Dwarf diversity in $Λ$CDM with baryons

TL;DR

This study addresses the longstanding problem of rotation-curve diversity in dwarf galaxies within $\Lambda$CDM by employing the Marvelous Massive Dwarf and Marvel Dwarf zoom-in simulations with baryonic physics. By exploring multiple subgrid physics models, including superbubble and blastwave feedback, and conducting NIHAO-like reruns, the authors demonstrate that dwarfs can simultaneously develop dark-matter cores and quickly rising rotation curves, and that the simulated dwarfs reproduce the observed size–$M_*$ scatter. The results reveal a nuanced dependence of RC shape on baryonic distribution and feedback history, with higher central baryon surface density yielding faster rising RCs in higher-$V_{ m max}$ systems, but with substantial scatter and potential non-circular motions in lower-mass dwarfs. The work concludes that the ability to form compact, high-surface-density dwarfs hinges on allowing dense central gas to persist while still sustaining clustered, bursty star formation, highlighting the critical role of subgrid physics in matching observed RC diversity and size distributions under CDM.

Abstract

Observed rotation curves of dwarf galaxies exhibit significant diversity at fixed halo mass, challenging galaxy formation within the Cold Dark Matter (CDM) model. Previous cosmological galaxy formation simulations with baryonic physics fail to reproduce the full diversity of rotation curves, suggesting either that there is a flaw in baryonic feedback models, or that an alternative to CDM must be invoked. In this work, we use the Marvelous Massive Dwarf zoom-in simulations, a suite of high-resolution dwarf simulations with $M_{200}~\sim 10^{10}-10^{11}$ ${\rm M}_{\odot}$ and $M_{*}\sim 10^{7}-10^{9}$ ${\rm M}_{\odot}$, designed to target the mass range where galaxy rotation curve diversity is maximized, i.e., between $V_{\rm max} \sim 70-100~ {\rm km/s}$. We add to this a set of low-mass galaxies from the Marvel Dwarf Zoom Volumes to extend the galaxy mass range to lower values. Our fiducial star formation and feedback models produce simulated dwarfs with a broader range of rotation curve shapes, similar to observations. These are the first simulations that can both create dark matter cores via baryonic feedback, reproducing the slower rising rotation curves, while also allowing for compact galaxies and steeply rising rotation curves. Our simulated dwarfs also reproduce the observed size$-M_*$ relation, including scatter, producing both extended and compact dwarfs for the first time in simulated field dwarfs. We explore star formation and feedback models and conclude that previous simulations may have had feedback that was too strong to produce compact dwarfs.

Figures (13)

• Figure 1: Mock UVI images of 40 of the Massive Dwarf zoom-in galaxies at $z = 0$, ordered by decreasing $M_{*}$ from top left to bottom right. The zoom-in name is annotated in the upper left of each subpanel. Each galaxy is shown face-on (i.e., with the angular momentum vector out of the page), and each square panel has a side length of 15 kpc. Visually, it is clear that the Massive Dwarf zooms have diverse morphologies.
• Figure 2: Rotation curves for sample galaxies from SPARC with $V_{\rm max} \sim 80$--$90$ km/s. Full SPARC RCs are shown in dark teal. In each panel, a simulated Massive Dwarf RC with similar shape and $V_{\rm max}$ to the SPARC RC is shown using spherical symmetry (Equation \ref{['eqn:vcirc']}) with a dashed line and from the midplane potential (Equation \ref{['eqn:vmid']}) with a solid line. The full simulated RCs are shown for both cases with dark gray lines ($V^{\rm circ}$ and $V^{\rm mid}$), and the HI-limited RCs are shown in black ($V^{\rm circ+HI}$ and $V^{\rm mid+HI}$). An NFW RC with $V_{\rm max} \sim 87~{\rm km/s}$, intermediate to the observed RCs, is shown with scatter in the NFW mass-concentration relationship by the light-gray shaded region. For the two simulations with disks (r492 and r569), the rotation speeds from the midplane potential peak higher and at larger radii than the RCs from the mass-enclosed method. Both the midplane and mass-enclosed RCs exhibit diversity that mimics the observed RCs; the RCs in the upper-left and lower-right panels are more quickly rising than the NFW case. In contrast, the upper right is more slowly rising, and the lower left is in rough agreement with the NFW RC.
• Figure 3: Fiducial velocity versus maximum circular velocity for SPARC in gray diamonds, the Massive Dwarf zooms in purple circles, and the Marvel dwarfs in teal squares. The NFW prediction is shown by the gray line with the shaded region representing the root-mean-square scatter in the mass-concentration relation reported in Ludlow2016. The dashed black $1:1$ line represents where $V_{\rm fid}$ is equal to $V_{\rm max}$. The results correspond to the following velocities $V^{\rm circ+HI}$ (upper left), $V^{\rm circ}$ (upper right), $V^{\rm mid+HI}$ (lower left), and $V^{\rm mid}$ (lower right). For some of the lowest-mass Marvel galaxies, there is limited HI and $R_{1} < r_{\rm conv}$; we mark these galaxies with black outlines on the teal squares to denote that they are HI-limited and excluded from the left panels. A larger spread in inner velocities at $r_{\rm fid}$ at a fixed $V_{\rm max}$ is predicted for the midplane potential compared to the spherical case. RCs that are more quickly and more slowly rising than NFW are predicted with the midplane method down to $V_{\rm max}^{\rm mid+HI} \sim 50$ km/s in the Massive Dwarf zooms.
• Figure 4: Left:Midplane RCs colored by baryonic surface density. The RCs are cut off at $R_1$ and are chosen in a $V_{\rm max}$ range that matches with Ren2019. The HI-limited midplane method is used to calculate $V_{\rm fid}$ and $V_{\rm max}$. The most quickly rising RCs (violet) have higher surface densities, whereas the more slowly rotating RCs (orange) have lower surface densities. This trend is consistent with observed RCs in the same $V_{\rm max}$ range Lelli2016. Right:$\eta_{\rm rot}$ versus $\Sigma_{\rm bar}$, with simulated galaxies colored by $V_{\rm max}$ and SPARC data in gray. In the simulated and observed galaxies, higher-mass systems have higher baryonic surface densities and have RCs that tend to rise more quickly. There is large scatter in $\eta_{\rm rot}$ at lower $\Sigma_{\rm bar}$. Systems with $V_{\rm max} \sim 79$--91 km/s that appear in the left panel are highlighted with star markers.
• Figure 5: $\eta_{\rm rot}$ versus baryon mass fraction within $r_{\rm fid}$, colored by $V_{\rm max}$, for Left: SPARC data in thin colored diamonds with mass modeling from Lelli2016. Objects in the upper-left region have low central baryon mass fractions, but are still more quickly rising in their central regions than the NFW prediction. Previous CDM and baryon simulations have struggled to reproduce these objects. This is demonstrated for NIHAO data SantosSantos2020 in light blue diamonds, including galaxies with $V_{\rm max} > 100$ km/s and measured using enclosed mass. Right: The Massive Dwarf zooms in circles and the Marvel sample in squares, using the midplane potential to calculate velocities. Most of the simulated galaxies have $\eta_{\rm bar} \lesssim 0.5$. There is a significant spread in $\eta_{\rm rot}$ in this region, including some systems with $\eta_{\rm rot} \gtrsim 0.75$, higher than the NFW prediction.