Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 50 St George Street, Toronto M5S 3H4, Ontario, Canada.
Nature. 2012 Jul 18;487(7407):338-40. doi: 10.1038/nature11256.
Although grand-design spiral galaxies are relatively common in the local Universe, only one has been spectroscopically confirmed to lie at redshift z > 2 (HDFX 28; z = 2.011); and it may prove to be a major merger that simply resembles a spiral in projection. The rarity of spirals has been explained as a result of disks being dynamically 'hot' at z > 2 (refs 2-5), which may instead favour the formation of commonly observed clumpy structures. Alternatively, current instrumentation may simply not be sensitive enough to detect spiral structures comparable to those in the modern Universe. At z < 2, the velocity dispersion of disks decreases, and spiral galaxies are more numerous by z ≈ 1 (refs 7, 13-15). Here we report observations of the grand-design spiral galaxy Q2343-BX442 at z = 2.18. Spectroscopy of ionized gas shows that the disk is dynamically hot, implying an uncertain origin for the spiral structure. The kinematics of the galaxy are consistent with a thick disk undergoing a minor merger, which can drive the formation of short-lived spiral structure. A duty cycle of <100 Myr for such tidally induced spiral structure in a hot massive disk is consistent with its rarity.
尽管大设计螺旋星系在本地宇宙中相对常见，但仅有一个被光谱证实位于红移 z > 2（HDFX 28；z = 2.011）；而且它可能被证明是一个简单地在投影中类似于螺旋的主要合并。螺旋星系的稀有性可以解释为在 z > 2 时磁盘动态地“热”（参考文献 2-5），这可能反而有利于形成通常观察到的块状结构。或者，当前的仪器可能根本不够敏感，无法检测到与现代宇宙中相当的螺旋结构。在 z < 2 处，磁盘的速度分散度减小，并且在 z ≈ 1 时螺旋星系更加丰富（参考文献 7、13-15）。在这里，我们报告了对 z = 2.18 的大设计螺旋星系 Q2343-BX442 的观测结果。对离子化气体的光谱分析表明，磁盘是动态热的，这意味着螺旋结构的起源不确定。星系的运动学与经历小合并的厚磁盘一致，这可以驱动短暂的螺旋结构的形成。在热大磁盘中，这种潮汐诱导的螺旋结构的工作周期 <100 Myr 与其稀有性一致。
