Modelling the formation of the circumnuclear ring in the Galactic centre

Abstract

Several thousand solar masses of molecular, atomic and ionized gas lie in the innermost ∼ 10 pc of our Galaxy. The most relevant structure of molecular gas is the circumnuclear ring (CNR), a dense and clumpy ring surrounding the supermassive black hole (SMBH), with a radius of ∼ 2 pc. We propose that the CNR formed through the tidal disruption of a molecular cloud, and we investigate this scenario by means of N-body smoothed-particle hydrodynamics simulations. We ran a grid of simulations with different cloud mass (4 × 10 4 , 1.3 × 10 5 M⊙), different initial orbital velocity (vin = 0.2 − 0.5 vesc, where vesc is the escape velocity from the SMBH), and different impact parameter (b = 8, 26 pc). The disruption of the molecular cloud leads to the formation of very dense and clumpy gas rings, containing most of the initial cloud mass. If the initial orbital velocity of the cloud is sufficiently low (vin < 0.4 vesc, for b = 26 pc) or the impact parameter is sufficiently small (b < ∼ 10 pc, for vin > 0.5 vesc), at least two rings form around the SMBH: an inner ring (with radius ∼ 0.4 pc) and an outer ring (with radius ∼ 2 − 4 pc). The inner ring forms from low-angular momentum material that engulfs the SMBH during the first periapsis passage, while the outer ring forms later, during the subsequent periapsis passages of the disrupted cloud. The inner and outer rings are misaligned by ∼ 24 degrees, because they form from different gas streamers, which are affected by the SMBH gravitational focusing in different ways. The outer ring matches several properties (mass, rotation velocity, temperature, clumpiness) of the CNR in our Galactic centre. We speculate that the inner ring might account for the neutral gas observed in the central cavity.

A common origin for the circumnuclear disc and the nearby molecular clouds in the Galactic Centre

“…The adopted density profile for both components is ρNB(r) = 2.8 × 10 6 (r/rNB) −γ M pc −3 (where r is the distance from the centre of the computational domain), with rNB = 0.22 pc, γ = 1.2 for r < rNB and γ = 1.75 for r > rNB (e.g., Schödel et al 2007). For completeness, we also included the Galactic bulge potential, as a Hernquist sphere with density profile ρGB(r) = 2.9 × 10 10 rGB/[2πr(r + rGB) 3 ] M , where rGB = 0.7 kpc (Mapelli & Trani 2016).…”

PHYSICAL CONTACT BETWEEN THE +20 km s−1 CLOUD AND THE GALACTIC CIRCUMNUCLEAR DISK

“…The age of the CND is suggested to be younger than ∼10 6 years (e.g., Requena-Torres et al 2012). The CND may have been formed by the tidal capture and disruption of a giant molecular cloud (GMC) by the central SMBH within 10 6 years (Sanders 1998;Wardle & Yusef-Zadeh 2008;Liu et al 2012;Mapelli et al 2012;Mapelli & Trani 2016). Indeed, two GMCs, M-0.13-0.08 and M-0.02-0.07 (+20 and +50 km s −1 clouds, respectively), are located on the southwest and southeast shunya@aysheaia.phys.keio.ac.jp sides of the CND in Galactic coordinates, respectively.…”

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

“…Measuring radial inward motions would need full orbital modeling in the future (Paumard et al 2004;Zhao et al 2009). For scenario (3), it is possible to obtain extended molecular gas structures within 0.5 pc if a cloud with homogeneous density falls in from 26 pc towards an SMBH on a nearly radial orbit with non-vanishing angular momentum (Mapelli & Trani 2016). In such a scenario, the infalling cloud is quickly disrupted by the SMBH, and the tidal force stretches the cloud to form a group of streamers within 2.5 × 10 5 yr.…”