电星系3C 84（NGC 1275）的观测结果。观测于2013年9月21日至22日开展，由24台地面射电望远镜组成的全球阵列在5 GHz和22 GHz频段进行观测，同时联合空间射电望远镜（SRT）参与。此外，甚长基线阵列（VLBA）和相控甚大阵（VLA）准同步地在15 GHz和43 GHz频段对该源进行了观测。地面阵列与SRT之间的干涉条纹在最长达地球直径8.1倍的基线上被探测到，为3C 84在这些波长下的观测提供了前所未有的分辨率。我们注意到，对应的条纹间距在5 GHz时为125微角秒，在22 GHz时为27微角秒。我们的空间VLBI图像揭示了约10年前喷出的1秒差距长致密喷流内部此前未被发现的子结构。在5 GHz图像中，我们首次探测到重启喷流周围茧状结构的低强度辐射。我们的结果表明，年轻喷流功率的提升正在其穿过星系中心区域周围介质的过程中，吹胀一个热等离子体泡。在此，我们估算了微型茧中储存的最小能量，以及它的压强、体积、膨胀速度、重粒子与相对论电子的比值，还有周围介质的密度。喷流输送的能量约有一半注入微型茧中，而该泡的近球形形态表明，这些能量可能被转移到星际介质中比准直喷流自身所能作用的大得多的体积内。热微型茧的压强也为在22 GHz RadioAstron图像中观测到的近圆柱形喷流轮廓提供了自然解释。
英文摘要：
We present RadioAstron space-based very long baseline interferometry (VLBI) observations of the nearby radio galaxy 3C84 (NGC1275) at the centre of the Perseus cluster. The observations were carried out on September 21-22, 2013 and involved a global array of 24 ground radio telescopes observing at 5 GHz and 22 GHz, together with the Space Radio Telescope (SRT). Furthermore, the Very Long Baseline Array (VLBA) and the phased Very Large Array (VLA) observed the source quasi-simultaneously at 15 GHz and 43 GHz. Fringes between the ground array and the SRT were detected on baseline lengths up to 8.1 times the Earth's diameter, providing unprecedented resolution for 3C 84 at these wavelengths. We note that the corresponding fringe spacing is 125 microarcsec at 5 GHz and 27 microarcsec at 22 GHz. Our space-VLBI images reveal a previously unseen sub-structure inside the compact 1 pc long jet that was ejected about ten years earlier. In the 5 GHz image, we detected, for the first time, low-intensity emission from a cocoon-like structure around the restarted jet. Our results suggest that the increased power of the young jet is inflating a bubble of hot plasma as it carves its way through the ambient medium of the central region of the galaxy. Here, we estimate the minimum energy stored in the mini-cocoon, along with its pressure, volume, expansion speed, and the ratio of heavy particles to relativistic electrons, as well as the density of the ambient medium. About half of the energy delivered by the jet is dumped into the mini-cocoon and the quasi-spherical shape of the bubble suggests that this energy may be transferred to a significantly larger volume of the interstellar medium than what would be accomplished by the well-collimated jet on its own. The pressure of the hot mini-cocoon also provides a natural explanation for the almost cylindrical jet profile seen in the 22 GHz RadioAstron image.
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