Letter to the Editor

A high pitch angle structure in the Sagittarius Arm

¹ California Institute of Technology, Pasadena, CA 91125, USA
e-mail: mkuhn@astro.caltech.edu
² Department of Physics, University of Wisconsin-Whitewater, 800 W Main St, Whitewater, WI 53190, USA
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁴ Donald Bren School of Information and Computer Sciences, University of California, Irvine, CA 92697, USA
⁵ CENTRA/SIM, Faculdade de Ciéncias, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
⁶ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Rd., Shanghai 200030, PR China
⁷ Institut de Ciéncies del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franqués 1, 08028 Barcelona, Spain
⁸ Université Clermont Auvergne, CNRS/IN2P3, LPC, 63000 Clermont-Ferrand, France
⁹ Department of Physics and Astronomy, California State Polytechnic University Pomona, 3801 West Temple Avenue, Pomona, CA 91768, USA

Received: 28 April 2021
Accepted: 28 June 2021

Abstract

Context. In spiral galaxies, star formation tends to trace features of the spiral pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging to connect individual star-forming regions to their larger Galactic environment owing to our perspective from within the disk. One feature in nearly all modern models of the Milky Way is the Sagittarius Arm, located inward of the Sun with a pitch angle of ∼12°.

Aims. We map the 3D locations and velocities of star-forming regions in a segment of the Sagittarius Arm using young stellar objects (YSOs) from the Spitzer/IRAC Candidate YSO (SPICY) catalog to compare their distribution to models of the arm.

Methods. Distances and velocities for these objects are derived from Gaia EDR3 astrometry and molecular line surveys. We infer parallaxes and proper motions for spatially clustered groups of YSOs and estimate their radial velocities from the velocities of spatially associated molecular clouds.

Results. We identify 25 star-forming regions in the Galactic longitude range ℓ ∼ 4.​ ° 0–18.​ ° 5 arranged in a narrow, ∼1 kpc long linear structure with a high pitch angle of ψ = 56° and a high aspect ratio of ∼7:1. This structure includes massive star-forming regions such as M8, M16, M17, and M20. The motions in the structure are remarkably coherent, with velocities in the direction of Galactic rotation of |V_φ|≈240 ± 3 km s⁻¹ (slightly higher than average) and slight drifts inward (V_R ≈ −4.3 km s⁻¹) and in the negative Z direction (V_Z ≈ −2.9 km s⁻¹). The rotational shear experienced by the structure is ΔΩ = 4.6 km s⁻¹ kpc⁻¹.

Conclusions. The observed 56° pitch angle is remarkably high for a segment of the Sagittarius Arm. We discuss possible interpretations of this feature as a substructure within the lower pitch angle Sagittarius Arm, as a spur, or as an isolated structure.