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Abstract Analyzing longitude–velocity diagrams (LVDs) in the CS (J = 2–1) and H$$^{13}$$CN (J = 1–0) molecular lines from the internal release data of the ALMA Central Molecular Zone (CMZ) Exploration Survey and in the $$^{13}$$CO (J = 1–0) line from the Nobeyama Galactic Center (GC) survey, we identify six GC arms as prominent straight LV ridges. In addition to the currently known Arms I to IV, we identify a new inner arm, Arm V, and further highlight the circumnuclear disk (CND) as Arm VI. Integrated intensity maps of the arms on the sky suggest that most of the arms compose ring-like structures inclined from the Galactic plane. We determine the radii (curvatures) of the arms using the velocity-gradient ($dv/dl$) method, assuming that the arms are rotating in circular orbits at a constant velocity of $$\sim$$150 km s$$^{-1}$$. We show that Arms I and II compose the main ring structure of the CMZ with radii $$\sim$$100–120 pc; Arm III is a dense arm 42 pc from the GC; Arm IV is a clear and narrow arm 20 pc from the GC; and Arm V is a faint, long arm of 8.2 pc radius. We show that the CND comprises the sixth arm, Arm VI, of radius $$\sim$$2.3 pc associated with bifurcated spiral fins. We also discuss the association of the 20 and 50 km s$$^{-1}$$ clouds with these arms. The radii of the arms fall on an empirical relation $$R\sim 630 (2/5)^N$$ for $N=1$ (Arm I) to 6 (VI), suggesting discrete rings or a logarithmic spiral with pitch angle $${\sim }22^{\circ }$$. The vertical full extent of the arm increases with radius and is represented by $$z\sim 0.7 (R/1 {\rm \ pc} )^{0.7}$$ pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC.
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A high pitch angle structure in the Sagittarius Arm
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.
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- PAR ID:
- 10313513
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 651
- ISSN:
- 0004-6361
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1051/0004-6361/202141198
