Star Cluster Formation in Cosmological Simulations. II. Effects of Star Formation Efficiency and Stellar Feedback

TL;DR

This paper investigates how star formation efficiency and stellar feedback influence galaxy and star cluster properties in cosmological simulations, refining models to better match observed cluster formation and feedback effects.

Contribution

It introduces improvements to the star formation and feedback algorithm, and explores how varying efficiency and feedback parameters affect galaxy and star cluster characteristics.

Findings

Star formation history is sensitive to feedback boost parameter $f_{boost}$.

Cluster properties are highly sensitive to star formation efficiency $\e_{ff}$.

Optimal feedback parameter $f_{boost}$ is approximately 5 for current setup.

Abstract

The implementation of star formation and stellar feedback in cosmological simulations plays a critical role in shaping galaxy properties. In the first paper of the series, we presented a new method to model star formation as a collection of star clusters. In this paper, we improve the algorithm by eliminating accretion gaps, boosting momentum feedback, and introducing a subgrid initial bound fraction, $f_i$, that distinguishes cluster mass from stellar particle mass. We perform a suite of simulations with different star formation efficiency per freefall time $\epsilon_{\rm ff}$ and supernova momentum feedback intensity $f_{\rm boost}$. We find that the star formation history of a Milky Way-sized galaxy is sensitive to $f_{\rm boost}$, which allows us to constrain its value, $f_{\rm boost}\approx5$, in the current simulation setup. Changing $\epsilon_{\rm ff}$ from a few percent to 200\% has little effect on global galaxy properties. However, on smaller scales, the properties of star clusters are very sensitive to $\epsilon_{\rm ff}$. We find that $f_i$ increases with $\epsilon_{\rm ff}$ and cluster mass. Through the dependence on $f_i$, the shape of the cluster initial mass function varies strongly with $\epsilon_{\rm ff}$. The fraction of clustered star formation and maximum cluster mass increase with the star formation rate surface density, with the normalization of both relations dependent on $\epsilon_{\rm ff}$. The cluster formation timescale systematically decreases with increasing $\epsilon_{\rm ff}$. Local variations in the gas accretion history lead to a 0.25~dex scatter for the integral cluster formation efficiency. Joint constraints from all the observables prefer the runs that produce a median integral efficiency of 16\%.