2025PASJ...77..687S

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.