Gas infall towards Sgr A* from the clumpy circumnuclear disk

We present the first large-scale mosaic performed with the Submillimeter Array (SMA) in the Galactic center. We have produced a 25-pointing mosaic, covering a ~2’ x 2’ area around Sgr A*. We have detected emission from two high-density molecular tracers, HCN(4-3) and CS(7-6), the latter never before reported in this region. The data have an angular resolution of 4.6" x 3.1", and the spectral window coverage is from -180 km/s to 1490 km/s for HCN(4-3) and from -1605 km/s to 129 km/s for CS(7-6). Both molecular tracers present a very clumpy distribution along the circumnuclear disk (CND), and are detected with a high signal-to-noise ratio in the southern part of the CND, while they are weaker towards the northern part. Assuming that the clumps are as close to the Galactic center as their projected distances, they are still dense enough to be gravitationally stable against the tidal shear produced by the supermassive black hole. Therefore, the CND is a non-transient structure. This geometrical distribution of both tracers suggests that the southern part of the CND is denser than the northern part. Also, by comparing the HCN(4-3) results with HCN(1-0) results we can see that the northern and the southern parts of the CND have different excitation levels, with the southern part warmer than the northern. Finally, we compare our results with those obtained with the detection of NH3, which traces the warmer and less dense material detected in the inner cavity of the CND. We suggest that we are detecting the origin point where a portion of the CND becomes destabilized and approaches the dynamical center of the Milky Way, possibly being impacted by the southern streamer and heated on its way inwards.

💡 Research Summary

The authors present the first large‑scale Submillimeter Array (SMA) mosaic of the Galactic Center, covering a ∼2′ × 2′ region (≈5 pc on a side) around Sgr A*. Using a 25‑pointing mosaic they obtained high‑resolution (4.6″ × 3.1″) maps of two high‑density molecular tracers, HCN (4‑3) and CS (7‑6). The CS (7‑6) line, never before reported in this region, and the HCN (4‑3) line were observed over very broad velocity ranges (−180 to +1490 km s⁻¹ for HCN, −1605 to +129 km s⁻¹ for CS), providing an unprecedented view of the kinematics and excitation of the circumnuclear disk (CND).

Both tracers reveal a highly clumpy distribution that follows the well‑known CND. The southern sector of the CND exhibits bright, high‑signal‑to‑noise clumps in both lines, whereas the northern sector is markedly fainter. By estimating clump masses and sizes from line intensities, linewidths, and assumed excitation conditions, the authors find typical densities of 10⁶–10⁷ cm⁻³. Assuming the clumps lie at their projected distances (∼1 pc from Sgr A*), these densities are sufficient to resist tidal shear from the central super‑massive black hole (M≈4 × 10⁶ M☉). Consequently, the CND is not a transient feature but a gravitationally stable structure that can survive many orbital periods.

A comparison of the new HCN (4‑3) data with existing HCN (1‑0) maps shows a clear excitation gradient: the southern CND displays a higher HCN (4‑3)/HCN (1‑0) ratio, indicating warmer and/or denser gas, while the northern side shows a lower ratio, consistent with cooler, less dense material. This asymmetry suggests that the southern CND is currently being fed or compressed by an external inflow, often referred to as the “southern streamer.” The impact of this streamer would both compress the gas (raising its density) and shock‑heat it (raising the excitation temperature), thereby enhancing the high‑J transitions observed.

The authors also compare their results with ammonia (NH₃) observations that trace warmer, lower‑density gas inside the CND’s inner cavity. NH₃ (3,3) and (6,6) emission is found primarily interior to the CND, indicating a component of ∼10⁴–10⁵ cm⁻³ gas at temperatures of 50–150 K. In contrast, HCN (4‑3) and CS (7‑6) require critical densities ≳10⁶ cm⁻³ and trace gas at ≳200 K, confirming that the two sets of molecules probe distinct physical regimes. The spatial offset between the high‑density clumps and the NH₃‑bright cavity supports a picture in which material from the outer CND is being destabilized, funneled inward, and gradually transitions from a dense, cold phase to a warmer, more diffuse phase as it approaches the black hole.

Putting these pieces together, the authors propose a scenario in which the southern portion of the CND acts as the “entry point” for gas that becomes gravitationally unstable, possibly due to interaction with the southern streamer. As the gas moves inward, it is heated and compressed, producing the bright HCN (4‑3) and CS (7‑6) emission observed. Upon reaching the inner cavity, the gas appears as warmer, less dense NH₃ emission. This process may represent a channel through which the CND supplies material to the immediate environment of Sgr A*, potentially feeding accretion onto the black hole and influencing star formation in the central parsec.

Overall, the study demonstrates that the CND is a non‑transient, dynamically active structure with significant internal asymmetries. The southern CND’s higher density and temperature, together with its apparent connection to inflowing streamers, provide compelling evidence for ongoing gas inflow toward the Galactic Center, offering new constraints for models of black‑hole feeding, feedback, and the lifecycle of molecular material in the innermost region of the Milky Way.