Forming Circumnuclear Disks and Rings in Galactic Nuclei: A Competition Between Supermassive Black Hole and Nuclear Star Cluster

Abstract

We investigate the formation of circumnuclear gas structures from the tidal disruption of molecular clouds in galactic nuclei, by means of smoothed particle hydrodynamics simulations. We model galactic nuclei as composed of a supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider different mass ratios between the two components. We find that the relative masses of the SMBH and the NSC have a deep impact on the morphology of the circumnuclear gas. Extended disks form only inside the sphere of influence of the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere of influence, where the gravity is dominated by the NSC. This result is in agreement with the properties of the Milky Way's circumnuclear ring, which orbits outside the SMBH sphere of influence. Our results indicate that compact circumnuclear rings can naturally form outside the SMBH sphere of influence.

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

“…The cloud reaches its first apocentre after roughly 0.4 Myr, though the parts of the cloud with lowest angular momentum already start falling back towards the SMBH and whirl around it (panel b and c). At this stage, the apocentre position has significantly changed, compared to the initial one, due to very fast orbital precession induced by the massive stellar cusp (Gualandris et al 2012;Trani et al 2018). A circumnuclear ring is already clearly visible after around 0.5 Myr from the beginning of the simulation, while most of the cloud is still elongated towards the SMBH and has not fallen back yet.…”

“…A connection between the inner few parsecs of the galaxy and the close-by molecular clouds is also supported by pure theory. Molecular clouds "engulfing" the SMBH are often invoked to explain the formation of the disc of young stars (Sanders 1998;Wardle & Yusef-Zadeh 2008;Bonnell & Rice 2008;Hobbs & Nayakshin 2009;Alig et al 2011;Mapelli et al 2012;Lucas et al 2013;Alig et al 2013) and/or the origin of the CND (Sanders 1998;Wardle & Yusef-Zadeh 2008;Mapelli & Trani 2016;Trani et al 2018; however, other authors modeled the CND as a long-lived stable structure, "carved" by feedback from the young stars or by tidal shear, see, e.g., Vollmer & Duschl 2001, 2002Blank et al 2016).…”

Gas inflow and star formation near supermassive black holes: the role of nuclear activity

“…Following the analytical model of , we impose a uniform inflow velocity of 100 km s −1 for all models. This velocity is comparable to the Keplerian orbital velocity of the circumnuclear disk (CND), which is the innermost molecular gas reservoir in the GC (Christopher et al 2005) and may have formed through an infall event similar to that modeled here (Mapelli & Trani 2016;Trani et al 2018). Velocity perturbations are excluded as they are expected to be unimportant due to the highly supersonic bulk flow .…”

Feeding of active galactic nuclei by dynamical perturbations

“…In the central cavity of the CNR there is a minispiral (Ferrière 2012). Trani et al (2018) hints that this central structure forms during the formation of the CNR. Our simulations suggest that similar structures could also form after collision with preexisting CNR.…”

“…Known as the Circum-nuclear ring (CNR), it has an inner radius of about 2 pc (Ferrière 2012). Hydrodynamical simulations (Mapelli & Trani 2015;Trani et al 2018) show that the CNR-like feature can form if there is an infall of matter to the central region, for example from a tidally disrupted molecular cloud (MC).…”