ALMA [CI] Image of the Circumnuclear Disk of the Milky Way: Inflowing Low-density Molecular Gas

Abstract

We present ALMA [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ imaging of the central $6.6\times 4.2{\mathrm{pc}}^2$ region of the Galaxy encompassing the circumnuclear disk (CND). The data reveal low-density ($n_{{\mathrm{H}}_2}\sim 10^3$cm${}^{-3}$) molecular gas with inward motion, widespread both inside and outside the CND. The normalized [\ion{C}{1}] 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${}^{\ast }$, an outer disk at $R\sim 3$--6~pc, and the rotating ring at $R=2$~pc. $P$--$P$--$V$ correlation between the inner streamers and H42$\alpha$ indicates gas supply to the mini-spiral through the western arc (WA) and northern arm (NA). The total inflowing mass is $1.5\times 10^4{M}_{\odot }$, 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~$M_{\odot }{\mathrm{yr}}^{-1}$), implying a CND dwelling time comparable to its orbital period and supporting the CND's transient nature. We also identify two [\ion{C}{1}]-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.

26 pages, 17 figures. Accepted for publication in the Astrophysical Journal