The Galactic-Centre Arms inferred from ACES (ALMA CMZ Exploration Survey)
Sofue, Y. ; Oka, Tomo. ; Longmore, S. N. ; Walker, D. ; Ginsburg, A. ; Henshaw, J. D. ; Bally, J. ; Barne, A. T. ; Battersby, C. ; Colzi, L. ; Ho, P. ; Jimenez-Serra, I. ; Kruijssen, J. M. D. ; Mills, E. ; Petkova, M. A. ; Sormani, M. C. ; Wallace, J. ; Armijos-Abendano, J. ; Dutkowska, K. M. ; Enokiya, R. ; Fukui, Y. ; Garcia, P. ; Guzman, A. ; Henkel, C. ; Hsieh, P. -Y. ; Hu, Y. ; Immer, K. ; Jeff, D. ; Klessen, R. S. ; Kohno, K. ; Krumholz, M. R. ; Lipman, D. ; Martin, S. ; Morris, M. R. ; Nogueras-Lara, F. ; Nonhebel, M. ; Otto, J. ; Pineda, J. E. ; Requena-Torres, M. A. ; Rivilla, V. M. ; Riquelme-Vasquez, D. ; Sanchez-Monge, A. ; Santa-Maria, M. G. ; Smith, H. A. ; Tanvir, T. S. ; Tolls, V. ; Wang, Q. D.
Analyzing longitude-velocity diagrams (LVDs) in the CS(J=2-1) and H13CN(J=1-0) molecular lines from the internal release data of the ALMA Central-Molecular-Zone Exploration Survey (ACES) and in the 13CO (J=1-0) line from the Nobeyama Galactic-Centre (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 circum-nuclear disc (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 on circular orbits at a constant velocity of ∼150 km/s. We show that Arms I and II compose the main ring structure of the CMZ with radii ∼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 circum-nuclear disc (CND) composes the sixth arm, Arm VI, of radius ∼2.3 pc associated with bifurcated spiral fins. We also discuss the association of the 20- and 50-km/s clouds with these Arms. The radii of the arms fall on an empirical relation R∼630(2/5)N for N=1 (Arm I) to 6 (VI), suggesting either discrete rings or a logarithmic spiral with pitch angle ∼22∘. The vertical full extent of the arm increases with radius and is represented by z∼0.7(R/1pc)0.7 pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC.