High velocity dispersion in a rare grand-design spiral galaxy at redshift z=2.18

Although grand-design spiral galaxies are relatively common in the local Universe, only one has been spectroscopically confirmed(1) 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(6-10). Alternatively, current instrumentation may simply not be sensitive enough to detect spiral structures comparable to those in the modern Universe(11). At z < 2, the velocity dispersion of disks decreases(12), and spiral galaxies are more numerous by z approximate to 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(16-18). A duty cycle of <100 Myr for such tidally induced spiral structure in a hot massive disk is consistent with its rarity.