Mapping Spiral Structure on the far side of the Milky Way

TL;DR

This paper measures the distance to a star-forming region on the far side of the Milky Way using VLBA parallax, revealing the spiral arm structure and validating a kinematic distance method.

Contribution

It provides the first direct parallax measurement of a far-side Galactic region, clarifying spiral structure and testing a new distance estimation technique.

Findings

Confirmed the location of the Scutum-Centaurus arm on the far side.

Validated the use of transverse motions for distance estimation.

Improved understanding of the Galaxy's spiral structure.

Abstract

Little is known about the portion of the Milky Way lying beyond the Galactic center at distances of more than 9 kilo-parsec from the Sun. These regions are opaque at optical wavelengths due to absorption by interstellar dust, and distances are very large and hard to measure. We report a direct trigonometric parallax distance of 20.4_{-2.2}^{+2.8} kilo-parsec obtained with the Very Long Baseline Array to a water maser source in a region of active star formation. These measurements allow us to shed light on Galactic spiral structure by locating the Scutum-Centaurus spiral arm as it passes through the far side of the Milky Way, and to validate a kinematic method for determining distances in this region based on transverse motions.