High-speed molecular cloudlets around the Galactic Center supermassive black hole

Abstract

We present 1"-resolution ALMA observations of the circumnuclear disk (CND) and the environment around SgrA*. The images unveil the presence of small spatial scale CO (J=3-2) molecular "cloudlets" within the central pc of the Milky Way, moving at high speeds, up to 300 km/s along the line-of-sight. The CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (v_LSR < -150 km/s), more localized and clumpy at extreme positive-velocities (v_LSR > +200 km/s). Based on the pencil-beam CO absorption spectrum toward SgrA* synchrotron emission, we also present evidence for a diffuse gas component producing absorption features at more extreme negative-velocities (v_LSR < -200 km/s). The CND shows a clumpy spatial distribution. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks are lower than the local Roche limit. This supports that CND molecular cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around SgrA*. The location and velocities of the CO cloudlets 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, tidally disrupted, clouds that fall into the cavity, or that they originate from instabilities in the inner rim of the CND and infall from there. Molecular cloudlets, all together with a mass of several 10 M_Sun, exist around SgrA*. Most of them must be short-lived: photoevaporated by the intense stellar radiation field, blown away by winds from massive stars, or disrupted by strong gravitational shears.