Self-interacting Dark Matter Models Research Articles

An N-body/hydrodynamical simulation study of the merging cluster El Gordo: A compelling case for self-interacting dark matter?

We used a large set N-body/hydrodynamical simulations to study the physical properties of the merging cluster El Gordo. We find that the observed X-ray structures, along with other data, can be matched fairly well by simulations with collision velocities 2000 km s−1 ≲ V ≲ 2500 km s−1 and impact parameters 600 kpc ≲ P ≲ 800 kpc. The mass of the primary is constrained to be between ∼1015 M⊙ and ∼1.6 × 1015 M⊙, in accordance with recent lensing-based mass measurements. Moreover, a returning, post-apocenter, scenario is not supported by our head-on simulations. We also considered merger models that incorporate dark matter self-interactions. The simulation results show that the observed spatial offsets between the different mass components are well reproduced in self-interacting dark matter models with an elastic cross-section in the range σDM/mX ∼ 4 − 5 cm2 gr−1. In addition, the mean relative line-of-sight radial velocity between the two brightest cluster galaxies is found to be on the order of several hundred km s−1. We argue that these findings provide an unambiguous signature of a dark matter behavior that exhibits collisional properties in a very energetic high-redshift cluster collision. The range of allowed values we find for σDM/mX is, however, inconsistent with present upper limits. To resolve this tension, we suggest the possibility that the self-interacting dark matter model used here be considered as only a low-order approximation, and that the underlying physical processes that describe the interaction of dark matter in major cluster mergers are more complex than can be adequately represented by the commonly assumed approach based on the scattering of dark matter particles.

Cosmological and idealized simulations of dark matter haloes with velocity-dependent, rare and frequent self-interactions

ABSTRACT Dark matter self-interactions may have the capability to solve or at least mitigate small-scale problems of the cosmological standard model, Lambda cold dark matter. There are a variety of self-interacting dark matter models that lead to distinguishable astrophysical predictions and hence varying success in explaining observations. Studies of dark matter (DM) density cores on various mass scales suggest a velocity-dependent scattering cross-section. In this work, we investigate how a velocity dependence alters the evolution of the DM distribution for frequent DM scatterings and compare to the velocity-independent case. We demonstrate that these cases are qualitatively different using a test problem. Moreover, we study the evolution of the density profile of idealized DM haloes and find that a velocity dependence can lead to larger core sizes and different time-scales of core formation and core collapse. In cosmological simulations, we investigate the effect of velocity-dependent self-interaction on haloes and satellites in the mass range of ≈1011–$10^{14} \, \mathrm{M_\odot }$. We study the abundance of satellites, density, and shape profiles and try to infer qualitative differences between velocity-dependent and velocity-independent scatterings as well as between frequent and rare self-interactions. We find that a strongly velocity-dependent cross-section can significantly amplify the diversity of rotation curves, independent of the angular dependence of the differential cross-section. We further find that the abundance of satellites in general depends on both the velocity dependence and the scattering angle, although the latter is less important for strongly velocity-dependent cross-sections.

R-modes as a new probe of dark matter in neutron stars

In this work, we perform the first systematic investigation of effects of the presence of dark matter on r-mode oscillations in neutron stars (NSs). Using a self-interacting dark matter (DM) model based on the neutron decay anomaly and a hadronic model obtained from the posterior distribution of a recent Bayesian analysis, we impose constraints on the DM self-interaction strength using recent multimessenger astrophysical observations. We also put new constraints on the DM fraction for this model of DM. The constrained DM interaction strength is then used to estimate DM self-interaction cross section and shear viscosity resulting from DM, which is found to be several orders of magnitude smaller than shear viscosity due to hadronic matter. Assuming chemical equilibrium among DM fermions and neutrons, we estimate the bulk viscosity resulting from the dark decay of neutrons considering different scenarios for the temperature dependence of the reaction rate and investigate the effect on the r-mode instability window. We conclude that DM shear and bulk viscosity may significantly modify the r-mode instability window compared with the minimal hadronic viscosities, depending on the temperature dependence of the process. We also found that for the window to be compatible with the X-ray and pulsar observational data, the rate for the dark decay process must be fast.

