Isolated Binary Black Hole Formation and Merger Rates from Galaxy Evolution

The Astrophysical Journal · 2026-01-27

Abstract 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.

Survival of the most compact: the life and death of satellite halos in self-interacting dark matter

arXiv (Cornell University) · 2026-03-19

Self-interacting dark matter (SIDM) models feature short-range interactions between dark matter (DM) particles that lead to larger diversity in the inner parts of galactic rotation curves and potentially unique gravitational lensing signatures. Satellite galaxies and dark subhalos provide a valuable testing ground for such models. We develop a simulation framework to explore subhalo evolution and its gravothermal collapse for velocity- and angle-dependent self-interacting cross section in these SIDM models. Our results are essential for testing these models. We perform N-body simulations, treating the host halo analytically and modelling the scattering-induced subhalo-halo interaction process using virtual host particles, a central innovation of our work. We use the Eddington inversion method to accurately model the local velocity distribution in the halo. Our approach is significantly less computationally expensive than simulations with a fully resolved host, while incorporating tidal stripping and tidal heating. We test both isotropic and forward-dominated self-scattering, which represent limiting cases for the angular dependence of the self-interaction cross section. Environmental effects, especially the scattering-induced subhalo-halo interaction, have a strong impact on the subhalo evolution and drive a complex structural evolution. As a result, SIDM subhalos have a larger range of central densities and density profile slopes compared to collisionless DM. Our cost-efficient simulation framework enables modelling of SIDM subhalos in realistic environments. Our results highlight the necessity of accurately modelling the scattering-induced subhalo-halo interaction to predict SIDM subhalo density profiles. For the SIDM models we investigate, the enhanced diversity in the mass profiles of subhalos would leave an observable imprint on strong lensing systems and satellite galaxies.

The Preference for Evolving Dark Energy from Cosmological Distance Measurements and Possible Signatures in the Growth Rate of Perturbations

arXiv (Cornell University) · 2025-02-18 · 1 citations

In this study, we use a flexible parametrization of the equation of state of dark energy to explore its possible evolution with datasets from the Dark Energy Spectroscopic Instrument (DESI), Planck cosmic microwave background, and either the 5-year Dark Energy Survey (DES) or the Pantheon+ (PP) supernova (SN) compilation. This parametrization, called transitional dark energy, allows for rapid changes in the equation of state but also changes like that in the Chevallier-Polarski-Linder parametrization. We find a 3.8σ preference for evolving dark energy over ΛCDM with the DES dataset and a weaker 2.4σ preference when using the PP dataset. This corroborates the finding of the DESI Collaboration, who found that their baryon acoustic oscillation data preferred evolving dark energy when fit with the CPL parametrization of the equation of state. Our analysis reveals no significant outliers in the DESI data around the TDE best-fit, while the data is asymmetrically distributed around the ΛCDM best-fit model such that the measured distances are on average smaller. The DESI and SN data both prefer an expansion history that implies a higher dark energy density around z=0.5 than in the Planck-ΛCDM model, with the inferred equation of state being greater than -1 around z=0 and close to or below -1 at z>0.5. We show that when the expansion rate is greater than that in the Planck-ΛCDM model (around z=0.5), the growth rate calculated assuming General Relativity is suppressed relative to the Planck-ΛCDM model, and it rebounds as the expansion rate differences between the models become smaller closer to the present time. The resulting flattening of the $fσ_8(z)$ curve compared to the ΛCDM model could be an independent signature of the temporal evolution of dark energy.

Theoretical Predictions for the Inner Dark Matter Distribution in the Milky Way Informed by Simulations

ArXiv.org · 2025-01-24 · 1 citations

We build a theoretical range for the Milky Way's (MW) inner dark matter (DM) distribution informed by the FIRE-2, Auriga, VINTERGATAN-GM, and TNG50 simulation suites assuming the canonical cold dark matter (CDM) model. The DM density profiles in Auriga, VINTERGATAN-GM, and TNG50 can be approximately modeled using the adiabatic contraction prescription of Gnedin et al. 2004, while FIRE-2 has stronger baryonic feedback, leading to a departure from the adiabatic contraction model. The simulated halos that are adiabatically contracted are close to spherical (axis ratio $q \in [0.75-0.9]$ at $5^\circ$), whereas halos that experience strong baryonic feedback are oblate ($q \in [0.5-0.7]$). Using the adiabatic contraction and strong baryonic feedback models, along with the observed stellar distribution of the MW, the inner logarithmic density slope for CDM in the MW is predicted to range from $ -0.5$ to $-1.3$. The $J$-factor, which determines the DM-annihilation flux, averaged over a solid angle of $5^\circ$ ($10^\circ$) is predicted to span the range $0.8$-$30$ ($0.6$-$10$) $\times 10^{23} \rm{GeV}^2/\rm{cm}^5$. The $D$-factor, which determines the flux due to DM decay, is predicted to be in the range $0.6$-$2$ ($0.5-1$) $\times10^{23} \rm{GeV}/\rm{cm}^2$. GitHub: The results for this work can be found at https://github.com/abdelazizhussein/MW-Inner-DM-Profile.

