The Most Ancient Spiral Galaxy: A 2.6-Gyr-old Disk with a Tranquil Velocity Field

We report an integral-field spectroscopic (IFS) observation of a gravitationally lensed spiral galaxy A1689B11 at redshift z = 2.54. It is the most ancient spiral galaxy discovered to date and the second kinematically confirmed spiral at z greater than or similar to 2. Thanks to gravitational lensing, this is also by far the deepest IFS observation with the highest spatial resolution (similar to 400 pc) on a spiral galaxy at a cosmic time when the Hubble sequence is about to emerge. After correcting for a lensing magnification of 7.2 +/- 0.8, this primitive spiral disk has an intrinsic star formation rate of 22 +/- 2 M-circle dot yr(-1), a stellar mass of 10(9.8 +/- 0.3) M-circle dot, and a half-light radius of r(1/2) = 2.6 +/- 0.7 kpc, typical of a main-sequence star-forming galaxy at z similar to 2. However, the Ha kinematics show a surprisingly tranquil velocity field with an ordered rotation (V-c = 200 +/- 12 km s(-1)) and uniformly small velocity dispersions (V-sigma,V-mean = 23 +/- 4 km s(-1) and V-sigma,V-outer-disk = 15 +/- 2 km s(-1)). The low gas velocity dispersion is similar to local spiral galaxies and is consistent with the classic density wave theory where spiral arms form in dynamically cold and thin disks. We speculate that A1689B11 belongs to a population of rare spiral galaxies at z greater than or similar to 2 that mark the formation epoch of thin disks. Future observations with the James Webb Space Telescope will greatly increase the sample of these rare galaxies and unveil the earliest onset of spiral arms.