East Asian VLBI Network astrometry toward the star-forming region G040.96+02.48 in the Extreme Outer Galaxy

TL;DR

This study uses the East Asian VLBI Network to measure the astrometry of a star-forming region in the far outer Galaxy, revealing its distance, warp of the Galactic disk, and complex kinematics.

Contribution

First precise astrometric measurement of a far-side outer Galaxy star-forming region using East Asian VLBI, expanding the sample in the Extreme Outer Galaxy.

Findings

Distance to the region is 20.2±3.2 kpc.

The Galactic disk shows a significant warp at this location.

The region exhibits a large outward radial velocity of -32±18 km/s.

Abstract

Accurate astrometric measurements for star-forming regions located on the far side of the Milky Way remain scarce. In this work, we present the astrometric results for a 22\,GHz water maser associated with star-forming region G040.96+02.48 located on the far side of the Milky Way, using the East Asian VLBI Network. The target water maser's proper motion was determined to be ($\mu_{\alpha}\cos\delta, \mu_{\delta}$) = ($-2.06_{-0.51}^{+0.53}$, $-2.95_{-0.44}^{+0.45}$)~mas~yr$^{-1}$. The derived three-dimensional kinematic distance to the star-forming region is 20.2$\pm$3.2\,kpc, placing it slightly outside the Outer Scutum$-$Centaurus Arm. The corresponding vertical height of 872$\pm$139\,pc indicates a significant warp of the outer Galactic disk, which is in good agreement with the latest precessing warp model. Moreover, the resulting peculiar motions reveal a complex kinematic pattern, characterized by a large outward radial velocity of $-32\pm$18\,km~s$^{-1}$. Our observations substantially expand the valuable sample of star-forming regions with accurate astrometric measurements in the Extreme Outer Galaxy.