EDGE: A new model for Nuclear Star Cluster formation in dwarf galaxies

Abstract: Nuclear Star Clusters (NSCs) are amongst the densest stellar systems in the Universe and are found at the centres of many bright spiral and elliptical galaxies, and up to ${\sim}$40% of dwarf galaxies. However, their formation mechanisms, and possible links to globular clusters (GCs), remain debated. This paper uses the EDGE simulations - a collection of zoom-in, cosmological simulations of isolated dwarf galaxies -- to present a new formation mechanism for NSCs. We find that, at a gas spatial and mass resolution of ${\sim}3\,$pc and ${\sim}161$ M$\odot$, respectively, NSCs naturally emerge in a subset of our EDGE dwarfs with redshift-zero halo masses of $\rm{M}{\rm{r}200\rm{c}} \sim 5 \times 10^9$ M$_\odot$. These dwarfs are quenched by reionisation, but retain a significant reservoir of gas that is unable to cool and form stars. Sometime after reionisation, the dwarfs then undergo a major (${\sim}$1:1) merger that excites rapid gas cooling, leading to a significant starburst. An NSC forms in this starburst that then quenches star formation thereafter. The result is a nucleated dwarf that has two stellar populations with distinct age: one pre-reionisation and one post-reionisation. Our mechanism is unique for two key reasons. Firstly, the low mass of the host dwarf means that NSCs, formed in this way, can accrete onto galaxies of almost all masses, potentially seeding the formation of NSCs everywhere. Secondly, our model predicts that NSCs should have at least two stellar populations with a large ($\gtrsim$1 billion year) age separation. This yields a predicted colour magnitude diagram for our nucleated dwarfs that has two distinct main sequence turnoffs. Several GCs orbiting the Milky Way, including Omega Centauri and M54, show exactly this behaviour, suggesting that they may, in fact, be accreted NSCs.

• The paper introduces a new model for Nuclear Star Cluster (NSC) formation in dwarf galaxies, positing that they form from starbursts triggered by major mergers in reionization-quenched dwarfs, using EDGE cosmological simulations.
• NSCs formed via this mechanism display at least two distinct stellar populations separated by significant age and chemical composition differences.
• The model implies that low-mass NSCs formed this way can be accreted by larger galaxies, potentially explaining the complex stellar populations seen in some observed globular clusters like Omega Centauri.

An Innovative Approach to Understanding Nuclear Star Cluster Formation

The study outlined in the paper titled "EDGE: A new model for Nuclear Star Cluster formation in dwarf galaxies" provides a novel mechanism explaining the formation of Nuclear Star Clusters (NSCs) in low-mass dwarf galaxies through cosmological simulations from the EDGE (Engineering Dwarfs at Galaxy formation's Edge) project. NSCs are recognized for their dense stellar configurations and central positions in a significant fraction of galaxies, yet their formation processes and connections with globular clusters (GCs) remain contentious. This research introduces a compelling model positing that NSCs in dwarf galaxies naturally arise due to specific cosmic events and mechanisms.

The model suggested by the authors proposes that NSCs form as a result of major mergers in dwarf galaxies that underwent quenching due to reionization. Through a comprehensive simulation approach using EDGE, the authors identify a subset of dwarf galaxies, characterized by their redshift-zero halo masses of approximately $5 \times 10^9$\,M$_\odot$, which exhibit NSC formation post-reionization. These dwarfs retain sufficient gas, which remains unable to cool effectively due to their low mass, yet a major merger invigorates rapid gas cooling, prompting a significant starburst. The starburst initiates the formation of a NSC that subsequently quenches additional star formation.

Key findings of this research include:

• NSCs formed within these dwarfs display at least two distinct stellar populations, separated by a substantial age gap approximating 1 billion years.
• The proposed mechanism is highly consequential for its potential universality: Low-mass NSCs formed through this method could be accreted by larger mass galaxies across various mass scales.
• The simulated NSCs display characteristic features such as distinct chemical composition differences (e.g., [Fe/H] and [O/Fe]) and varied kinematic properties between pre- and post-reionization populations.

The authors further validate their model by performing an ensemble of simulations which not only demonstrate the robustness of the method but also reveal the impact of stochastic elements like merger timing and initial conditions. These simulations yield NSCs that show similarities with real observations, such as in the structure and chemical profiles of certain GCs, such as Omega Centauri and M54, implying these GCs might themselves be accreted NSCs.

The implications of these results extend to predicting GC and NSC evolutionary pathways and accounting for specific complexity in main-sequence turnoffs observed in some GCs. This favors the interpretation of certain GCs as relics of nucleated dwarf mergers, thereby reshaping our understanding of the role of dynamical processes in galactic evolution.

Future avenues for exploration could include high-resolution simulations to refine rotational dynamics and the incorporation of additional physical processes not currently modeled, such as more detailed radiative feedback mechanisms. Additionally, observational efforts could focus on NSCs within isolated environments to directly test the predictions and implications indicated by the model presented here. This study invites deeper inquiry into the universal and diverse nature of NSC formation and evolution across cosmic timescales. Such work is essential for broadening our comprehension of galactic formation and the conditions that influence the lifecycle of stellar clusters in different astrophysical contexts.