Revisiting the Milky Way stellar long bar and the 3 kpc arm

TL;DR

This study refines the understanding of the Milky Way's central long bar and 3 kpc arm by analyzing star counts and distances, confirming their morphology, orientation, and connection within the inner Galaxy.

Contribution

It provides new measurements of the long bar's angle, size, and its contact with the 3 kpc arm, clarifying their structure and relationship in the Milky Way.

Findings

The long bar has an average angle of 27.4° with the Sun-Galactic center line.

The long bar's semi-major axis is approximately 4 kpc.

The 3 kpc arm is detected at heliocentric distances around 5 kpc and 9-12 kpc.

Abstract

CONTEXT. One of the most difficult and unexplored regions of the Milky Way is the highly extincted in-plane central region within the Galactic coordinates $10^\circ \lesssim |\ell |\lesssim 30^\circ $, $|b|\lesssim 3^\circ $, where we have the long-bar and 3 kpc arm with intermediate-age stellar population, whose morphological properties are still unclear. AIMS. We aim to advance our knowledge of the morphology of these two components. METHODS. We examined star counts of bright M giants in WISE-4.6$\mu $m and its distribution of distances derived from spectroscopic parallaxes with APOGEE-DR17. We also examined the distribution of distances of young OGLE-O-rich Mira variable stars, and reviewed the literature on red clump distance determination within that area. RESULTS. We corroborate the asymmetry between positive and negative longitudes in in-plane regions, thus confirming the necessity to include a long bar. We obtain an average angle between the major axis of the long bar and the line Sun-Galactic centre of $\alpha =27.4^\circ \pm 1.5^\circ $, aligned with the triaxial bulge and a semi-major-axis length $\approx 4$ kpc. The tips of the long bar are in contact with the elliptical 3 kpc arm, with the major axis again aligned with the bulge and the long bar's major axes, whose tangential lines of sight correspond to $\ell =-22^\circ $ and $\ell=+27^\circ $. In the range of 50 degrees in the sky between these two longitudes, the stellar near 3 kpc arm is clearly detected at heliocentric distances around 5 kpc, and the stellar far 3 kpc arm is tentatively detected at heliocentric distances of 9-12 kpc.