Disk Formation and the Size-sSFR Relation of Dwarf Galaxies

TL;DR

This study investigates how isolated dwarf galaxies grow in size and form rotation-supported stellar disks within a cosmological context by analyzing 39 high-resolution zoom-in dwarfs from the Marvelous Massive Dwarfs suite. Using detailed measurements of angular momentum transfer, gas accretion, and merger dynamics, the authors identify a dichotomy: compact dwarfs remain small ($R_e < 2$ kpc) while extended dwarfs grow secularly through the buildup of AM-supported stellar disks that originate from gas-rich mergers on high-angular-momentum, spiraling-in orbits. They find that about 30% of the z=0 cold gas and ~33% of the star-forming gas were contributed by merging satellites, and that disk formation requires both a sustained gas supply and a quiescent merger history to preserve the disk. The work places these results in context with simulations like FIRE and IllustrisTNG, highlighting the central role of angular momentum transfer from satellites in driving disk growth and shaping the size–sSFR relation in low-mass galaxies, with implications for interpreting observations from upcoming surveys. All mathematical notation is presented with appropriate delimiters, and the findings offer a concrete mechanism for the emergence of extended disks in dwarf galaxies and a framework for future observational tests.

Abstract

Dwarf galaxies are dark matter-dominated systems that are sensitive to feedback and display a diversity of baryonic morphologies. This makes them excellent probes for understanding dark matter and galaxy evolution. This work investigates the physical processes that influence the sizes of isolated dwarf galaxies using high-resolution cosmological zoom-in simulations of $39$ dwarf galaxies drawn from the Marvelous Massive Dwarfs simulation suite ($7.5 < \log(M_{\star}/M_{\odot}) < 9.1$). Our simulations show that dwarf galaxies initially form as compact galaxies ($R_e < 2$ kpc). However, several of these galaxies ($54\%$) experience periods of gradual size growth at relatively stable sSFR, allowing them to become extended galaxies. We find that the growth of rotation-supported stellar disks is the primary means by which isolated dwarfs become extended in size. These stellar disks are formed by mergers with high orbital angular momentum satellites on high angular momentum (spiraling-in) orbits, which spin up the gas surrounding the central galaxy and contribute $\approx 30 \%$ of the cold gas mass at $z=0$. For these systems, star formation in the angular momentum supported gas and the gradual build up of stars in the disk result in secular size growth.

Figures (14)

• Figure 1: The mass-size, mass-SFR, and size-sSFR relations of our final sample of galaxies at present day (z=$0$). We add a $2$ kpc reference line in the mass-size panel (further discussed in Section \ref{['sub:Secular Size Growth']}). In the size-sSFR panel, we include galaxies from patel18 for reference. Mass-size measurements by Cruz2025 of our sample indicate that the Marvelous Massive Dwarfs simulations agree better with SPARC SPARK in the size-mass plane than other simulations 2019MNRAS.487.5272J, and with the size-mass relationship for dwarfs as derived in Carlsten2021.
• Figure 2: The evolution of galaxies in our compact (top row) and extended (bottom row) sub-samples. Similar to Figure \ref{['fig:z0_tri_panel']}, the first three columns show the mass-size, mass-SFR, and size-sSFR relations for every galaxy in each sub-sample, at every snapshot up to $z=3$. The last column shows the evolution of a single galaxy example in each sub-sample across cosmic time. In the compact sample, galaxies generally maintain a small size of less than $2$ kpc (with some spikes above $2$ kpc due to halo finder confusion). Extended dwarf galaxies begin as compact galaxies but eventually experience secular size growth over time at relatively stable sSFRs.
• Figure 3: The evolution of half-light radii (top) and stellar masses (bottom) of galaxies in our final sample. We show these quantities up to $z=3$. We do not filter out halo finder confusion in which mergers appear as spikes in size right before coalescence (usually appears before $6$ Gyr). compact galaxies are shown in red and extended galaxies are in blue. We provide a histogram of both quantities at z=$0$ to the right of each panel. The compact and extended galaxies are separated in size by definition and cover a range of up to $4$ kpc. Though the two sub-samples are bifurcated by size, they cover a diverse range of masses. In particular, compact galaxies span both the low and high end of the stellar mass range.
• Figure 4: Our disk and not-disk sub-samples. The first two panels from the left show the size-sSFR and size-$j_\star$ (where $j_\star \equiv |\vec{j_\star}|$) relations. The last two panels show stellar axis ratios of our sample measured at $R_e$Keith2025 as a function of size ($A$ is the semi-major axis, $C$ is the semi-minor axis, and $B$ is the intermediate axis). Disk galaxies are shown in orange, while galaxies that are not-disks are in black. Almost all extended galaxies are disk galaxies, and disk galaxies tend to have higher stellar $j_\star$ since they are AM supported systems.
• Figure 5: Evolution in galaxy properties for r492 (extended disk), r968 (compact disk), and r615 (disrupted disk). We show the absolute value of the median $\hat{j_x}$ and $\hat{j_y}$ components in gray and $\hat{j_z}$ in solid blue for all the stars in the galaxy. We also include the absolute value of the median $\hat{j}_{z,100{\rm Myr}}$ for stars that formed within $100$ Myr of the snapshot as a dashed blue line. The half-light radius, normalized by the maximum value, is displayed in orange. To indicate the point at which the galaxy transitions into an extended galaxy, we include an orange horizontal line corresponding to $2$ kpc. The stellar mass and gas mass within the virial radius normalized to their maximum values are shown as solid and dashed teal lines, respectively. Lastly, the beginning of mergers that are responsible for morphological transformations are shown as red lines.