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Galactic rotation curves exhibit diverse behavior in the inner regions while obeying an organizing principle; i.e., they can be approximately described by a radial acceleration relation or the modified Newtonian dynamics phenomenology. We analyze the rotation curve data from the SPARC sample and explicitly demonstrate that both the diversity and uniformity are naturally reproduced in a hierarchical structure formation model with the addition of dark matter self-interactions. The required concentrations of the dark matter halos are fully consistent with the concentration-mass relation predicted by the Planck cosmological model. The inferred stellar mass-to-light (3.6μm) ratios scatter around0.5M⊙/L⊙, as expected from population synthesis models, leading to a tight radial acceleration relation and a baryonic Tully-Fisher relation. The inferred stellar-halo mass relation is consistent with the expectations from abundance matching. These results provide compelling arguments in favor of the idea that the inner halos of galaxies are thermalized due to dark matter self-interactions.

The LIGO–Virgo–KAGRA collaboration has detected over 150 confirmed gravitational-wave events through Observing Run 4a. Binary black hole (BBH) systems represent the overwhelming majority of these observations. We construct a model for the population of BBHs based on the distribution of metallicities in galaxies and state-of-the-art stellar evolution models implemented through the Stellar Evolution N-body code. We calculate the redshift evolution of the total merger rate of BBHs and the differential rates with respect to primary mass, secondary mass, and the mass ratio. We explore variations in the delay-time distribution’s power-law index and show that it affects the total merger rate’s spectral shape, but primarily acts as an amplitude shift on the differential rates. When comparing to the primary mass distribution, our results indicate that either the average initial mass function in dwarf galaxies must be top heavy, or most of the 30–40 M⊙ BHs must be formed through a dynamical capture mechanism. For masses greater than about 50 M⊙, the predicted number of BBH systems plummets to zero, revealing the well-known mass gap due to the pair instability mechanism and mass loss in binary systems.

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 code SatGen, 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 with Gala. We find that subhalos with masses in the range 2 × 106 M⊙–108 M⊙ at the time of the stream–subhalo encounter, corresponding to masses of about 2 × 107 M⊙–109 M⊙ 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.

Milky way satellites: galaxies with a dark side Sophisticated optical astronomy projects such as the Sloan Digital Sky Survey are reaching a new threshold in detecting the least luminous galaxies in the Universe, and now at least twenty-three faint satellite galaxies are known in the region of the Milky Way. They range in luminosity from about a thousand to more than 100 million times that of the Sun. The velocities of the stars in these galaxies reveal that despite this variation in luminosity, each of the galaxies is similar in mass, at about 10 million times the mass of the Sun within their central 300 parsecs. The faintest of the Milky Way satellites are accordingly the most dark-matter-dominated galaxies known in the Universe. The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered 1 , 2 in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales 3 , 4 and probes the nature of the dark matter that dominates the mass density of the Universe 5 . Here we use new measurements of the velocities of the stars in these galaxies 6 , 7 to show that they are consistent with them having a common mass of about 10 7 within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark-matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

We analyze circular velocity profiles of seven ultradiffuse galaxies (UDGs) that are isolated and gas-rich. Assuming that the dark matter halos of these UDGs have a Navarro–Frenk–White (NFW) density profile or a Read density profile (which allows for constant-density cores), the inferred halo concentrations are systematically lower than the cosmological median, even as low as −0.6 dex (about 5σ away) in some cases. Alternatively, similar fits can be obtained with a density profile that scales roughly as 1/r 2 for radii larger than a few kiloparsecs. Both solutions require the radius where the halo circular velocity peaks ( Rmax ) to be much larger than the median expectation. Surprisingly, we find an overabundance of such large- Rmax halos in the IllustrisTNG dark-matter-only simulations compared to the Gaussian expectation. These halos form late and have higher spins compared to median halos of similar masses. The inner densities of the most extreme among these late-forming halos are higher than their NFW counterparts, leading to a ∼1/r 2 density profile. However, the two well-resolved UDGs in our sample strongly prefer lower dark matter densities in the center than the simulated ones. Comparing to IllustrisTNG hydrodynamical simulations, we also find a tension in getting both low enough circular velocities and high enough halo mass to accommodate the measurements. Our results indicate that the gas-rich UDGs present a significant challenge for galaxy formation models.