Dark matter effects on the properties of neutron stars: Optical radii

We study the effects of dark matter on the properties of neutron stars by employing a DM-admixed model. The Brueckner–Hartree–Fock theory with realistic three-body forces and a generic bosonic self-interacting dark matter model describe the equations of state for nuclear matter and DM, respectively. We study the complete set of stable ‘dark’ neutron stars and in particular the observable radii of these objects. A rich variety of stellar configurations is found and discussed in detail.

The impact of baryonic potentials on the gravothermal evolution of self-interacting dark matter haloes

ABSTRACT The presence of a central baryonic potential can have a significant impact on the gravothermal evolution of self-interacting dark matter (SIDM) haloes. We extend a semi-analytical fluid model to incorporate the influence of a static baryonic potential and calibrate it using controlled N-body simulations. We construct benchmark scenarios with varying baryon concentrations and different SIDM models, including constant and velocity-dependent self-interacting cross-sections. The presence of the baryonic potential induces changes in SIDM halo properties, including central density, core size, and velocity dispersion, and it accelerates the halo’s evolution in both expansion and collapse phases. Furthermore, we observe a quasi-universality in the gravothermal evolution of SIDM haloes with the baryonic potential, resembling a previously known feature in the absence of the baryons. By appropriately rescaling the physical quantities that characterize the SIDM haloes, the evolution of all our benchmark cases exhibits remarkable similarity. Our findings offer a framework for testing SIDM predictions using observations of galactic systems where baryons play a significant dynamical role.

Testing the Collisionless Nature of Dark Matter with the Radial Acceleration Relation in Galaxy Clusters

The radial acceleration relation (RAR) represents a tight empirical relation between the inferred total and baryonic centripetal accelerations, g tot = GM tot(< r)/r 2 and g bar = GM bar(< r)/r 2, observed in galaxies and galaxy clusters. The tight correlation between these two quantities can provide insight into the nature of dark matter. Here we use BAHAMAS, a state-of-the-art suite of cosmological hydrodynamical simulations, to characterize the RAR in cluster-scale halos for both cold and collisionless dark matter (CDM) and self-interacting dark matter (SIDM) models. SIDM halos generally have reduced central dark matter densities, which reduces the total acceleration in the central region when compared with CDM. We compare the RARs in galaxy clusters simulated with different dark matter models to the RAR inferred from CLASH observations. Our comparison shows that the cluster-scale RAR in the CDM model provides an excellent match to the CLASH RAR obtained by Tian et al. including the high-acceleration regime probed by the brightest cluster galaxies (BCGs). By contrast, models with a larger SIDM cross section yield increasingly poorer matches to the CLASH RAR. Excluding the BCG regions results in a weaker but still competitive constraint on the SIDM cross section. Using the RAR data outside the central r < 100 kpc region, an SIDM model with σ/m = 0.3 cm2 g−1 is disfavored at the 3.8σ level with respect to the CDM model. This study demonstrates the power of the cluster-scale RAR for testing the collisionless nature of dark matter.

Properties and observables of massive galaxies in self-interacting dark matter cosmologies

ABSTRACT We use hydrodynamic cosmological simulations to test the differences between cold and self-interacting dark matter models (CDM and SIDM) in the mass range of massive galaxies (1012 &amp;lt; M200/M⊙ h−1 &amp;lt; 1013.5). We consider two SIDM models: one with constant cross-section $\sigma /m_{\chi } = 1\, \mathrm{cm^2 \, g^{-1}}$ and one where the cross-section is velocity-dependent. Despite a weak trend in mass, we find that with the inclusion of baryons the differences between SIDM and CDM density profiles observed in the dark-matter-only case are almost erased. We also search for signatures of SIDM in the distribution of strong lensing Einstein radii and find that the distributions derived from CDM and SIDM hydro runs are both comparable to observational samples of strong lenses. We find that, looking at the total matter distribution, the interplay between self-interactions and baryons can greatly reduce the expected differences between CDM and SIDM models at this mass scale, making the discrimination between these DM models challenging. However, looking at the dark matter/baryonic fractions in the inner region of the haloes we show that the deviations of SIDM from CDM can still be found considering these components separately. These results highlight that one of the most promising paths to discriminate between CDM and SIDM is to focus on techniques able to distinguish between the dark matter and baryonic components in galaxies and clusters.

