A galaxy merger is expected to cause the formation of a supermassive black hole (SMBH) binary, which itself eventually coalesces through the anisotropic emission of gravitational waves. This may result in the merged SMBH receiving a recoil kick velocity ∼100–1000 kms−1, causing it to oscillate in the gravitational potential of the host galaxy. The luminous quasar E1821+643, identified as an SMBH recoil candidate via spectropolarimetry observations, shows Doppler shifting of the broad emission lines in direct and scattered light, consistent with a relative velocity of 2100 km s−1 between the quasar nucleus and host galaxy. In this paper, we attempt to detect the expected spatial displacement using a combination of optical spectroastrometry and Hubble Space Telescope (HST) narrow-band images. The spectroastrometry reveals a relative spatial displacement between the quasar nucleus and the gas emitting the [O iii] λλ4959, 5007 lines of ∼130 mas (∼580 pc) to the north-west. Our HST images resolve the [O iii] emission on sub-arcsecond scales, showing that it is asymmetrically distributed, extending to radial distances ∼0.5−0.6 arcsec from the nucleus in a wide arc running from the north-east around to the west. A simulated spectroastrometry observation based on the HST [O iii] image indicates that only a small fraction of the measured displacement can be attributed to the asymmetric [O iii] emission. This displacement therefore appears to be a real spatial offset of the quasar nucleus with respect to the narrow-line region, presumed to be located at the host galaxy centre, further supporting the interpretation that a post-merger gravitational recoil of the SMBH has occurred in E1821+643.

中文翻译：

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空间偏移类星体 E1821+643：引力反冲的新证据

预计星系合并将导致超大质量黑洞 (SMBH) 双星的形成，该双星本身最终会通过引力波的各向异性发射而合并。这可能导致合并的 SMBH 接收到 100-1000 kms-1 的反冲速度，使其在宿主星系的引力势中振荡。发光的类星体 E1821+643，通过分光偏振观测确定为 SMBH 反冲候选者，显示了直接和散射光中宽发射线的多普勒频移，与类星体核和宿主星系之间 2100 km s-1 的相对速度一致。在本文中，我们尝试结合使用光谱天文学和哈勃太空望远镜 (HST) 窄带图像来检测预期的空间位移。光谱分析揭示了类星体核与向西北发射约 130 mas (约 580 pc) 的 [O iii] λλ4959, 5007 线的气体之间的相对空间位移。我们的 HST 图像解析了亚弧秒尺度上的 [O iii] 发射，表明它是不对称分布的，在从东北向西延伸的宽弧中延伸到距原子核约 0.5-0.6 弧秒的径向距离。基于 HST [O iii] 图像的模拟光谱观测表明，只有一小部分测量位移可归因于不对称 [O iii] 发射。因此，这种位移似乎是类星体核相对于假定位于宿主星系中心的窄线区域的真实空间偏移，