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 UV to near-infrared, yet has an ordered rotation and lies on the star-formation main sequence. We combine new JWST/NIRCam and ALMA band 4 observations, together with previous VLT/SINFONI integral field spectroscopy and HST imaging to re-examine its nature. The high-resolution Hα kinematics are best fitted if the mass is distributed within a ring with total mass Mring = 2 × 10¹⁰ M_ and radius Rring = 4.6 kpc, together with a central undetected mass component (e.g., a "bulge") with a dynamical mass of Mbulge = 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 142GHz continuum emission imply a total molecular and dust gas masses of Mmol,gas = 7.1 × 10¹⁰M_ and Mdust = 3 × 10⁸ M_ over the entire galaxy, giving a dust-to-mass 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-UV to near-infrared. Sensitive high-resolution ALMA observations will be essential to confirm this scenario and study the gas and dust distribution.