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1. Warm dark matter chills out: constraints on the halo mass function and the free-streaming length of dark matter with eight quadruple-image strong gravitational lenses

   https://doi.org/10.1093/mnras/stz3480  

   Gilman, Daniel; Birrer, Simon; Nierenberg, Anna; Treu, Tommaso; Du, Xiaolong; Benson, Andrew (February 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The free-streaming length of dark matter depends on fundamental dark matter physics, and determines the abundance and concentration of dark matter haloes on sub-galactic scales. Using the image positions and flux ratios from eight quadruply imaged quasars, we constrain the free-streaming length of dark matter and the amplitude of the subhalo mass function (SHMF). We model both main deflector subhaloes and haloes along the line of sight, and account for warm dark matter free-streaming effects on the mass function and mass–concentration relation. By calibrating the scaling of the SHMF with host halo mass and redshift using a suite of simulated haloes, we infer a global normalization for the SHMF. We account for finite-size background sources, and marginalize over the mass profile of the main deflector. Parametrizing dark matter free-streaming through the half-mode mass mhm, we constrain the thermal relic particle mass mDM corresponding to mhm. At $$95 \, {\rm per\, cent}$$ CI: mhm < 107.8 M⊙ ($$m_{\rm {DM}} \gt 5.2 \ \rm {keV}$$). We disfavour $$m_{\rm {DM}} = 4.0 \,\rm {keV}$$ and $$m_{\rm {DM}} = 3.0 \,\rm {keV}$$ with likelihood ratios of 7:1 and 30:1, respectively, relative to the peak of the posterior distribution. Assuming cold dark matter, we constrain the projected mass in substructure between 106 and 109 M⊙ near lensed images. At $$68 \, {\rm per\, cent}$$ CI, we infer $$2.0{-}6.1 \times 10^{7}\, {{\rm M}_{\odot }}\,\rm {kpc^{-2}}$$, corresponding to mean projected mass fraction $$\bar{f}_{\rm {sub}} = 0.035_{-0.017}^{+0.021}$$. At $$95 \, {\rm per\, cent}$$ CI, we obtain a lower bound on the projected mass of $$0.6 \times 10^{7} \,{{\rm M}_{\odot }}\,\rm {kpc^{-2}}$$, corresponding to $$\bar{f}_{\rm {sub}} \gt 0.005$$. These results agree with the predictions of cold dark matter. 

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2. Effects of Galactic Environment on Size and Dark Matter Content in Low-mass Galaxies

   https://doi.org/10.3847/1538-4357/adbf07  

   Mercado, Francisco J; Moreno, Jorge; Feldmann, Robert; Zeender, Marckie; Benavides, José A; Piotrowska, Joanna M; Klein, Courtney; Wheeler, Coral; Necib, Lina; Bullock, James S; et al (April 2025, The Astrophysical Journal)

   Abstract We utilize the cosmological volume simulation FIREbox to investigate how a galaxy’s environment influences its size and dark matter content. Our study focuses on approximately 1200 galaxies (886 central and 332 satellite halos) in the low-mass regime, with stellar masses between 10⁶and 10⁹M_⊙. We analyze the size–mass relation (r₅₀–M_⋆), the inner dark matter mass–stellar mass ( $M_{\mathrm{DM}}^{50}$ –M_⋆) relation, and the halo mass–stellar mass (M_halo–M_⋆) relation. At fixed stellar mass, we find that galaxies experiencing stronger tidal influences, indicated by higher Perturbation Indices (PI > 1) are generally larger and have lower halo masses relative to their counterparts with lower Perturbation Indices (PI < 1). Applying a Random Forest regression model, we show that both the environment (PI) and halo mass (M_halo) are significant predictors of a galaxy’s relative size and dark matter content. Notably, becauseM_halois also strongly affected by the environment, our findings indicate that environmental conditions not only influence galactic sizes and relative inner dark matter content directly, but also indirectly, through their impact on halo mass. Our results highlight a critical interplay between environmental factors and halo mass in shaping galaxy properties, affirming the environment as a fundamental driver in galaxy formation and evolution. 

