The Expanding 3 kpc Arms Are Neither Expanding nor Spiral Arms but X1 Orbits Driven by the Galactic Bar

TL;DR

This study uses precise VLBI measurements of water masers to reveal that the so-called Expanding 3-kpc arms are actually X1 orbits driven by the Galactic Bar, challenging previous interpretations of their nature.

Contribution

It provides the first direct measurements of X1 orbits in the Milky Way, linking young stars to the Galactic Bar and clarifying the nature of the 3-kpc arms.

Findings

Expanding 3-kpc arms are likely X1 orbits around the Galactic Bar.

Young stars in these regions are on quasi-elliptical orbits.

Some stars previously thought to be in the Norma spiral arm are in these orbits.

Abstract

Near the center of our Milky Way is a bar-like structure and the so-called Expanding 3-kpc arms. We currently have limited knowledge of this important region, since we are about 8.2 kpc from the center and cannot directly observe it at optical wavelengths, owing to strong extinction from interstellar dust. Here we present extremely precise VLBI measurements of water maser sources from the BeSSeL Survey, where extinction is not a problem, which accurately determine the 3-dimensional locations and motions of three massive young stars. Combined with previous measurements, these stars delineate a trail of orbits outlining the Milky Way's Galactic Bar. We present the first measurements capturing the dynamics of quasi-elliptical (X1) orbits around the Galactic Bar. Our findings provide evidence substantiating the existence of such orbits populated by massive young stars. Our measurements of the position and velocity of a number of massive young stars, previously identified with the Expanding 3-kpc arms, show that they are more likely located in the X1 orbits about the Galactic Bar. Also, some stars previously assigned to the Norma spiral arm appear to be in these orbits, which suggests that this spiral arm does not extend past the end of the bar.