Dark-Matter-Deficient Galaxies from Collisions: A New Probe of Bursty Feedback and Dark Matter Physics

TL;DR

This study probes how dark-matter-deficient galaxies (DMDGs) can form in collisions of gas-rich ultra-diffuse galaxies (UDGs) and whether their demographics encode the underlying core-formation physics. The authors identify the baryonic binding energy $|E_{ m bind}|$ as the governing parameter and compare bursty baryonic feedback against elastic SIDM in controlled hydrodynamical experiments. They find that small reductions in $|E_{ m bind}|$ from bursty feedback can substantially increase DMDG masses (with $\gamma=0.1$ halos producing the most extreme cases) whereas SIDM-form cores do not lower $|E_{ m bind}|$, leading to distinct DMDG demographics. Extending the analysis to host halos shows delayed, environment-dependent tidal stripping producing DF2-like remnants under both scenarios but with different timescales and mass properties, offering a practical observational route to distinguish the two mechanisms; forthcoming wide-field imaging and HI surveys can test these predictions and help constrain dark matter physics and feedback processes.

Abstract

High-velocity collisions between gas-rich ultra-diffuse galaxies present a promising formation channel for dark-matter-deficient galaxies (DMDGs). Using hydrodynamical simulations, we show that the progenitors' baryonic binding energy, $|E_{\rm bind}|$, critically controls the outcome. Repeated potential fluctuations, e.g., from bursty feedback, inject energy and reduce $|E_{\rm bind}|$ by $\approx 15\%$, yielding fewer but substantially more massive DMDGs. By contrast, elastic self-interacting dark matter (SIDM) produces comparable cores without lowering $|E_{\rm bind}|$, perturbing DMDG masses without clear enhancement. This differs from what happens in host halos, where SIDM-induced cores enhance dark matter tidal stripping while keeping baryons compact and resilient to tidal effects. The contrasting roles of SIDM may provide a means to distinguish feedback-formed halo cores from those created by SIDM. Among 15 paired simulation runs, 13 show higher DMDG masses in the weakened-binding case, and about two thirds exhibit $>100\%$ mass enhancements. The simulations also predict systematically lower gas fractions due to sustained post-collision star formation, yielding a clean observational signature. Upcoming wide-field imaging (CSST, LSST), HI surveys (FAST), and kinematic follow-up will be crucial to test this scenario.

Figures (11)

• Figure 1: Effect of energy injection into dark matter halos. Repeated potential variations, such as those induced by bursty feedback, can flatten the inner halo densities (left), shallow the potential (middle), and decrease the absolute baryonic binding energy, $|E_{\rm bind}|$ (right). The final snapshot resembles a $\gamma=0.1$ (red) profile, which features an inner density core and a reduced $|E_{\rm bind}|$ relative to the initial cuspy $\gamma=1$ profile (black), and we adopt this in our simulations. For comparison, we overlay SIDM results (blue) from a simulation with the same initial $\gamma=1$ condition and a cross section per mass $\sigma/m = 20~\rm cm^2/g$. The initial conditions correspond to the 1a and 1b benchmarks listed in \ref{['Tab:1']}.
• Figure 2: Formation of DMDGs from the collision of gas-rich dwarf galaxies. The initial setup (left) has two progenitors approaching at a close separation. They collide within 0.2 Gyr (middle), displacing gas from the halo centers and triggering efficient star formation. By 5 Gyr (right), the expelled baryons have collapsed into several DMDGs that are far from their progenitors. The surface density distributions of dark matter (top), gas (middle), and stars (bottom) are presented at the corresponding snapshots for the 2b ($\gamma=1$) benchmark.
• Figure 3: Effect of tides on extended gas debris. The figure shows the largest eigenvalue ($\lambda_{\max}$) of the tidal tensor on the mid-plane for halo pairs with inner slopes of $\gamma=1$ (top), $\gamma=0.1$ (middle), and a cored SIDM profile (bottom), displayed at separations of $d=\{0,4,8\}~\rm kpc$ (columns). Warm colors denote tidal stretching along at least one principal direction ($\lambda_{\max}>0$), while cold colors indicate fully compressive tides with all eigenvalues negative ($\lambda_{\max}<0$). The halo parameters follow the first benchmark listed in \ref{['Tab:1']}.
• Figure 4: Characteristics of the most massive DMDGs in 15 paired simulations. The bar chart summarizes the ratios of the most massive DMDG masses in the $\gamma = 0.1$ (left) and SIDM (right) scenarios, each normalized relative to the $\gamma = 1$ case. Bars are arranged in ascending order of the DMDG mass ratio, $M_{b,\gamma=0.1}/M_{b,\gamma=1}$ or $M_{b,SIDM}/M_{b,\gamma=1}$. The bar width scales with the relative change in binding energy, $|\Delta E_{{\rm bind}}/E_{{\rm bind},\gamma=1}|$, while the color indicates the gas fraction, $f = M_{\rm gas}/(M_{\rm gas}+M_{\rm stars})$. With lowered gravitational binding, the increase in the DMDG masses is predominantly positive. In 9 out of the 15 simulations, this increase exceeds 100%, as reflected by the bars that rise above the line representing $\Delta M_b/M_{b,\gamma=1}=1$. In the SIDM scenario, mass increases and decreases occur with nearly equal frequency. There is also a weak anti-correlation between the magnitude of mass change and gas fraction.
• Figure 5: Tidal evolution of satellite systems in a $10^{13}~\rm M_{\rm \odot}$ host halo, with the initial conditions taken from the 2a, 2b, and 2c benchmarks from \ref{['Tab:1']} for the $\gamma=1$ (blue), $\gamma=0.1$ (red), and SIDM (orange) scenarios. Left: Evolution of the bound dark matter (solid) and stellar (dashed) masses. The region giving rise to a close DF2 analog is shaded in light purple. Right: Evolution of the stellar-to-halo mass ratio. DMDGs are often selected by $M_{\rm star}/M_{\rm DM}>1$. The shaded area corresponds to the same region as in the left panel.