Physics of the Galactic Center Cloud G2, on its Way towards the Super-Massive Black Hole

TL;DR

The paper models the origin, structure, and evolution of the G2 gas cloud approaching the Galactic supermassive black hole, explaining its observed properties and predicting its eventual disruption and impact on black hole activity.

Contribution

It proposes a formation scenario for G2 as part of a larger shell-like structure, reconciling observations with simulations, and predicts its future disruption and influence on the SMBH.

Findings

G2 formed in pressure equilibrium with surrounding gas in 1995.

G2's structure is consistent with a shell-like formation at apocenter.

G2 will break into droplets and potentially trigger AGN activity within 30 years.

Abstract

The origin, structure and evolution of the small gas cloud, G2, is investigated, that is on an orbit almost straight into the Galactic central supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion zone of Sgr A*, requiring gas temperatures and densities that agree well with models of captured shock-heated stellar winds. Its mass is equal to the critical mass below which cold clumps would be destroyed quickly by evaporation. Its mass is also constrained by the fact that at apocenter its sound crossing timescale was equal to its orbital timescale. Our numerical simulations show that the observed structure and evolution of G2 can be well reproduced if it formed in pressure equilibrium with the surrounding in 1995 at a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at apocenter in the 'clockwise' stellar disk as expected from its orbit, it would be torn into a very elongated spaghetti-like filament by 2011 which is not observed. This problem can be solved if G2 is the head of a larger, shell-like structure that formed at apocenter. Our numerical simulations show that this scenario explains not only G2's observed kinematical and geometrical properties but also the Br_gamma observations of a low surface brightness gas tail that trails the cloud. In 2013, while passing the SMBH G2 will break up into a string of droplets that within the next 30 years mix with the surrounding hot gas and trigger cycles of AGN activity.