Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Cambridge, Massachusetts 02138, USA.
Nature. 2011 Aug 31;477(7365):431-4. doi: 10.1038/nature10364.
The current picture of galaxy evolution advocates co-evolution of galaxies and their nuclear massive black holes, through accretion and galactic merging. Pairs of quasars, each with a massive black hole at the centre of its galaxy, have separations of 6,000 to 300,000 light years (refs 2 and 3; 1 parsec = 3.26 light years) and exemplify the first stages of this gravitational interaction. The final stages of the black-hole merging process, through binary black holes and final collapse into a single black hole with gravitational wave emission, are consistent with the sub-light-year separation inferred from the optical spectra and light-variability of two such quasars. The double active nuclei of a few nearby galaxies with disrupted morphology and intense star formation (such as NGC 6240 with a separation of about 2,600 light years and Mrk 463 with a separation of about 13,000 light years between the nuclei) demonstrate the importance of major mergers of equal-mass spiral galaxies in this evolution; such mergers lead to an elliptical galaxy, as in the case of the double-radio-nucleus elliptical galaxy 0402+379 (with a separation of about 24 light years between the nuclei). Minor mergers of a spiral galaxy with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active massive black-hole pairs, but have hitherto not been seen. Here we report the presence of two active massive black holes, separated by about 490 light years, in the Seyfert galaxy NGC 3393 (50 Mpc, about 160 million light years). The regular spiral morphology and predominantly old circum-nuclear stellar population of this galaxy, and the closeness of the black holes embedded in the bulge, provide a hitherto missing observational point to the study of galaxy/black hole evolution. Comparison of our observations with current theoretical models of mergers suggests that they are the result of minor merger evolution.
当前的星系演化图景提倡星系与其核心大质量黑洞通过吸积和星系合并协同演化。类星体对，每一对都有一个位于星系中心的大质量黑洞，它们的分离距离在 6000 到 300000 光年之间（参考文献 2 和 3；1 秒差距=3.26 光年），是这种引力相互作用的最初阶段的范例。通过双黑洞和最终引力波发射导致的单个黑洞坍缩，这一黑洞合并过程的最终阶段与从两个类星体的光学光谱和光变推断出的亚光年级分离相一致。少数几个具有破坏形态和强烈恒星形成的近邻星系的双活动核（例如 NGC 6240 核间距约为 2600 光年，Mrk 463 核间距约为 13000 光年），证明了等质量螺旋星系主要合并在这种演化中的重要性；这种合并会导致一个椭圆星系，就像双射电核椭圆星系 0402+379 （核间距约为 24 光年）那样。较小伴星系的螺旋星系的小合并应该更为常见，演变成具有活动大质量黑洞对的螺旋星系，但迄今为止尚未见到。在这里，我们报告在塞弗特星系 NGC 3393（50 Mpc，约 1.6 亿光年）中存在两个活动大质量黑洞，它们之间的距离约为 490 光年。这个星系具有规则的螺旋形态和主要是古老的核周恒星种群，以及嵌入在星系核中的黑洞的接近程度，为星系/黑洞演化的研究提供了一个迄今为止缺失的观测点。将我们的观测结果与当前的合并理论模型进行比较表明，它们是小合并演化的结果。
