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We investigated the detailed radio structure of the jet of 1H 0323+342 usinghigh-resolution multi-frequency Very Long Baseline Array observations. Thissource is known as the nearest $\gamma$-ray emitting radio-loud narrow-lineSeyfert 1 (NLS1) galaxy. We discovered that the morphology of the inner jet iswell characterized by a parabolic shape, indicating the jet being continuouslycollimated near the jet base. On the other hand, we found that the jet expandsmore rapidly at larger scales, resulting in a conical-like shape. The locationof the "collimation break" is coincident with a bright quasi-stationary featureat 7 mas from core (corresponding to a deprojected distance of the order of$\sim$100pc), where the jet width locally contracts together with highlypolarized signals, suggesting a recollimation shock. We found that thecollimation region is coincident with the region where the jet speed graduallyaccelerates, suggesting the coexistence of the jet acceleration and collimationzone, ending up with the recollimation shock, which could be a potential siteof high-energy $\gamma$-ray flares detected by the Fermi-LAT. Remarkably, theseobservational features of the 1H 0323+342 jet are overall very similar to thoseof the nearby radio galaxy M87 and HST-1 as well as some blazars, suggestingthat a common jet formation mechanism might be at work. Based on the similarityof the jet profile between the two sources, we also briefly discuss the mass ofthe central black hole of 1H 0323+342, which is also still highly controversialon this source and NLS1s in general.

Collimation, Acceleration and Recollimation Shock in the Jet of Gamma-Ray-emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy 1H 0323+342