一对相互作用的星系在光回波的起源
An interacting galaxy pair at the origin of a light echo
摘要：
在沙普利超星系团的一个低密度区域，我们发现了一个红移z=0.04865的相互作用星系对，其中主星系（ShaSS 073）的塞弗特2核激发了质量较小的伴星（ShaSS 622）盘中的一个扩展发射线区域（EELR，~170 kpc ^2）。新的积分场光谱学和多波段数据集，从远紫外到远红外和无线电波长，使我们能够获得ShaSS 622-073系统的详细描述。气体运动学显示了相互作用的迹象，尽管整体速度场显示两个星系都有相当规则的旋转，这表明我们正在观察它们的第一次相遇，这一点得到了两个星系中心之间21 kpc的估计距离的证实。在ShaSS 622中探测到的~2-3 kpc活动星系核外流和EELR的几何形状支持中空双锥结构的存在。通过光电离模型和贝叶斯方法分析了整个系统的电离状态和电离源，证明活动星系核和EELR之间存在明确的联系。活动星系核的光度（2.4 × 10^44 erg/s）比激发EELR中气体所需的能量（4.6 × 10^45 erg/s）低20倍，这表明活动星系核在过去的3 × 10^4年中发生了剧烈的衰落。ShaSS 073-622提供了光回波配方中列出的所有成分，其中高度电离的区域保持着对现在褪色的活动星系核先前更活跃阶段的记忆。这是第一次在两个星系之间观察到光回波。
英文摘要：
In a low-density region of the Shapley supercluster we identified an interacting galaxy pair at redshift z=0.04865 in which the Seyfert-2 nucleus of the main galaxy (ShaSS 073) is exciting an extended emission line region (EELR, ~170 kpc^2) in the disk of the less massive companion (ShaSS 622). New integral-field spectroscopy and the multi-band data-set, spanning from far-ultraviolet to far-infrared and radio wavelengths, allowed us to obtain a detailed description of the ShaSS 622-073 system. The gas kinematics shows hints of interaction, although the overall velocity field shows a quite regular rotation in both galaxies, thus suggesting that we are observing their first encounter as confirmed by the estimated distance of 21 kpc between the two galaxy centers. The detected ~2-3 kpc AGN outflow and the geometry of the EELR in ShaSS 622 support the presence of a hollow bicone structure. The status and sources of the ionization across the whole system have been analysed through photoionization models and a Bayesian approach which prove a clear connection between the AGN and the EELR. The luminosity of the AGN (2.4x10^44 erg/s) is a factor 20 lower than the power needed to excite the gas in the EELR (4.6x10^45 erg/s) indicating a dramatic fading of the AGN in the last 3x10^4 yr. ShaSS 073-622 provides all the ingredients listed in the recipe of a light echo where a highly ionised region maintains memory of a preceding more energetic phase of a now faded AGN. This is the first case of a light echo observed between two galaxies.
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