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 a few 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 to 104.3, blown away by winds from massive stars in the central cluster, or disrupted by strong gravitational shears.

Effects of turbulence in the Circumnuclear Disk

“…Such high densities would imply extremely large CND masses of 10 5 − 10 6 M , which are inconsistent with the optically thin far-IR and submillimeter fluxes from the CND (Genzel et al 2010;Etxaluze et al 2011). The densities required for tidal stability are also much higher than have been inferred in many recent studies (Oka et al 2011;Requena-Torres et al 2012;Lau et al 2013;Mills et al 2013;Smith & Wardle 2014;Harada et al 2015;Goicoechea et al 2018;Tsuboi et al 2018). We conclude that clumps and streams in the CND must truly be transient features.…”

“…A popular model to account for the transient structures within the CND is that a passing cloud was gravitationally captured by the black hole. This "infalling cloud" was then tidally disrupted into what is now the CND (Sanders 1998;Bradford et al 2005;Bonnell & Rice 2008;Wardle & Yusef-Zadeh 2008;Hobbs & Nayakshin 2009;Alig et al 2011;Oka et al 2011;Mapelli et al 2012;Mapelli & Trani 2016;Trani et al 2018;Goicoechea et al 2018;Ballone et al 2019).…”

The Nuclear Filaments inside the Circumnuclear Disk in the Central 0.5 pc of the Galactic Center

“…This implies that the nuclear filaments (and the inner cavity of the CND) could have a face-on geometry with intrinsic velocities ≥ 300 km s −1 at a (de-projected) radius of 0.15 pc. This range of the inclination angle of the nuclear filaments is quite different from that of the CND (65 • to 80 • ; Martín et al 2012;Hsieh et al 2017;Goicoechea et al 2018) if the assumption of Keplerian motion holds. Figure 4 illustrates the nuclear structures including the CND, the nuclear filaments, and the extended molecular gas that we detected.…”

“…This newly detected structural connection of the CND with the nuclear filaments appears clearer and more robust than in previous maps (Moser et al 2017;Tsuboi et al 2018). Moreover, the high-excitation CS(7-6) line also better discriminates against lower-excitation material and reveals the nuclear filaments more clearly than in CO(3-2) (Goicoechea et al 2018). In order to enhance the extended emission, we smooth the CS(7-6) data to 1.3 ′′ .…”

The Circumnuclear Disk Revealed by ALMA. I. Dense Clouds and Tides in the Galactic Center

“…Within the central few pc, previous observations show that the concentration of molecular gas is influenced by the gravitational potential of a SMBH and follows a quasi-Keplerian rotation, which is often called the circumnuclear disk/ring (CND/CNR, e.g. Martín et al 2012;Liu et al 2012;Mills et al 2013;Hsieh et al 2017;Imanishi et al 2018;Tsuboi et al 2018;Goicoechea et al 2018;Izumi et al 2018;Combes et al 2019). The CND/CNR, being the largest and closest 2012) are labeled on the SMA HCN(4-3) map (Liu et al 2012).…”