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

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.

💡 Research Summary

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The paper investigates a novel formation channel for dark‑matter‑deficient galaxies (DMDGs) through high‑velocity collisions of gas‑rich ultra‑diffuse galaxies (UDGs). Using a suite of controlled hydrodynamical simulations with the Gadget‑4 code, the authors identify the baryonic binding energy of the progenitors, (|E_{\rm bind}|), as the decisive parameter that governs whether a collision will produce a massive DMDG.

Physical picture and methodology
Two families of initial conditions are constructed: (i) cuspy dark‑matter halos with inner slope (\gamma=1) (strong binding) and (ii) cored halos with (\gamma=0.1) (weakened binding). Both have identical total halo mass ((M_{200}=1!-!2\times10^{10},M_\odot)) and concentration ((c=4!-!14)). The gas component is modeled as a rotating exponential disk (mass fraction 0.1–0.22, scale radius 1.5–3.5 kpc, vertical thickness 0.15–0.35 kpc) stabilized with Toomre‑(Q>1.5). Collisions are set with relative velocities (v_r=300!-!600) km s(^{-1}) (fiducial 400 km s(^{-1})), a separation of 60 kpc and an impact parameter of 2 kpc.

In addition to the CDM runs, a set of simulations includes elastic self‑interacting dark matter (SIDM) with a cross‑section per unit mass (\sigma/m=20) cm(^2) g(^{-1}). SIDM cores are generated in isolation for 2 Gyr before the collision, allowing a direct comparison between feedback‑driven cores and SIDM‑driven cores while keeping all other physics identical.

Energy injection model
The authors model bursty stellar feedback as a series of impulsive potential fluctuations, each rescaling the gravitational potential by (\Delta V/V_0=0.14). Using analytic expressions for the change in particle energy (Eq. 1) and reconstructing the density profile after each impulse, they demonstrate that such fluctuations flatten the inner density, shallow the potential, and reduce (|E_{\rm bind}|) by roughly 10–15 % after 2 Gyr (Figure 1). By contrast, the SIDM run produces a core of comparable size but the potential remains deep; (|E_{\rm bind}|) actually rises slightly.

Results
Fifteen paired simulations (each pair: one with (\gamma=1), one with (\gamma=0.1), plus the SIDM counterpart) are summarized in Table 1. Key findings are:

1. Weakened binding ((\gamma=0.1)) dramatically boosts DMDG mass. In 13 of the 15 pairs the DMDG formed from the cored progenitors is more massive than its cuspy counterpart. About two‑thirds of the cases show mass enhancements exceeding 100 %. The number of DMDGs per run is lower (fewer fragments) but each fragment is substantially heavier.

2. SIDM cores do not increase DMDG mass. Although SIDM creates a central density core, the unchanged (or slightly increased) (|E_{\rm bind}|) means that the gas is not as easily expelled during the collision. Consequently, DMDG masses are comparable to or smaller than the CDM‑cusp case, and no systematic mass boost is observed.

3. Gas fractions are systematically lower in the weakened‑binding runs. Post‑collision star formation, sustained for several Gyr, consumes a large fraction of the expelled gas. The resulting DMDGs have gas‑to‑stellar mass ratios 30–50 % lower than in the SIDM or cuspy cases. This provides a clean observational discriminator.

4. Tidal analysis. By evaluating the largest eigenvalue (\lambda_{\rm max}) of the tidal tensor (-\nabla\nabla\Phi) on the mid‑plane, the authors show that a reduced (|E_{\rm bind}|) makes the gas debris more susceptible to tidal stretching and subsequent collapse, enhancing star formation efficiency during the collision.

Interpretation and implications
The study argues that the baryonic binding energy is the controlling variable linking energy injection (via bursty feedback) to DMDG production. Feedback‑driven core formation simultaneously lowers (|E_{\rm bind}|) and makes the gas more vulnerable to tidal removal, leading to massive, gas‑poor DMDGs. SIDM, by contrast, modifies only the dark‑matter distribution while leaving the baryonic binding essentially unchanged; thus it cannot replicate the same DMDG mass boost.

This distinction offers a novel observational test of dark‑matter physics. If a population of field DMDGs exhibits (i) low gas fractions, (ii) recent star‑formation signatures (blue colors, elevated Hα/UV), and (iii) relatively large effective radii with low central velocity dispersions, it would favor a feedback‑driven core scenario. Conversely, a DMDG sample with normal gas content and older stellar populations would be more consistent with SIDM‑induced cores.

Observational prospects
The authors highlight upcoming facilities: the Chinese Space Station Telescope (CSST) and the Vera C. Rubin Observatory (LSST) for wide‑field imaging, the Five‑hundred‑meter Aperture Spherical Telescope (FAST) for HI surveys, and integral‑field spectrographs (e.g., MUSE, KCWI) for kinematic follow‑up. Joint measurements of stellar kinematics, gas content, and star‑formation histories could directly test the predicted signatures and, crucially, discriminate between bursty feedback and SIDM as the origin of cores in dwarf‑scale halos.

Conclusion
The paper establishes that the reduction of baryonic binding energy by ~15 % via bursty feedback is the key lever that turns high‑speed UDG collisions into efficient factories of massive, gas‑poor DMDGs. SIDM produces comparable dark‑matter cores but does not lower (|E_{\rm bind}|) and therefore fails to generate the same DMDG mass enhancement. This duality provides a concrete, observable pathway to probe both stellar feedback processes and the microphysics of dark matter on dwarf‑galaxy scales.