JWST + ALMA ubiquitously discover companion systems within ≲18 kpc around four

Mergers play important roles in galaxy evolution at and beyond cosmic noon (z ∼ 3). They have been found to be a trigger of active galactic nuclei (AGN) activity and a process for growing stellar mass and black hole mass. High-z radio galaxies (HzRGs = type-2 radio-loud AGN) are among the most massive galaxies known, and they reside in dense environments on scales of tens of kiloparsecs to Megaparsecs. We present the first search for kiloparsec-scale companions using matched 0.2″ resolution ALMA and JWST/NIRSpec integral field unit data in a sample of four z ∼ 3.5 HzRGs with many supporting datasets. We discovered a total of ∼12 companion systems within ≲18 kpc across all four HzRG fields using two independent detection methods: peculiar [O <ns0:sc>III</ns0:sc>]4959, 5007 kinematics offset from the main (systemic) ionized gas component and [C <ns0:sc>II</ns0:sc>]158 μm emitters. We examined the velocity fields of these companions and find evidence of disk rotation along with more complex motions. We estimate the dynamical masses of these nearby systems to be Mdyn ∼ 109 − 11 M⊙, which may indicate a minor merger scenario. Our results indicate that these companions may be the trigger of the powerful radio-loud AGN. We discuss the roles of the discovered companion systems in galaxy evolution for these powerful jetted AGN and indicate that they may impede jet launch and deflect the jet.