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3. Closing the Gap between Observed Low-mass Galaxy H i Kinematics and Cold Dark Matter Predictions

   https://doi.org/10.3847/1538-4357/ad250f  

   Sardone, Amy; Peter, Annika_H G; Brooks, Alyson M; Kaczmarek, Jane (March 2024, The Astrophysical Journal)

   Abstract Testing the standard cosmological model (ΛCDM) at small scales is challenging. Galaxies that inhabit low-mass dark matter halos provide an ideal test bed for dark matter models by linking observational properties of galaxies at small scales (low mass, low velocity) to low-mass dark matter halos. However, the observed kinematics of these galaxies do not align with the kinematics of the dark matter halos predicted to host them, obscuring our understanding of the low-mass end of the galaxy–halo connection. We use deep Hiobservations of low-mass galaxies at high spectral resolution in combination with cosmological simulations of dwarf galaxies to better understand the connection between dwarf galaxy kinematics and low-mass halos. Specifically, we use Hiline widths to directly compare to the maximum velocities in a dark matter halo and find that each deeper measurement approaches the expected one-to-one relationship between the observed kinematics and the predicted kinematics in ΛCDM. We also measure baryonic masses and place these on the baryonic Tully–Fisher relation (BTFR). Again, our deepest measurements approach the theoretical predictions for the low-mass end of this relation, a significant improvement on similar measurements based on line widths measured at 50% and 20% of the peak. Our data also hint at the rollover in the BTFR predicted by hydrodynamical simulations of ΛCDM for low-mass galaxies. 

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4. Quantifying Scatter in Galaxy Formation at the Lowest Masses

   https://doi.org/10.3847/1538-4357/ac0db6  

   Munshi, Ferah; Brooks, Alyson M.; Applebaum, Elaad; Christensen, Charlotte R.; Quinn, T.; Sligh, Serena (December 2021, The Astrophysical Journal)

   Abstract We predict the stellar mass–halo mass (SMHM) relationship for dwarf galaxies, using simulated galaxies with peak halo masses of M peak = 10 11 M ⊙ down into the ultra-faint dwarf range to M peak = 10 7 M ⊙ . Our simulated dwarfs have stellar masses of M star = 790 M ⊙ to 8.2 × 10 8 M ⊙ , with corresponding V -band magnitudes from −2 to −18.5. For M peak > 10 10 M ⊙ , the simulated SMHM relationship agrees with literature determinations, including exhibiting a small scatter of 0.3 dex. However, the scatter in the SMHM relation increases for lower-mass halos. We first present results for well-resolved halos that contain a simulated stellar population, but recognize that whether a halo hosts a galaxy is inherently mass resolution dependent. We thus adopt a probabilistic model to populate “dark” halos below our resolution limit to predict an “intrinsic” slope and scatter for the SMHM relation. We fit linearly growing log-normal scatter in stellar mass, which grows to more than 1 dex at M peak = 10 8 M ⊙ . At the faintest end of the SMHM relation probed by our simulations, a galaxy cannot be assigned a unique halo mass based solely on its luminosity. Instead, we provide a formula to stochastically populate low-mass halos following our results. Finally, we show that our growing log-normal scatter steepens the faint-end slope of the predicted stellar mass function. 

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5. MARVELously Dark: the density profile evolution of dwarf halos in velocity-dependent SIDM

   https://doi.org/10.1088/1475-7516/2026/08/059  

   Engelhardt, Anna; Munshi, Ferah; Peter, Annika HG; Nadler, Ethan O; Cruz, Akaxia; Brooks, Alyson M; Zeng, Zhichao Carton; Quinn, Thomas R; Keith, Blake (August 2026, Journal of Cosmology and Astroparticle Physics)

   Abstract Self-interacting dark matter (SIDM) with a sufficiently large cross section has been shown to naturally produce constant dark matter (DM) cores, as well as core-collapse, at the centers of dwarf halos on cosmic timescales, potentially reducing tensions with observation. Here, we present halos from a new dark matter only (DMO) cosmological (SIDM) simulation: Ms.Marvel DMO with a velocity-dependent self-interaction cross section withσ/m_max= 50 cm²/g atv_max= 35 km/s. We compare these to the CDM suite of Storm simulations including both DMO and dark matter + hydrodynamics runs, in order to test core-formation (and core-collapse) across different dark matter models. We show that Ms.Marvel DMO can reproduce core slopes consistent with observations of isolated dwarf galaxies and more massive (M_vir≳ 10¹⁰M_⊙) CDM dwarf halos that include stellar feedback from the matched CDM run (Storm CDM+baryons). We identify nine Ms.Marvel SIDM DMO halos in the core-collapse phase of gravothermal evolution with halo masses below 2 × 10⁹M_⊙. We find that using the inner density slope to measure the core-collapse timescales of Ms.Marvel DMO halos agrees well with predicted collapse times estimated with the parametric model for SIDM halos introduced by [1]. Additionally, compared to central density, the inner density slope is less sensitive to both the radius of measurement and halo merger history. These results indicate that the slope of the inner DM density profile more cleanly differentiates core-collapsed versus core-forming halos than central density amplitude. 

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