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Semi-analytic modeling furnishes an efficient avenue for characterizing the properties of dark matter halos associated with satellites of Milky Way-like systems, as it easily accounts for uncertainties arising from halo-to-halo variance, the orbital disruption of satellites, baryonic feedback, and the stellar-to-halo mass (SMHM) relation. We use the SatGen semi-analytic satellite generator -- which incorporates both empirical models of the galaxy-halo connection in the field as well as analytic prescriptions for the orbital evolution of these satellites after they enter a host galaxy -- to create large samples of Milky Way-like systems and their satellites. By selecting satellites in the sample that match the observed properties of a particular dwarf galaxy, we can then infer arbitrary properties of the satellite galaxy within the Cold Dark Matter paradigm. For the Milky Way's classical dwarfs, we provide inferred values (with associated uncertainties) for the maximum circular velocity \\(v_max\\) and the radius \\(r_max\\) at which it occurs, varying over two choices of feedback model and two prescriptions for the SMHM relation that populate dark matter halos with physically distinct galaxies. While simple empirical scaling relations can recover the median inferred value for \\(v_max\\) and \\(r_max\\), this approach provides realistic correlated uncertainties and aids interpretability through variation of the model. For these different models, we also demonstrate how the internal properties of a satellite's dark matter profile correlate with its orbit, and we show that it is difficult to reproduce observations of the Fornax dwarf without strong baryonic feedback. The technique developed in this work is flexible in its application of observational data and can leverage arbitrary information about the satellite galaxies to make inferences about their dark matter halos and population statistics.

When dark matter has a large cross section for self scattering, halos can undergo a process known as gravothermal core collapse, where the inner core rapidly increases in density and temperature. To date, several methods have been used to implement Self-Interacting Dark Matter~(SIDM) in N-body codes, but there has been no systematic study of these different methods or their accuracy in the core-collapse phase. In this paper, we compare three different numerical implementations of SIDM, including the standard methods from the GIZMO and Arepo codes, by simulating idealized dwarf halos undergoing significant dark matter self interactions (\\(/m = 50\\)~cm\\(^2\\)/g). When simulating these halos, we also vary the mass resolution, time-stepping criteria, and gravitational force-softening scheme. The various SIDM methods lead to distinct differences in a halo's evolution during the core-collapse phase, as each results in spurious scattering rate differences and energy gains/losses. The use of adaptive force softening for gravity can lead to numerical heating that artificially accelerates core collapse, while an insufficiently small simulation time step can cause core evolution to stall or completely reverse. Additionally, particle numbers must be large enough to ensure that the simulated halos are not sensitive to noise in the initial conditions. Even for the highest-resolution simulations tested in this study (\\(10^6\\) particles per halo), we find that variations of order \\(10\\%\\) in collapse time are still present. The results of this work underscore the sensitivity of SIDM modeling on the choice of numerical implementation and motivate a careful study of how these results generalize to halos in a cosmological context.

The LIGO-Virgo-KAGRA (LVK) collaboration has detected over 150 confirmed gravitational wave events through O4a. Binary black hole (BBH) systems represent the overwhelming majority of these observations. We construct a model for the population of the BBHs based on the distribution of metallicities in galaxies and state-of-the-art stellar evolution models implemented through the Stellar EVolution N-body (SEVN) code. We calculate the redshift evolution of the total merger rate of BBHs and the differential rates with respect to primary mass, secondary mass, and the mass ratio. We explore variations in the delay-time distribution's (DTD) power-law index and show that it affects the total merger rate's spectral shape, but primarily acts as an amplitude shift on the differential rates. When comparing to the primary mass distribution, our results indicate that either the average IMF in dwarf galaxies must be top heavy, or most of the 30-40 \\( M_\\) black holes must be formed through a dynamical capture mechanism. For masses greater than about \\(50 \\, M_\\), the predicted number of BBH systems plummet to zero, revealing the well-known mass gap due to the pair instability mechanism and mass loss in binary systems.

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 \\(\\)CDM, 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 semi-analytic code SatGen, 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 with Gala. We find that subhalos with masses in the range \\(2 10^6 M_ - 10^8 M_\\) at the time of the stream-subhalo encounter, corresponding to masses of about \\(2 10^7 M_ - 10^9 M_\\) 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)\\)~deg 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. We also investigate the effects of increasing the host-halo mass on the gap properties and formation rate.