East Asian VLBI Network astrometry toward the extreme outer Galaxy: Kinematic distance with the proper motion of G034.84−00.95.

This article focuses on the astrometric study of the star-forming region G034.84−00.95 to investigate the structure and kinematics of the Outer Scutum–Centaurus (OSC) arm in the far outer Milky Way using very long baseline interferometry (VLBI) with the KaVA array, which combines the Korean VLBI Network (KVN) and VLBI Exploration of Radio Astrometry (VERA). The authors measured the proper motion of a 22 GHz H₂O maser associated with G034.84−00.95 as (−1.61 ± 0.18, −4.29 ± 0.16) mas yr⁻¹ and estimated its 2D kinematic distance to be 18.6 ± 1.0 kpc, placing it in the OSC arm with a significant vertical velocity of −38 ± 16 km s⁻¹ away from the Galactic plane. The study evaluates and rejects the possibility that G034.84−00.95 is a nearby object (~2.5 kpc) based on its kinematics and luminosity, and discusses potential origins of its vertical motion, including supernova-driven superbubble expansion and cloud-disk collisions, but finds no conclusive evidence for these scenarios. The findings highlight the need for further VLBI astrometry of OSC arm sources to better understand the outer Galactic disk and its interaction with the halo.