The role of baryons in self-interacting dark matter mergers

ABSTRACTMergers of galaxy clusters are promising probes of dark matter (DM) physics. For example, an offset between the DM component and the galaxy distribution can constrain DM self-interactions. We investigate the role of the intracluster medium (ICM) and its influence on DM–galaxy offsets in self-interacting dark matter models. To this end, we employ Smoothed Particle Hydrodynamics  + N-body simulations to study idealized setups of equal- and unequal-mass mergers with head-on collisions. Our simulations show that the ICM hardly affects the offsets arising shortly after the first pericentre passage compared to DM-only simulations. But later on, e.g. at the first apocentre, the offsets can be amplified by the presence of the ICM. Furthermore, we find that cross-sections small enough not to be excluded by measurements of the core sizes of relaxed galaxy clusters have a chance to produce observable offsets. We found that different DM models affect the DM distribution and also the galaxy and ICM distribution, including its temperature. Potentially, the position of the shock fronts, combined with the brightest cluster galaxies, provides further clues to the properties of DM. Overall our results demonstrate that mergers of galaxy clusters at stages about the first apocentre passage could be more interesting in terms of DM physics than those shortly after the first pericentre passage. This may motivate further studies of mergers at later evolutionary stages.

Constraining resonant dark matter self-interactions with strong gravitational lenses

We devise a method to constrain self-interacting dark matter (SIDM) from observations of quadruply imaged quasars, and apply it to five self-interaction potentials with a long-range dark force. We consider several SIDM models with an attractive potential that allows for the formation of quasibound states, giving rise to resonant features in the cross section localized at particular velocities below $50\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. We propose these resonances, which amplify or suppress the cross section amplitude by over an order of magnitude, accelerate or delay the onset of core collapse in low-mass dark matter halos, and derive constraints on the timescale for core collapse for the five interaction potentials we consider. Our data strongly disfavors scenarios in which a majority of halos core collapse, with the strongest constraints obtained for cross section strengths exceeding $100\text{ }\text{ }{\mathrm{cm}}^{2}\text{ }{\mathrm{g}}^{\ensuremath{-}1}$ at relative velocities below $30\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. This work opens a new avenue to explore the vast landscape of possible SIDM theories.

Fitting a self-interacting dark matter model to data ranging from satellite galaxies to galaxy clusters

We present a fit to observational data in an asymmetric self-interacting dark matter model using our recently calculated cross sections that incorporate both $t$-channel and $u$-channel exchanges in the scattering of identical particles. We find good fits to the data ranging from dwarf galaxies to galaxy clusters, and equivalent relative velocities from $\sim 20$ km/sec to $\gtrsim 10^3$ km/s. We compare our results with previous fits that used only $t$-channel exchange contributions to the scattering.

Orbital evolution of satellite galaxies in self-interacting dark matter models

Dark matter self-interactions can leave distinctive signatures on the properties of satellite galaxies around Milky Way--like hosts through their impact on tidal stripping, ram pressure, and gravothermal collapse. We delineate the regions of self-interacting dark matter parameter space---specified by interaction cross section and a velocity scale---where each of these effects dominates and show how the relative mass loss depends on the satellite's initial mass, density profile, and orbit. We obtain novel, conservative constraints in this parameter space using Milky Way satellite galaxies with notably high central densities and small pericenter distances. Our results for self-interacting dark matter models, in combination with constraints from clusters of galaxies, favor either velocity-dependent cross sections that lead to gravothermal core collapse in the densest satellites or small cross sections which more closely resemble cold and collisionless dark matter.

