Dark Matter Self-interaction Cross Section Research Articles

Simulations of galaxy cluster mergers with velocity-dependent, rare, and frequent self-interactions

ABSTRACT Self-interacting dark matter (SIDM) has been proposed to solve small-scale problems in $\rm {\Lambda CDM}$ cosmology. In previous work, constraints on the self-interaction cross-section of dark matter have been derived assuming that the self-interaction cross-section is independent of velocity. However, a velocity-dependent cross-section is more natural in most theories of SIDM. Using idealized N-body simulations without baryons, we study merging clusters with velocity-dependent SIDM. In addition to the usual rare scattering in the isotropic limit, we also simulate these systems with anisotropic, small-angle (frequent) scatterings. We find that the collisionless brightest cluster galaxy (BCG) has an offset from the DM peak that grows at later stages. Finally, we also extend the existing upper bounds on the velocity-independent, isotropic self-interaction cross-section to the parameter space of rare and frequent velocity-dependent self-interactions by studying the central densities of dark matter-only isolated haloes. For these upper-bound parameters, the DM-BCG offsets just after the first pericentre in the dark matter-only simulations are found to be ≲10 kpc. On the other hand, because of BCG oscillations, we speculate that the distribution of BCG offsets in a relaxed cluster is a statistically viable probe. Therefore, this motivates further studies of BCG off-centring in hydrodynamic cosmological simulations.

Formation of Massive Black Holes with M = (103–108) M ⊙ at Pre-quasar Epochs through Hoyle–Lyttleton–Bondi Accretion of Self-interacting Dark Matter onto a Moving Seed

The presence of supermassive black holes with M ∼ 109 M ⊙ hosted by the luminous quasars at cosmological redshift z ≥ 6 is still an open problem in astrophysical cosmology. Here we study the formation of massive black holes at high redshift (z ≫ 7) through Hoyle–Lyttleton–Bondi accretion of self-interacting dark matter (SIDM) onto a 20 M ⊙ seed black hole moving with a velocity ∼100 km s−1 inside the short, mean-free path region of an SIDM halo. We consider observational constraints on a specific SIDM cross section, σ/m dm = (0.1–5) cm2 g−1. Formation timescale of massive black holes with M = (103–108) M ⊙ is calculated for the universal Navarro–Frenk–White (NFW) profile, singular isothermal sphere (SIS), other power-law profiles with a cusp index 2.19 ≤ γ ≤ 2.5 of accreted dark matter, and modified-core isothermal profiles. The ambient sound speed is taken as C s = (10–100) km s−1. It is found that an NFW profile with halo concentration C = 4.75–32.58 estimated at z = 20 and 30 for halo masses M 200 = (1012–1014) M ⊙ favors formation of massive black holes with M = (103–108) M ⊙ at high redshift, well before quasar epoch. In this profile, these black holes grow within timescales (0.1–69) Myr at z = 16–20. For the SIS profile, the black hole formation timescales are short compared to NFW. For power-law profiles, massive black holes with M = (106–108) M ⊙ grow within a few tens to 100 Myr (z = 5–30). For modified-core isothermal profiles, the timescale of massive black hole formation is in the range (0.79–464.08) Myr (z = 8–30).

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.

Anisotropic strong lensing as a probe of dark matter self-interactions

ABSTRACT Galaxy-scale strongly lensed systems have been shown to provide a unique technique for exploring the underlying physics of dark matter at sub-galactic scales. In the past, much attention was given to detecting and studying individual haloes in a strong lens system. In addition to the subhaloes, line-of-sight haloes contribute significantly to the small perturbations in lensed images. In prior work, we demonstrated that these line-of-sight haloes imprint a distinctive anisotropic signature and hence give rise to a detectable non-zero parity-even quadrupole moment in the effective convergence field’s two-point correlation function. In this study, we show that these line-of-sight haloes also produce a non-zero curl component of the effective deflection field with a parity-odd quadrupole moment of the two-point function. These multipole moments have the ability to statistically separate line-of-sight haloes from dark matter substructure. In this paper, we examine how these multipole moments evolve in the presence of warm dark matter and self-interacting dark matter in terms of central density evolution and dark matter halo abundance. Importantly, we show that these different multipole moments display exquisite sensitivity to both the amplitude and the velocity dependence of the dark matter self-interaction cross-section. Our approach opens the door for strong lensing observations to probe dark matter self-interaction over a broad range of relative velocities.

