High-speed molecular cloudlets around the Galactic center’s supermassive black hole

Abstract

We present 1″-resolution ALMA observations of the circumnuclear disk (CND) and the interstellar environment around Sgr A*. The images unveil the presence of small spatial scale 12CO (J = 3–2) molecular “cloudlets” (≲20 000 AU size) within the central parsec of the Milky Way, in other words, inside the cavity of the CND, and moving at high speeds, up to 300 km s−1 along the line-of-sight. The 12CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (vLSR ≲−150 km s−1), more localized and clumpy at extreme positive-velocities (vLSR ≳+200 km s−1). Based on the pencil-beam 12CO absorption spectrum toward Sgr A* synchrotron emission, we also present evidence for a diffuse molecular gas component producing absorption features at more extreme negative-velocities (vLSR < −200 km s−1). The CND shows a clumpy spatial distribution traced by the optically thin H13CN (J = 4–3) emission. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks, molecular cores of several 105 cm−3, are lower than the local Roche limit. This supports that CND cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around Sgr A*. The location and velocities of the 12CO cloudlets inside the cavity are inconsistent with the spiral model, and only two of them are consistent with the Keplerian ellipse model. Most cloudlets, however, show similar velocities that are incompatible with the motions of the ionized streamers or with gas bounded to the central gravity. We speculate that they are leftovers of more massive molecular clouds that fall into the cavity and are tidally disrupted, or that they originate from instabilities in the inner rim of the CND that lead to fragmentation and infall from there. In either case, we show that molecular cloudlets, all together with a mass of several 10 M⊙, exist around Sgr A*. Most of them must be short-lived, ≲104 yr: photoevaporated by the intense stellar radiation field, G0 ≃ 105.3–104.3, blown away by winds from massive stars in the central cluster, or disrupted by strong gravitational shears.