ALMA [C <scp>I</scp> ] Image of the Circumnuclear Disk of the Milky Way: Inflowing Low-density Molecular Gas

Abstract We present Atacama Large Millimeter/submillimeter Array [C i ] 3 P 1 – 3 P 0 imaging of the central 6.6 × 4.2 pc 2 region of the Galaxy encompassing the circumnuclear disk (CND). The data reveal low-density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> </mml:math> cm −3 ) molecular gas with inward motion, widespread both inside and outside the CND. The normalized [C i ] to CS 7–6 intensity difference decreases inwardly from R = 4 pc to 1.7 pc and azimuthally along the CND’s rotation, likely tracing paths of low-density gas inflow. By projecting spaxels into orbital coordinates assuming a velocity field model, we identify four kinematic features: a pair of spiral outer streamers toward the CND, inner streamers extending to 0.5 pc from Sgr A * , an outer disk at R ∼ 3–6 pc, and the rotating ring at R = 2 pc. P – P – V correlation between the inner streamers and H42 α indicates gas supply to the mini-spiral through the western arc (WA) and northern arm (NA). The total inflowing mass is 1.5 × 10 4 M ⊙ , 1.7 times the mass of the rotating ring. The identified flows can be organized into two main pathways connecting the CND exterior and interior: “WA flow” feeding the mini-spiral WA via the CND, and “NA flow” bypassing the purely rotating orbit. The inflow rate along the former is approximately constant (0.1–0.16 M ⊙ yr −1 ), implying a CND dwelling time comparable to its orbital period and supporting the CND’s transient nature. We also identify two [C i ]-bright clumps (CBCs) lacking dense-gas counterparts near the contact point between the northern outer streamer and the CND. Apparently intact against tidal disruption despite subcritical densities, the CBCs may represent a chemically young phase shortly after formation in colliding flows.