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 \gtrsim 2 .
Thanks to gravitational lensing , this is also by far the deepest IFS observation with the highest spatial resolution ( \sim 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 \pm 0.8 , this primitive spiral disk has an intrinsic star formation rate of 22 \pm 2 M _ { \odot } yr ^ { -1 } , a stellar mass of 10 ^ { 9.8 \pm 0.3 } M _ { \odot } and a half-light radius of r _ { 1 / 2 } = 2.6 \pm 0.7 kpc , typical of a main-sequence star-forming ( SF ) galaxy at z \sim 2 .
However , the H \alpha kinematics show a surprisingly tranquil velocity field with an ordered rotation ( V _ { c } = 200 \pm 12 km s ^ { -1 } ) and uniformly small velocity dispersions ( V _ { \sigma,mean } = 23 \pm 4 km s ^ { -1 } and V _ { \sigma,outer - disk } = 15 \pm 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 \gtrsim 2 that mark the formation epoch of thin disks .
Future observations with JWST will greatly increase the sample of these rare galaxies and unveil the earliest onset of spiral arms .