Evolution of Nuclear Star Cluster in Dwarf Galaxy through Mergers and In-Situ Star Formation

Abstract

Nuclear Star Clusters (NSCs) are dense stellar systems located at the centers of galaxies. Employing Enzo-Abyss, which integrates hydrodynamics with a direct N-body solver, we introduce a simulation capable of resolving the evolution of NSCs within a live galaxy. This includes live dark matter, gaseous dynamics, star formation and feedback, collisional dynamics for star clusters. The evolution of NSCs is typically shaped by two main processes: mergers of star clusters and in-situ star formation. Our simulation enables investigation of the contributions of these mechanisms to the growth of NSCs. This work focuses on the impact of stellar physics and gas content on the growth of NSCs within a dwarf galaxy. To this end, we carry out four simulations, a fiducial simulation, one without supernova feedback, one with low star formation efficiency, and one with higher galactic gas content. This study shows a likelihood that both mergers and in-situ star formation contribute to NSC evolution comparably. In addition, mergers result in disruption of dense gas clumps within star clusters, indicating that in-situ star formation is suppressed when mergers occur. However, the limitations -- such as the lack of individual star physics and limited spatial/particle mass resolution -- hinder drawing a definite conclusion. Nevertheless, with further development, our simulations will serve as a cornerstone that untangles the complex interplay between mergers and in-situ star formation in shaping the structure and mass of NSCs, thereby providing insights into their formation and evolution.

Figures (16)

• Figure 1: Two dimensional disk-on projections of stars and gas in two different views---a galaxy view of a 2 kpc box ( first and second columns) and a cluster view of a 400 pc box ( third and fourth columns)---at $t =$ 50 Myr ( top), 100 Myr ( middle), and 150 Myr ( bottom).
• Figure 2: Surface density of the NSC, stars and dark matters at 0 Myr ( upper left), 50 Myr ( upper right), 100 Myr ( lower left), and 150 Myr ( lower right). The guidelines ( grey dotted) are at 10 pc and $10^3 \,{\rm M}_\odot/\mathrm{pc}^2$. After 50 Myr, the NSC have become denser and concentrated. The dark matters have been able to fall towards the center of the NSC (refer to Sec. \ref{['sec:appendix_dark_matter']} for further discussion on the dark matter profiles). The recent merger at around 140 Myr results in the disrupted inner profile of NSC at 150 Myr despite the mass growth (refer to Fig. \ref{['fig:mass_growth_merger']}).
• Figure 3: Top: the temporal evolution of the total mass of newly formed stars within the host galaxy, excluding the initial condition. Middle: the ratio of the aggregate mass of star clusters to the total mass of newly formed stars over time. Bottom the total number of star clusters within the host galaxy as a function of time. The fiducial, no feedback, low star formation, and double gas mass simulations are represented by black solid, red dashed, blue dotted, and green dot-dashed lines, respectively.
• Figure 4: Trajectory of the center of mass of the NSC over 150 Myr ( top). The distance between the centers of mass of the NSC and the host galaxy with time ( bottom). The initial position of the center of mass of the NSC at 0 Myr and the final position at 150 Myr are represented with the blue and red dot, respectively. The two points are connected through trajectory of the center of mass of the NSC ( black dotted). The initial position coincides with the center of mass of the host galaxy.
• Figure 5: Upper panels: The merger trees and the mass growths of the NSCs for the three simulations---Fiducial ( 1st row), No Feedback ( 2nd row), Low Star Formation ( 3rd row), and $2\times$ Gas ( 4th row). The NSCs---primary of merger trees---are represented by the red dots in the merger trees, while the black dots represent the nearby star clusters. The vertical lines indicate NSC-star cluster mergers. Lower panels: The stellar mass evolution of the NSCs given by the HDBSCAN ( blue).