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1. Globular Clusters and Streaming Velocities: Testing the New Formation Channel in High-resolution Cosmological Simulations

   https://doi.org/10.3847/1538-4357/ac27aa  

   Schauer, Anna T. P.; Bromm, Volker; Boylan-Kolchin, Michael; Glover, Simon C. O.; Klessen, Ralf S. (November 2021, The Astrophysical Journal)

   Abstract The formation of globular clusters and their relation to the distribution of dark matter have long puzzled astronomers. One of the most recently proposed globular cluster formation channels ties ancient star clusters to the large-scale streaming velocity of baryons relative to dark matter in the early universe. These streaming velocities affect the global infall of baryons into dark matter halos, the high-redshift halo mass function, and the earliest generations of stars. In some cases, streaming velocities may result in dense regions of dark matter-free gas that becomes Jeans unstable, potentially leading to the formation of compact star clusters. We investigate this hypothesis using cosmological hydrodynamical simulations that include a full chemical network and the formation and destruction of H₂, a process crucial for the formation of the first stars. We find that high-density gas in regions with significant streaming velocities is indeed somewhat offset from the centers of dark matter halos, but this offset is typically significantly smaller than the virial radius. Gas outside of dark matter halos never reaches Jeans-unstable densities in our simulations. We postulate that low-level (Z≈ 10⁻³Z_⊙) metal enrichment by Population III supernovae may enable cooling in the extra-virial regions, allowing gas outside of dark matter halos to cool to the cosmic microwave background temperature and become Jeans unstable. Follow-up simulations that include both streaming velocities and metal enrichment by Population III supernovae are needed to understand if streaming velocities provide one path for the formation of globular clusters in the early universe. 

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2. Primordial Black Hole Dark Matter Simulations Using PopSyCLE

   https://doi.org/10.3847/1538-4357/ad3df0  

   Pruett, Kerianne; Dawson, William; Medford, Michael S; Lu, Jessica R; Lam, Casey; Perkins, Scott; McGill, Peter; Golovich, Nathan; Chapline, George (July 2024, The Astrophysical Journal)

   Abstract Primordial black holes (PBHs), theorized to have originated in the early Universe, are speculated to be a viable form of dark matter. If they exist, they should be detectable through photometric and astrometric signals resulting from gravitational microlensing of stars in the Milky Way. Population Synthesis for Compact-object Lensing Events, orPopSyCLE, is a simulation code that enables users to simulate microlensing surveys, and is the first of its kind to include both photometric and astrometric microlensing effects, which are important for potential PBH detection and characterization. To estimate the number of observable PBH microlensing events, we modifyPopSyCLEto include a dark matter halo consisting of PBHs. We detail our PBH population model, and demonstrate ourPopSyCLE+ PBH results through simulations of the Optical Gravitational Lensing Experiment-IV (OGLE-IV) and Nancy Grace Roman Space Telescope (Roman) microlensing surveys. We provide a proof-of-concept analysis for adding PBHs intoPopSyCLE, and thus include many simplifying assumptions, such asf_DM, the fraction of dark matter composed of PBHs, and ${\overline{m}}_{\mathrm{PBH}}$, mean PBH mass. Assuming ${\overline{m}}_{\mathrm{PBH}}=30$M_⊙, we find ∼3.6f_DMtimes as many PBH microlensing events than stellar evolved black hole events, a PBH average peak Einstein crossing time of ∼91.5 days, estimate on order of 10²f_DMPBH events within the 8 yr OGLE-IV results, and estimate Roman to detect ∼1000f_DMPBH microlensing events throughout its planned microlensing survey. 

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3. Free streaming of warm wave dark matter in modified expansion histories

   https://doi.org/10.1088/1475-7516/2025/06/043  

   Long, Andrew J; Venegas, Moira (June 2025, Journal of Cosmology and Astroparticle Physics)

   In models of warm dark matter, there is an appreciable population of high momentum particles in the early universe, which free stream out of primordial over/under densities, thereby prohibiting the growth of structure on small length scales. The distance that a dark matter particle travels without obstruction, known as the free streaming length, depends on the particle's mass and momentum, but also on the cosmological expansion rate. In this way, measurements of the linear matter power spectrum serve to probe warm dark matter as well as the cosmological expansion history. In this work, we focus on ultra-light warm wave dark matter (WWDM) characterized by a typical comoving momentumq_*and massm. We first derive constraints on the WWDM parameter space (q_*,m) using Lyman-αforest observations due to a combination of the free-streaming effect and the white-noise effect. We next assess how the free streaming of WWDM is affected by three modified expansion histories: early matter domination, early dark energy, and very early dark energy. 

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4. Milky Way Satellite Census. IV. Constraints on Decaying Dark Matter from Observations of Milky Way Satellite Galaxies

   https://doi.org/10.3847/1538-4357/ac6e65  

   Mau, S.; Nadler, E. O.; Wechsler, R. H.; Drlica-Wagner, A.; Bechtol, K.; Green, G.; Huterer, D.; Li, T. S.; Mao, Y. -Y.; Martínez-Vázquez, C. E.; et al (June 2022, The Astrophysical Journal)

   Abstract We use a recent census of the Milky Way (MW) satellite galaxy population to constrain the lifetime of particle dark matter (DM). We consider two-body decaying dark matter (DDM) in which a heavy DM particle decays with lifetimeτcomparable to the age of the universe to a lighter DM particle (with mass splittingϵ) and to a dark radiation species. These decays impart a characteristic “kick velocity,”V_kick=ϵc, on the DM daughter particles, significantly depleting the DM content of low-mass subhalos and making them more susceptible to tidal disruption. We fit the suppression of the present-day DDM subhalo mass function (SHMF) as a function ofτandV_kickusing a suite of high-resolution zoom-in simulations of MW-mass halos, and we validate this model on new DDM simulations of systems specifically chosen to resemble the MW. We implement our DDM SHMF predictions in a forward model that incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk using an empirical model for the galaxy–halo connection. By comparing to the observed MW satellite population, we conservatively exclude DDM models withτ< 18 Gyr (29 Gyr) forV_kick= 20 kms⁻¹(40 kms⁻¹) at 95% confidence. These constraints are among the most stringent and robust small-scale structure limits on the DM particle lifetime and strongly disfavor DDM models that have been proposed to alleviate the Hubble andS₈tensions. 

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5. Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf

   https://doi.org/10.3847/1538-4357/ad9b89  

   Foote, Hayden_R; Besla, Gurtina; Garavito-Camargo, Nicolás; Patel, Ekta; Thomas, Guillaume_F; Bonaca, Ana; Price-Whelan, Adrian_M; Peter, Annika_H_G; Zaritsky, Dennis; Conroy, Charlie (January 2025, The Astrophysical Journal)

   Abstract Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2σwidth) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( $O\left(1\right)$kpc) than are probed by its stars ( $O\left(10\right)$pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 10⁹M_⊙, the CPS's velocity dispersion will be ∼ 40 km s⁻¹larger atϕ₁ > 20° than atϕ₁ < 0°. If no such heating is detected, Segue 2’s mass cannot exceed 10⁹M_⊙within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations. 

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