Gravothermal collapse and the diversity of galactic rotation curves

Physical review. D/Physical review. D. · 2025-05-20 · 14 citations

The rotation curves of spiral galaxies exhibit a great diversity that challenges our understanding of galaxy formation and the nature of dark matter. Previous studies showed that in self-interacting dark matter (SIDM) models with a cross section per unit mass of $\ensuremath{\sigma}/m\ensuremath{\approx}\mathcal{O}(1)\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{g}$, the predicted dark matter central densities are a good match to the observed densities in galaxies. In this work, we explore a regime with a larger cross section of $\ensuremath{\sigma}/m\ensuremath{\approx}20\ensuremath{-}40\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{g}$ in dwarf galactic halos. We will show that such strong dark matter self-interactions can further amplify the diversity of halo densities inherited from their assembly history. High concentration halos can enter the gravothermal collapse phase within 10 Gyr, resulting in a high density, while low concentration ones remain in the expansion phase and have a low density. We fit the rotation curves of 14 representative low surface brightness galaxies and demonstrate how the large range of observed central densities is naturally accommodated in the strong SIDM regime of $\ensuremath{\sigma}/m\ensuremath{\approx}20--40\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{g}$. Galaxies that are outliers in the previous studies, due to their high halo central densities, are no longer outliers in this SIDM regime as their halos would be in the collapse phase. For galaxies with a low density, the SIDM fits are robust to the variation of the cross section. Our findings open up a new window for testing gravothermal collapse, the unique signature of strong dark matter self-interactions, and exploring a broader SIDM model space. As an example, we illustrate how the larger cross sections favored by our fits, together with upper limits from strong lensing observations in clusters, pick out the preferred SIDM model space for a dark matter particle coupled to a light gauge boson in the Born regime.

Semianalytic Modeling of Dark Matter Subhalo Encounters with Thin Stellar Streams: Statistical Predictions for GD-1-like Streams in Cold Dark Matter

The Astrophysical Journal · 2025-09-16 · 1 citations

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 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)° 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.

Semi-Analytic Modeling of Dark Matter Subhalo Encounters with Thin Stellar Streams: Statistical Predictions for GD-1-like Streams in CDM

arXiv (Cornell University) · 2024-12-17

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 $Λ$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 $2\times 10^6 M_{\odot} - 10^8 M_{\odot}$ at the time of the stream-subhalo encounter, corresponding to masses of about $2 \times 10^7 M_{\odot} - 10^9 M_{\odot}$ at the time of infall, are the likeliest to produce gaps in a GD-1-like stream. We find that gaps occur on average $\sim$3~times per realization of the host system. These gaps have typical widths of $\sim(5 - 27)$~deg and fractional underdensities of $\sim (10 - 30)\%$, 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 $\sim(200 - 410)$~km/s. We also investigate the effects of increasing the host-halo mass on the gap properties and formation rate.

Gravothermal collapse and the diversity of galactic rotation curves

arXiv (Cornell University) · 2024-07-20 · 2 citations

The rotation curves of spiral galaxies exhibit a great diversity that challenge our understanding of galaxy formation and the nature of dark matter. Previous studies showed that in self-interacting dark matter (SIDM) models with a cross section per unit mass of $σ/m\approx{\cal O}(1)~{\rm cm^2/g}$, the predicted dark matter central densities are a good match to the observed densities in galaxies. In this work, we explore a regime with a larger cross section of $σ/m\approx20\text{-}40~{\rm cm^2/g}$ in dwarf galactic halos. We will show that such strong dark matter self-interactions can further amplify the diversity of halo densities inherited from their assembly history. High concentration halos can enter the gravothermal collapse phase within $10~{\rm Gyr}$, resulting in a high density, while low concentration ones remain in the expansion phase and have a low density. We fit the rotation curves of $14$ representative low surface brightness galaxies and demonstrate how the large range of observed central densities are naturally accommodated in the strong SIDM regime of $σ/m\approx20\text{-}40~{\rm cm^2/g}$. Galaxies that are outliers in the previous studies due to their high halo central densities, are no longer outliers in this SIDM regime as their halos would be in the collapse phase. For galaxies with a low density, the SIDM fits are robust to the variation of the cross section. Our findings open up a new window for testing gravothermal collapse, the unique signature of strong dark matter self-interactions, and exploring a broader SIDM model space. As an example, we illustrate how the larger cross sections favored by our fits, together with upper limits from strong lensing observations in clusters, pick out the preferred SIDM model space for a dark matter particle coupled to a light gauge boson in the Born regime.

On the late-time evolution of velocity-dependent self-interacting dark matter halos

Journal of Cosmology and Astroparticle Physics · 2024-05-01 · 15 citations

Abstract We study the evolution of isolated self-interacting dark matter (SIDM) halos that undergo gravothermal collapse and are driven deep into the short-mean-free-path regime. We assume spherical Navarro-Frenk-White (NFW) halos as initial conditions and allow for elastic dark matter self-interactions. We discuss the structure of the halo core deep in the core-collapsed regime and how it depends on the particle physics properties of dark matter, in particular, the velocity dependence of the self-interaction cross section. We find an approximate universality deep in this regime that allows us to connect the evolution in the short- and long-mean-free-path regimes, and approximately map the velocity-dependent self-interaction cross sections to constant ones for the full gravothermal evolution. We provide a semi-analytic prescription based on our numerical results for halo evolution deep in the core-collapsed regime. Our results are essential for estimating the masses of the black holes that are likely to be left in the core of SIDM halos.

Numerical challenges in modeling gravothermal collapse in Self-Interacting Dark Matter halos

Journal of Cosmology and Astroparticle Physics · 2024-09-01 · 17 citations

Abstract 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 ( σ / 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.