On the Structure of the Sagittarius Spiral Arm in the Inner Milky Way

TL;DR

This study uses precise maser measurements to refine the structure of the Sagittarius spiral arm in the Milky Way, indicating it extends farther than previously modeled and may merge with the Perseus arm in the first Galactic quadrant.

Contribution

First direct parallax and proper motion measurements of masers in the Sagittarius arm's far segment, providing new insights into its structure and relation to other spiral arms.

Findings

Sagittarius arm extends beyond 8 kpc from the Sun in the first Galactic quadrant.

The arm's segment should be modeled as extending radially outward.

Evidence suggests the Sagittarius and Perseus arms may merge in the first quadrant.

Abstract

We report measurements of trigonometric parallax and proper motion for two 6.7 GHz methanol and two 22 GHz water masers located in the far portion of the Sagittarius spiral arm as part of the BeSSeL Survey. Distances for these sources are estimated from parallax measurements combined with 3-dimensional kinematic distances. The distances of G033.64$-$00.22, G035.57$-$00.03, G041.15$-$00.20, and G043.89$-$00.78 are $9.9\pm0.5$, $10.2\pm0.6$, $7.6\pm0.5$, and $7.5\pm0.3$ kpc, respectively. Based on these measurements, we suggest that the Sagittarius arm segment beyond about 8 kpc from the Sun in the first Galactic quadrant should be adjusted radially outward relative to previous models. This supports the suggestion of Xu et al. (2023) that the Sagittarius and Perseus spiral arms might merge in the first quadrant before spiraling inward to the far end of the Galactic bar.