Formation of Accretion Disks around Intermediate-Mass Black Holes in Ultra Compact Dwarf Galaxies and Nuclear Stellar Clusters

Abstract: Recent observations have found many ultra-compact dwarf galaxies (UCDs) that contain massive black holes (MBHs) typically consisting of $10\%-15\%$ of their total mass, with some as high as $40\%$. However, whether UCDs initially started with MBHs or grew from intermediate-mass black holes (IMBHs) is unclear. We propose a mechanism in which AGB stars have their gas ejected which forms an accretion disk around an IMBH to fuel BH growth. We used smooth particle hydrodynamical simulations to (i) model the creation and evolution of gas disks that can finally evolve into accretion disks and to investigate (ii) how the properties of the gas disk depend on the model. We find that the amount of mass in the disk increases more with UCD mass than with IMBH mass. For lower-mass IMBHs of $100 \text{ M}_\odot$, a majority of the gas mass present can be trapped in a disk, leading to the idea that lower-mass BHs can utilise the proposed mechanism. The main mechanism working against accretion disk formation is the creation of new stars within the disk, not just through the removal of gas but through gravitational disruption as well. Finally, an estimation of the total BH growth provides a list of observed UCDs which could have formed MBHs using the proposed mechanism which can be useful for verifying the proposed mechanism. Future simulations aimed at modelling the formation of such UCDs can be employed to provide a full picture of the results presented in this paper.