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Abstract This work explores how assumptions regarding the particle-physics nature of dark matter can alter the evolution of the Sagittarius (Sgr) dwarf spheroidal galaxy and its expansive stellar stream. We run a large suite ofN-body simulations to model the infall of a spherically symmetric Sgr-like dwarf, exploring how the presence of dark matter self-interactions impacts its evolution. For a scattering cross section ofσ/m_χ = 30 cm²g⁻¹(at orbital velocity scales), these interactions result in significantly less stellar mass and little to no dark matter bound to the progenitor at the present day. To isolate the cause of this mass loss, we introduce a novel technique for controlling which pairs of dark matter simulation particles can interact. This enables us to identify ram-pressure evaporation—the scattering of satellite and host dark matter particles—as the primary source of the enhanced mass loss. The rapid disintegration of the Sgr progenitor when self-interactions are allowed alters some key properties of the resulting stellar stream, most dramatically suppressing the presence of a “spur” on the apocenter of the trailing stream arm that correlates with the mass of the satellite at last pericenter. We demonstrate how the effects on the Sgr system scale with the particular choice of self-interaction cross section, which affects the degree of ram-pressure evaporation. These findings generalize beyond the Sgr system, underscoring that dwarf stellar streams and dwarf galaxies with close passages may serve as sensitive probes for dark matter self-interactions. 

Abstract Stellar streams from disrupted globular clusters are dynamically cold structures that are sensitive to perturbations from dark matter subhalos, allowing them in principle to trace the dark matter substructure in the Milky Way. We model, within the context of Λ cold dark matter, the likelihood of dark matter subhalos to produce a significant feature in a GD-1-like stream and analyze the properties of such subhalos. We generate many realizations of the subhalo population within a Milky Way mass host halo using the semianalytic codeSatGen, accounting for effects such as tidal stripping and dynamical friction. The subhalo distributions are combined with a GD-1-like stream model, and the impact of subhalos that pass close to the stream are modeled withGala. We find that subhalos with masses in the range 2 × 10⁶M_⊙–10⁸M_⊙at the time of the stream–subhalo encounter, corresponding to masses of about 2 × 10⁷M_⊙–10⁹M_⊙at the time of infall, are the likeliest to produce gaps in a GD-1-like stream. We find that gaps occur on average ∼3 times per realization of the host system. These gaps have typical widths of ∼(5–27)° and fractional underdensities of ∼(10–30)%, with larger gaps being caused by heavier subhalos. The stream–subhalo encounters responsible for these have impact parameters (0.1–1.5) kpc and relative velocities ∼(200–410) km s⁻¹. We also investigate the effects of increasing the host-halo mass on the gap properties and formation rate. 

Abstract Models of a dark radiation sector with a mass threshold (WZDR+) have proved to be an appealing alternative to ΛCDM. These models provide simple comparison models, grounded in well-understood particle physics and with limited additional parameters. In addition, they have shown relevance in easing existing cosmological tensions, specifically theH₀tension and theS₈tension. Recently, measurements of CMB lensing by the ACT collaboration have provided strong additional information on clustering at late times. Within ΛCDM, these results yield a high value ofS₈at odds with weak-lensing measurements. In this work, we study this in the context of WZDR+, and find a much wider range of allowed values ofS₈, and in particular much better agreement between data sets and an overall improvement of fit versus ΛCDM. We expand our analyses to include a wide set of data, including the ACT-DR6 lensing data, as well as primary CMB information from ACT-DR4 and SPT-3G, scale-dependent power spectra from DES and measurements ofH₀from SH0ES. We find that there is little to no tension in measurements of structure within the data sets, and the inferred value ofS₈is generally lower than that in ΛCDM. We find that the inclusion of DES generally favors a higherH₀, but there is some direct tension between the high-ℓ multipole data and this result. Future data should clarify whether this is a statistical artifact, or a true incompatibility of these datasets within this model.