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

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ Institute of Astronomy and Physics, Inner Mongolia University, Hohhot 010021, China
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
⁵ Local Universe and Time-Domain Astronomy Laboratory, China West Normal University, Nanchong 637002, China
⁶ Luoxiahong Institute of Astronomy, China West Normal University, Nanchong 637002, China
⁷ Department of Astronomy, China West Normal University, Nanchong 637002, China

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 December 2025
Accepted: 10 March 2026

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^◦ ≲ |ℓ| ≲ 30^◦, |b| ≲ 3^◦, 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 µm 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 α = 27.4^◦ ± 1.5^◦, aligned with the triaxial bulge and a semi-major-axis length ≈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 ℓ = −22^◦ and ℓ = +27^◦. 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.