CROCODILE-DWARF: Assembly and Kinematics of Field Dwarf Galaxies with GADGET4-OSAKA

TL;DR

This work investigates how assembly history drives the diversity of field dwarf galaxies in a cosmological context. Using CROCODILE-DWARF zoom-in simulations with gadget4-osaka and enhanced SN feedback, the authors map SHMR, MZR, sizes, and gas/stellar kinematics for halos around 10^10 solar masses, revealing that early-assembled, high-concentration halos become gas-poor and dispersion-dominated, while late-assembling halos remain gas-rich and can host rotating disks after mergers. A robust anti-correlation between gas rotational support and cumulative merger mass fraction shows that merger-driven dynamical heating is the dominant factor shaping kinematic diversity, with rare late major mergers capable of forming extended gas disks. The results emphasize assembly history as a critical driver of dwarf galaxy properties, offering testable predictions for upcoming surveys and informing feedback implementations in the low-mass regime.

Abstract

We present results from CROCODILE-DWARF, a suite of cosmological zoom-in hydrodynamic simulations of isolated field dwarf galaxies with halo masses of $\sim10^{10}\,M_\odot$ at $z=0$, performed with the \textsc{gadget4-osaka} code. The simulations include detailed modeling of star formation, chemical enrichment, and supernova feedback using the \textsc{CELib} and \textsc{grackle} libraries, achieving baryonic resolutions of $\sim2\times10^3\,M_\odot$. Our study focuses on how assembly history governs the structural and kinematic diversity of dwarf galaxies within the $Λ$CDM framework. The simulated galaxies reproduce the observed stellar-to-halo mass, mass--metallicity, and size--mass relations, yielding stellar masses of $10^6-10^8\,M_\odot$ and metallicities consistent with those of Local Group dwarf galaxies. The galaxies display a broad range of rotational support, where gas is generally more rotationally supported than stars. Differences in morphology and kinematics primarily reflect variations in halo assembly timescales and merger activity. Early-assembling, high-concentration halos form stars efficiently and become gas-poor by $z=0$, while late-assembling, low-concentration halos remain gas-rich due to delayed star formation and rejuvenated gas accretion. We identify a clear anti-correlation between rotational support and the cumulative merger mass fraction, demonstrating that dynamical heating by mergers is the dominant factor shaping kinematic diversity. In some cases, late-time mergers induce the formation of extended gas disks by delivering fresh gas and angular momentum. These results demonstrate that assembly history, rather than halo mass alone, critically shapes the present-day kinematic and morphological diversity of dwarf galaxies.

Figures (11)

• Figure 1: Stellar-to-halo mass relation at $z=0$, with evolutionary tracks for dwarf galaxies in our simulations. We highlight two sample galaxies, AGORA-1e10q and AGORA-1e10v, and show their relations at redshifts $z=0.25$, $0.5$, $1.0$, $2.0$, and $5.0$. Our results are compared with empirical relations derived from abundance matching and their extrapolations by Behroozi2013 and Moster2013 at $z=0.1$, as well as with the relation inferred from H i rotation curve observations of field dwarf galaxies Read2017. For reference, we also show results from other simulations in the literature, along with lines corresponding to $10\,\%$ and $1\,\%$ of the baryon fraction of the universe $f_\mathrm{bar}$.
• Figure 2: Mass--metallicity relations at $z=0$ for dwarf galaxies in our simulations. The top panel shows the relation for gas, while the bottom panel shows that for stars. In the bottom panel, light-colored small triangles, small pluses, and error bars represent the mean, median, and 25th--75th percentile ranges of stellar metallicities of star particles, respectively. Observational data for gas are taken from the SDSS sample AndrewsMartini2013, the DESI sample Scholte2024, and individual nearby galaxies Berg2012, while those for stars are from Local Group dwarf galaxies Kirby2013Kirby2020Vargas2014 and Fornax dwarf galaxies Romero-Gomez2023.
• Figure 3: Stellar half-mass radius as a function of stellar mass for dwarf galaxies in our simulations. The observational data are from nearby dwarf galaxies McConnachie2012
• Figure 4: Median orbital circularity parameter of gas (circle) and stars (square) at $z=0$ as a function of stellar mass for dwarf galaxies in our simulations.
• Figure 5: Left panel: Evolution of the stellar-to-halo mass ratio over time for dwarf galaxies in our simulations. Right panel: Relation between the halo concentration in the dark-matter-only re-simulations and the stellar-to-halo mass ratio at $z=0$. Lines on the left panel and symbols on the right panel are color-coded by the gas fraction of galaxies $f_\mathrm{gas}=M_\mathrm{gas}/(M_\mathrm{gas}+M_\star)$ at $z=0$.