An excursion into the core of the cluster lens Abell 1689

ABSTRACT Abell 1689 is a well-studied cluster of galaxies and one of the largest gravitational lens systems ever observed. We have obtained a reconstruction of the cluster Abell 1689 using grale, a free-form lens inversion method that relies exclusively on the multiple image data. Non-inclusion of any data related to cluster member galaxies ensures an unbiased measure of the mass distribution, which is the most notable feature of free-form methods like grale. We used two different sets of multiple image systems from the available strong lensing data – one containing only the secure systems (107 images) and the other containing all available systems, only excluding some very non-secure systems (151 images). For the very well-constrained central ∼100 kpc region of the cluster, we made a detailed comparison of the grale reconstructed lensing mass and stellar mass retrieved by the Spectral Energy Distribution (SED) fitting software fast++. We found a light-unaccompanied mass peak in this region, whose existence, while tentative, is favoured by the distribution of nearby images that are local maxima in the Fermat potential. However, further tests, using different methodologies are needed to confirm the reality of this feature. If it is shown to be real, this light-unaccompanied mass peak is consistent with dark matter self-interaction cross-section σ ≲ 1 cm2 g−1, while being in tension with larger cross-sections.

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.

Constraints on Self-Interacting dark matter from relaxed galaxy groups

Self-interacting dark matter (SIDM) has been proposed as an alternative to the standard collisionless cold dark matter to explain the diversity of galactic rotation curves and core-cusp problems seen at small scales. Here, we estimate the constraints on SIDM for a sample of 11 relaxed galaxy groups with X-ray observations from Chandra and XMM-Newton. We fit the dark matter density distribution to the Einasto profile and use the estimated Einasto α parameter to constrain the SIDM cross-section, based on the empirical relation between the two, which was obtained in Eckert et al. (2022). We obtain a non-zero central estimate for the cross-section per unit mass (σ/m) for seven groups, with the most precise estimate obtained for NGC 5044, given by σ/m=0.165±0.025cm2/g, for dark matter velocity dispersion of about 300 km/s. For the remaining four groups, we obtain 95% c.l. upper limits on σ/m< 0.16-6.61 cm2/g with dark matter velocity dispersions between 200–500 km/s, with the most stringent limit for our sample obtained for the group MKW 4, given by σ/m<0.16cm2/g for dark matter velocity dispersion of about 350 km/s.

Strong Dark Matter Self-interactions Diversify Halo Populations within and surrounding the Milky Way

We perform a high-resolution cosmological zoom-in simulation of a Milky Way (MW)–like system, which includes a realistic Large Magellanic Cloud analog, using a large differential elastic dark matter self-interaction cross section that reaches ≈100 cm2 g−1 at relative velocities of ≈10 km s−1, motivated by the diverse and orbitally dependent central densities of dwarf galaxies within and surrounding the MW. We explore the effects of dark matter self-interactions on satellite, splashback, and isolated halos through their abundance, central densities, maximum circular velocities, orbital parameters, and correlations between these variables. We use an effective constant cross section model to analytically predict the stages of our simulated halos’ gravothermal evolution, demonstrating that deviations from the collisionless – relation can be used to select deeply core-collapsed halos, where is a halo’s maximum circular velocity, and is the radius at which it occurs. We predict that a sizable fraction (≈20%) of subhalos with masses down to ≈108 M ⊙ is deeply core collapsed in our SIDM model. Core-collapsed systems form ≈10% of the isolated halo population down to the same mass; these isolated, core-collapsed halos would host faint dwarf field galaxies with extremely steep central density profiles. Finally, most halos with masses above ≈109 M ⊙ are core-forming in our simulation. Our study thus demonstrates how self-interactions diversify halo populations in an environmentally dependent fashion within and surrounding MW-mass hosts, providing a compelling avenue to address the diverse dark matter distributions of observed dwarf galaxies.

Exploring the self interacting dark matter properties from low redshift observations

We calculate the shear (eta ) and bulk viscosity (zeta ) of Self Interacting Dark Matter (SIDM) fluid using the kinetic theory formalism. Using the astrophysical constraints on dark matter self-interaction cross section over mass sigma /m , we demonstrate that viscous SIDM fluid violates the lower bound on the ratio of shear viscosity to its entropy density, eta /mathfrak {s}=frac{1}{4pi }. Then, considering the eta /mathfrak {s} bound as universal, we derive a theoretical upper limit on the ratio of velocity average dark matter self interaction cross-section to its mass and also estimate an upper limit on SIDM mass. We report that mass of the SIDM particle should be sub-GeV scale. Furthermore, with the assumption of a power-law form of eta and zeta , we study its evolution in the light of low redshift observations. We show that at the large redshift, the SIDM viscosity is small, but at the small redshift, it becomes sufficiently large and contributes significantly to cosmic dissipation. As a consequence, viscous SIDM can explain the low redshift observations and also consistent with the standard cosmological prediction.

