A Large-scale Ring Galaxy at z = 2.2 Revealed by JWST/NIRCam: Kinematic Observations and Analytical Modelling

A unique galaxy at z = 2.2, zC406690, has a striking clumpy large-scale ring structure that persists from rest-frame UV to near-infrared, yet has an ordered rotation and lies on the star formation main sequence. We combine new JWST/NIRCam and Atacama Large Millimeter/submillimeter Array (ALMA) band 4 observations, together with previous Very Large Telescope/SINFONI integral field spectroscopy and Hubble Space Telescope imaging to reexamine its nature. The high-resolution Hα kinematics is best fitted if the mass is distributed within a ring with total mass M_ring ≈ 2 × 10¹⁰ M_⊙ and radius R_ring = 4.6 kpc, together with a central undetected mass component (e.g., a “bulge”) with a dynamical mass of M_bulge = 8 × 10¹⁰ M_⊙. We also consider a purely flux-emitting ring superposed over a faint exponential disk, or a highly “cuspy” dark matter halo, both disfavored against a massive ring model. The low-resolution CO(4-3) line and 142 GHz continuum emission imply total molecular and dust gas masses of M_mol,gas = 7.1 × 10¹⁰ M_⊙ and M_dust = 3 × 10⁸ M_⊙, respectively, over the entire galaxy, giving a dust-to-gas ratio of 0.7%. We estimate that roughly half the gas and dust mass lie inside the ring, and that ∼10% of the total dust is in a foreground screen that attenuates the stellar light of the bulge in the rest-frame UV to near-infrared. Sensitive high-resolution ALMA observations will be essential to confirm this scenario and study the gas and dust distribution.