Starbursting disk with a spiral morphology in a Hyper Luminous Infrared Galaxy at redshift of 4.4

We have not yet understood the physical origin of a “starburst” with a star formation rate exceeding 1000M_sun/yr, seen in submillimeter galaxies. ALMA’s high sensitivity allows us to obtain the most detailed [CII] gas kinematics for an unlensed, hyper luminous infrared galaxy (HyLIRG) at redshift 4.4, BRI 1335-0417. Analyzing the [CII] kinematics (~1.1kpc), we discovered a rotating disk, a central compact structure like a bulge, and spiral structure on the disk in a galaxy at the redshift of 4.4 (over 12 billion years ago), long before the peak of cosmic star formation. HyLIRGs are thought to be formed mainly through major mergers, but surprisingly the internal structures revealed in this study are similar to those of a spiral disk galaxy, with star-forming bulge and disk. These results may give us a clue to the detailed formation scenario of HyLIRGs and massive galaxies in cosmic history.