The local dark matter distribution in self-interacting dark matter halos

We study the effects of dark matter self-interactions on the local dark matter distribution in selected Milky Way-like galaxies in the EAGLE hydrodynamical simulations. The simulations were run with two different self-interacting dark matter models, a constant and velocity-dependent self-interaction cross-section. We find that the local dark matter velocity distribution of the Milky Way-like halos in the simulations with dark matter self-interactions and baryons are generally similar to those extracted from cold collisionless dark matter simulations with baryons. In both cases, the local dark matter speed distributions agree well with their best fit Maxwellian distributions. Including baryons in the simulations with or without dark matter self-interactions increases the local dark matter density and shifts the dark matter speed distributions to higher speeds. To study the implications for direct detection, we compute the dark matter halo integrals obtained directly from the simulations and compare them to those obtained from the best fit Maxwellian velocity distribution. We find that a Maxwellian distribution provides a good fit to the halo integrals of most halos, without any significant difference between the results of different dark matter self-interaction models.

Matching the mass function of Milky Way satellites in competing dark matter models

ABSTRACT Any successful model of dark matter must explain the diversity of observed Milky Way (MW) satellite density profiles, from very dense ultrafaints to low-density satellites so large that they could be larger than their inferred dark matter haloes. Predictions for these density profiles are complicated by the limitations of simulation resolution in the stripping of subhaloes by the MW system. We consider cold dark matter (CDM), warm dark matter (WDM, 3.3 keV thermal relic power spectrum), and a self-interacting dark matter model (SIDM) that induces gravothermal collapse in low-mass subhaloes. Using N-body simulations combined with a halo stripping algorithm, we find that most CDM and WDM subhaloes of mass &amp;gt;108 ${\, \rm M_\odot }$ are large enough after stripping to fit most satellites; however, the required amount of stripping often requires a stronger tidal field than is available on the subhalo’s orbit. The lower concentrations of WDM subhaloes enable more stripping to take place, even on orbits with large pericentres. SIDM cores offer the best fits to massive, low-density satellites at the expense of predicting &amp;gt;109 ${\, \rm M_\odot }$ subhaloes to host low-density satellites with no observed analogue. The agreement of the total number of satellites with observations in CDM and WDM depends strongly on the assumptions made to draw the observational estimates. We conclude that an SIDM model must have a very high velocity-dependent cross-section in order to match all satellites, and that WDM offers a marginally better fit than CDM to the MW satellite mass function.

Dark matter halo cores and the tidal survival of Milky Way satellites

ABSTRACT The cuspy central density profiles of cold dark matter (CDM) haloes make them highly resilient to disruption by tides. Self-interactions between dark matter particles, or the cycling of baryons, may result in the formation of a constant-density core that would make haloes more susceptible to tidal disruption. We use N-body simulations to study the evolution of Navarro–Frenk–White (NFW)-like ‘cored’ subhaloes in the tidal field of a massive host, and identify the criteria and time-scales for full disruption. Our results imply that the survival of Milky Way satellites places constraints on the sizes of dark matter cores. We find that no subhaloes with cores larger than 1 per cent of their initial NFW scale radius can survive for a Hubble time on orbits with pericentres ${\lesssim} 10\, \mathrm{kpc}$. A satellite like Tucana 3, with pericentre ${\sim} 3.5\, \mathrm{kpc}$, must have a core size smaller than ${\sim} 2\, \mathrm{pc}$ to survive just three orbital periods on its current orbit. The core sizes expected in self-interacting dark matter models with a velocity-independent cross-section of $1\, \mathrm{cm^2}\,\mathrm{g}^{-1}$ seem incompatible with ultrafaint satellites with small pericentric radii, such as Tuc 3, Seg 1, Seg 2, Ret 2, Tri 2, and Wil 1, as these should have fully disrupted if accreted on to the Milky Way ${\gtrsim} 10\, \mathrm{Gyr}$ ago. These results suggest that many satellites have vanishingly small core sizes, consistent with CDM cusps. The discovery of further Milky Way satellites on orbits with small pericentric radii would strengthen these conclusions and allow for stricter upper limits on the core sizes.

