2025ApJ...979L..44K

Recent JWST observations have revealed the prevalence of spiral structures at z > 1. Unlike in the local Universe, the origin and the consequence of spirals at this epoch remain unexplored. We use public JWST/NIRCam data from the COSMOS-Web survey to map spiral structures in eight massive (>10^10.5M_⊙) star-forming galaxies at z_spec ∼ 1.5. We present a method for systematically quantifying spiral arms at z > 1, enabling direct measurements of flux distributions. Using rest-frame near-IR images, we construct morphological models accurately tracing spiral arms. We detect offsets (∼0.2-0.8 kpc) between the rest-frame optical and near-IR flux distributions across most arms. Drawing parallels to the local Universe, we conclude that these offsets reflect the presence of density waves. For 9 out of 18 arms, the offsets indicate spiral shocks triggered by density waves. In all, 5 arms have offsets in the opposite direction and are likely associated with tidal interactions. For the remaining cases with no detected offsets, we suggest that stochastic “clumpy” star formation is the primary driver of their formation. In conclusion, we find a multifaceted nature of spiral arms at z > 1, similar to that in the local Universe.