ALMA [CI] Image of the Circumnuclear Disk of the Milky Way: Inflowing Low-density Molecular Gas
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| Formato: | Preprint |
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2026
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| author | Tanaka, Kunihiko Nagai, Makoto Kamegai, Kazuhisa |
| author_facet | Tanaka, Kunihiko Nagai, Makoto Kamegai, Kazuhisa |
| contents | We present ALMA [\ion{C}{1}]~$^3P_1$--$^3P_0$ imaging of the central $6.6\times4.2~\mathrm{pc}^2$ region of the Galaxy encompassing the circumnuclear disk (CND). The data reveal low-density ($n_\mathrm{H_2}\sim10^3~$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$^*$, an outer disk at $ R\sim3$--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 $1.5\times10^4~M_\odot$, 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_09251 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | ALMA [CI] Image of the Circumnuclear Disk of the Milky Way: Inflowing Low-density Molecular Gas Tanaka, Kunihiko Nagai, Makoto Kamegai, Kazuhisa Astrophysics of Galaxies We present ALMA [\ion{C}{1}]~$^3P_1$--$^3P_0$ imaging of the central $6.6\times4.2~\mathrm{pc}^2$ region of the Galaxy encompassing the circumnuclear disk (CND). The data reveal low-density ($n_\mathrm{H_2}\sim10^3~$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$^*$, an outer disk at $ R\sim3$--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 $1.5\times10^4~M_\odot$, 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. |
| title | ALMA [CI] Image of the Circumnuclear Disk of the Milky Way: Inflowing Low-density Molecular Gas |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2602.09251 |