Motivations for a large self-interacting dark matter cross-section from Milky Way satellites

ABSTRACT We explore the properties of Milky Way (MW) subhaloes in self-interacting dark matter models for moderate cross-sections of 1–5 cm2 g−1 using high-resolution zoom-in N-body simulations. We include the gravitational potential of a baryonic disc and bulge matched to the MW, which is critical for getting accurate predictions. The predicted number and distribution of subhaloes within the host halo are similar for 1 and 5 cm2 g−1 models, and they agree with observations of MW satellite galaxies only if subhaloes with peak circular velocity over all time &amp;gt;7 km s−1 are able to form galaxies. We do not find distinctive signatures in the pericentre distribution of the subhaloes that could help distinguish the models. Using an analytical model to extend the simulation results, we are able to show that subhaloes in models with cross-sections between 1 and 5 cm2 g−1 are not dense enough to match the densest ultrafaint and classical dwarf spheroidal galaxies in the MW. This motivates exploring velocity-dependent cross-sections with values larger than 5 cm2 g−1 at the velocities relevant for the satellites such that core collapse would occur in some of the ultrafaint and classical dwarf spheroidals.

Modelling self-interacting dark matter substructures – I.  Calibration with N-body simulations of a Milky-Way-sized halo and its satellite

ABSTRACT We study evolution of single subhaloes with their masses of ∼109 M⊙ in a Milky Way-sized host halo for self-interacting dark matter (SIDM) models. We perform dark-matter-only N-body simulations of dynamical evolution of individual subhaloes orbiting its host by varying self-scattering cross-sections (including a velocity-dependent scenario), subhalo orbits, and internal properties of the subhalo. We calibrate a gravothermal fluid model to predict time evolution in spherical mass density profiles of isolated SIDM haloes with the simulations. We find that tidal effects of SIDM subhaloes can be described with a framework developed for the case of collision-less cold dark matter (CDM), but a shorter typical time-scale for the mass loss due to tidal stripping is required to explain our SIDM simulation results. As long as the cross-section is less than $\sim \! 10\, \mathrm{cm}^2\,\mathrm{g}^{ -1}$ and initial states of subhaloes are set within a 2σ-level scatter at redshifts of ∼2 predicted by the standard ΛCDM cosmology, our simulations do not exhibit a prominent feature of gravothermal collapse in the subhalo central density for 10 Gyr. We develop a semi-analytic model of SIDM subhaloes in a time-evolving density core of the host with tidal stripping and self-scattering ram pressure effects. Our semi-analytic approach provides a simple, efficient, and physically intuitive prediction of SIDM subhaloes, but further improvements are needed to account for baryonic effects in the host and the gravothermal instability accelerated by tidal stripping effects.

Constraining SIDM with halo shapes: Revisited predictions from realistic simulations of early-type galaxies

ABSTRACT We study the effect of self-interacting dark matter (SIDM) and baryons on the shape of early-type galaxies (ETGs) and their dark matter haloes, comparing them to the predictions of the cold dark matter (CDM) scenario. We use five hydrodynamical zoom-in simulations of haloes hosting ETGs ($M_{\rm vir}\sim 10^{13}\, \mathrm{M}_{\odot }$ and $M_{*}\sim 10^{11}\, \mathrm{M}_{\odot }$), simulated in CDM and a SIDM model with constant cross-section of σT/mχ = 1 cm2g−1. We measure the 3D and projected shapes of the dark matter haloes and their baryonic content using the inertia tensor and compare our measurements to the results of three HST samples of gravitational lenses and Chandra and XMM–Newton X-ray observations. We find that the inclusion of baryons greatly reduces the differences between CDM and a SIDM, together with the ability to draw constraints based on shapes. Lensing measurements reject the predictions of CDM dark-matter-only simulations and prefer one of the hydro scenarios. When we consider the total sample of lenses, observational data prefer the CDM hydro scenario. The shapes of the X-ray emitting gas are compatible with observational results in both hydro runs, with CDM predicting higher elongations only in the very centre. Contrary to previous claims at the scale of elliptical galaxies, we conclude that both CDM and our SIDM model can still explain observed shapes once we include baryons in the simulations. Our results demonstrate that this is essential to derive realistic constraints and that new simulations are needed to confirm and extend our findings.

