PI: Debora Sijacki
While mounting observational evidence suggests that intermediate-mass black holes (IMBHs) may play an important role in shaping the properties of dwarf galaxies, both at high redshift and in the local Universe, our theoretical understanding of how these IMBHs form and grow remains largely incomplete.
IMBHs, with masses ranging from a hundred to a million solar masses, represent a critical missing piece in our understanding of the assembly of supermassive black holes (SMBHs), which have been detected in abundance across cosmic time. As potential analogues of massive galaxies hosting SMBHs, lower-mass galaxies, and dwarf galaxies in particular, are considered promising sites for IMBH formation and growth. This expectation is supported by the well-established scaling relations between black hole mass and host-galaxy properties, such as stellar velocity dispersion, bulge mass, and stellar mass. However, detecting such low-mass black holes remains challenging, as their limited dynamical influence and intrinsically low luminosities make them elusive observational targets. Nevertheless, the number of IMBH candidates identified in both local and high-redshift galaxies is steadily increasing, reflecting growing observational efforts and interest in this population.
Fundamental questions remain: how do IMBH seeds form and evolve within their host galaxies? How is multiphase gas funnelled into the vicinity of IMBHs? How is this inflow ultimately circularized into an IMBH accretion disc?
To address these questions, we performed extremely high-resolution simulations of an isolated dwarf galaxy harbouring an IMBH at its centre, reaching a peak spatial resolution of ≲ 0.01 pc thanks to our super-Lagrangian refinement technique. The simulations were performed on DiRAC’s Cambridge and Durham nodes as part of the dp379 project. Within a fully multi-phase interstellar medium (ISM), we incorporate explicit sampling of stars from the initial mass function, photoionisation, photoelectric heating, individual supernovae (SNe), as well as a Shakura–Sunyaev accretion-disc model to track the evolution of black hole mass and spin. The combination of super-Lagrangian refinement, resolved individual stars, and detailed black hole accretion modelling allows us to resolve the key processes governing angular momentum transport and mass transfer onto the IMBH, providing critical insights into its long-term growth and thermodynamic impact on the surrounding environment.
Interestingly, in our simulation models that include a nuclear star cluster (NSC), we find that the NSC effectively captures ISM gas and promotes the formation of a dense circumnuclear disc (CND) on scales of ≲ 7 pc. Simultaneously, gravitational torques from the NSC reduce the CND’s angular momentum on (sub-)parsec scales, circularizing the gas onto the α-accretion disc and promoting sustained IMBH growth at ∼ 0.01 of the Eddington rate.
While star formation is highly suppressed in the innermost regions (≲ 0.5 pc), the CND remains susceptible to fragmentation, leading to the formation of massive young stars. Despite an in situ SN rate of 0.3 per Myr, the dense CND persists, sustaining black hole accretion and leading to its net spin-up. Our study demonstrates the complex interplay between stellar feedback, NSCs, and dense (warped) CNDs, which can drive both nuclear in situ star formation and sustained black hole growth. This methodology will ultimately allow us to shed light on the build-up of the elusive high-redshift black hole population, which remains one of the most compelling open questions in astrophysics.
Figure 1. Visualization of the MandelZoom framework. Large left-hand panel shows the simulated dwarf galaxy, where the multi-phase ISM, shaped by stellar feedback is clearly visible. Panels in the middle show the CND (face-on and edge-on gas density) and in-situ young stars around the BH, zooming from galactic to sub-0.01~pc scales. Bottom middle and right-hand panels show how super-Lagrangian refinement resolves the α-disc self-gravity radius and enables a physically well posed sub-grid model, linking galactic inflow to IMBH mass and spin growth.