X-ray morphology of cluster-mass haloes in self-interacting dark matter

ABSTRACT We perform cosmological zoom-in simulations of 19 relaxed cluster-mass haloes with the inclusion of adiabatic gas in the cold dark matter (CDM) and self-interacting dark matter (SIDM) models. These clusters are selected as dynamically relaxed clusters from a parent simulation with $M_{\rm 200} \simeq (1\!-\!3)\times 10^{15}{\, \rm M_\odot }$. Both the dark matter and the intracluster gas distributions in SIDM appear more spherical than their CDM counterparts. Mock X-ray images are generated based on the simulations and are compared to the real X-ray images of 84 relaxed clusters selected from the Chandra and ROSAT archives. We perform ellipse fitting for the isophotes of mock and real X-ray images and obtain the ellipticities at cluster-centric radii of $r\simeq 0.1\!-\!0.2R_{\rm 200}$. The X-ray isophotes in SIDM models with increasing cross-sections are rounder than their CDM counterparts, which manifests as a systematic shift in the distribution function of ellipticities. Unexpectedly, the X-ray morphology of the observed non-cool-core clusters agrees better with SIDM models with cross-section $(\sigma /m)= 0.5\!-\!1\, {\rm cm}^2\, {\rm g}^{-1}$ than CDM and SIDM with $(\sigma /m)=0.1\, {\rm cm}^2\, {\rm g}^{-1}$. Our statistical analysis indicates that the latter two models are disfavoured at the $68{{\ \rm per\ cent}}$ confidence level (as conservative estimates). This conclusion is not altered by shifting the radial range of measurements or applying a temperature selection criterion. However, the primary uncertainty originates from the lack of baryonic physics in the adiabatic model, such as cooling, star formation and feedback effects, which still have the potential to reconcile CDM simulations with observations.

A critical assessment of solutions to the galaxy diversity problem

Galactic rotation curves exhibit a diverse range of inner slopes. Observational data indicates that explaining this diversity may require a mechanism that correlates a galaxy's surface brightness with the central-most region of its dark matter halo. In this work, we compare several concrete models that capture the relevant physics required to explain the galaxy diversity problem. We focus specifically on a Self-Interacting Dark Matter (SIDM) model with an isothermal core and two Cold Dark Matter (CDM) models with/without baryonic feedback. In contrast to the CDM case, the SIDM model can lead to the formation of an isothermal core in the halo, and is also mostly insensitive to baryonic feedback processes, which act on longer time-scales. Using rotation curves from 90 galaxies in the Spitzer Photometry & Accurate Rotation Curves (SPARC) catalog, we perform a comprehensive model comparison that addresses issues of statistical methodology from prior works. The best-fit halo models that we recover are consistent with standard CDM concentration-mass and abundance matching relations. We find that both the SIDM and feedback-affected CDM models are better than a CDM model with no feedback in explaining the rotation curves of low and high surface brightness galaxies in the sample. However, when compared to each other, there is no strong statistical preference for either the SIDM or the feedback-affected CDM halo model as the source of galaxy diversity in the SPARC catalog.

The Signatures of Self-interacting Dark Matter and Subhalo Disruption on Cluster Substructure

The abundance, distribution, and inner structure of satellites of galaxy clusters can be sensitive probes of the properties of dark matter. We run 30 cosmological zoom-in simulations with self-interacting dark matter (SIDM), with a velocity-dependent cross section, to study the properties of subhalos within cluster-mass hosts. We find that the abundance of subhalos that survive in the SIDM simulations are suppressed relative to their cold dark matter (CDM) counterparts. Once the population of disrupted subhalos—which may host orphan galaxies—is taken into account, satellite galaxy populations in CDM and SIDM models can be reconciled. However, even in this case, the inner structures of subhalos are significantly different in the two dark matter models. We study the feasibility of using the weak-lensing signal from the subhalo density profiles to distinguish between the cold and self-interacting dark matter while accounting for the potential contribution of orphan galaxies. We find that the effects of self-interactions on the density profile of subhalos can appear degenerate with subhalo disruption in CDM, when orphans are accounted for. With current error bars from the Subaru Hyper Suprime-Cam Strategic Program, we find that subhalos in the outskirts of clusters (where disruption is less prevalent) can be used to constrain dark matter physics. In the future, the Vera C. Rubin Observatory Legacy Survey of Space and Time will give precise measurements of the weak-lensing profile and can be used to constrain σ T /m at the ∼1 cm2 g−1 level at v ∼ 2000 km s−1.