New pathways to the relic abundance of vector-portal dark matter

We fully explore the thermal freezeout histories of a vector-portal dark matter model, in the region of parameter space in which the ratio of masses of the dark photon $A^{\prime}$ and dark matter $\chi$ is in the range $1 \lesssim m_{A^{\prime}}/m_{\chi} \lesssim 2$. In this region $2 \rightarrow 2$ and $3 \rightarrow 2$ annihilation processes within the dark sector, as well as processes that transfer energy between the dark sector and the Standard Model, play important roles in controlling the thermal freezeout of the dark matter. We carefully track the temperatures of all species, relaxing the assumption of previous studies that the dark and Standard Model sectors remain in thermal equilibrium throughout dark matter freezeout. Our calculations reveal a rich set of novel pathways which lead to the observed relic density of dark matter, and we develop a simple analytic understanding of these different regimes. The viable parameter space in our model provides a target for future experiments searching for light (MeV-GeV) dark matter, and includes regions where the dark matter self-interaction cross section is large enough to affect the small-scale structure of galaxies.

New thermal relic targets for inelastic vector-portal dark matter

We examine the vector-portal inelastic dark matter (DM) model with DM mass $m_\chi$ and dark photon mass $m_{A'}$, in the `forbidden dark matter' regime where $1 \lesssim m_{A'}/m_\chi \lesssim 2$, carefully tracking the dark sector temperature throughout freezeout. The inelastic nature of the dark sector relaxes the stringent cosmic microwave background (CMB) and self-interaction constraints compared to symmetric DM models. We determine the CMB limits on both annihilations involving excited states and annihilation into $e^+e^-$ through initial-state-radiation of an $A'$, as well as limits on the DM self-scattering, which proceeds at the one-loop level. The unconstrained parameter space serves as an ideal target for accelerator $A'$ searches, and provides a DM self-interaction cross section that is large enough to observably impact small-scale structure.

A stringent upper limit on dark matter self-interaction cross-section from cluster strong lensing

ABSTRACT We analyse strongly lensed images in eight galaxy clusters to measure their dark matter density profiles in the radial region between 10 kpc and 150 kpc, and use this to constrain the self-interaction cross-section of dark matter (DM) particles. We infer the mass profiles of the central DM haloes, bright central galaxies, key member galaxies, and DM subhaloes for the member galaxies for all eight clusters using the qlens code. The inferred DM halo surface densities are fit to a self-interacting dark matter model, which allows us to constrain the self-interaction cross-section over mass σ/m. When our full method is applied to mock data generated from two clusters in the Illustris-TNG simulation, we find results consistent with no dark matter self-interactions as expected. For the eight observed clusters with average relative velocities of $1458_{-81}^{+80}$ km s−1, we infer $\sigma /m = 0.082_{-0.021}^{+0.027} \rm cm^2\, g^{ -1}$ and $\sigma /m \lt 0.13~ \rm cm^2\, g^{ -1}$ at the 95 per cent confidence level.

Core fragmentation in simplest superfluid dark matter scenario

We study the structure of galactic halos within a scalar dark matter model, endowed with a repulsive quartic self-interaction, capable of undergoing the superfluid phase transition in high-density regions. We demonstrate that the thermalized cores are prone to fragmentation into superfluid droplets due to the Jeans instability. Furthermore, since cores of astrophysical size may be generated only when most of the particles comprising the halo reside in a highly degenerate phase-space, the well-known bound on the dark matter self-interaction cross section inferred from the collision of clusters needs to be revised, accounting for the enhancement of the interaction rate due to degeneracy. As a result, generation of kpc-size superfluid solitons, within the parameter subspace consistent with the Bullet Cluster bound, requires dark matter particles to be ultra-light.

The surprising accuracy of isothermal Jeans modelling of self-interacting dark matter density profiles

ABSTRACT Recent claims of observational evidence for self-interacting dark matter (SIDM) have relied on a semi-analytic method for predicting the density profiles of galaxies and galaxy clusters containing SIDM. We present a thorough description of this method, known as isothermal Jeans modelling, and then test it with a large ensemble of haloes taken from cosmological simulations. Our simulations were run with cold and collisionless dark matter (CDM) as well as two different SIDM models, all with dark matter only variants as well as versions including baryons and relevant galaxy formation physics. Using a mix of different box sizes and resolutions, we study haloes with masses ranging from 3 × 1010 to $3 \times 10^{15} \mathrm{\, M_\odot }$. Overall, we find that the isothermal Jeans model provides as accurate a description of simulated SIDM density profiles as the Navarro–Frenk–White profile does of CDM haloes. We can use the model predictions, compared with the simulated density profiles, to determine the input DM–DM scattering cross-sections used to run the simulations. This works especially well for large cross-sections, while with CDM our results tend to favour non-zero (albeit fairly small) cross-sections, driven by a bias against small cross-sections inherent to our adopted method of sampling the model parameter space. The model works across the whole halo mass range we study, although including baryons leads to DM profiles of intermediate-mass ($10^{12} - 10^{13} \mathrm{\, M_\odot }$) haloes that do not depend strongly on the SIDM cross-section. The tightest constraints will therefore come from lower and higher mass haloes: dwarf galaxies and galaxy clusters.

Neutron Stars and Dark Matter

Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic center, the density increase in dark matter results in increased accretion. At distances of some fraction of a parsec, the neutron star acquire enough dark matter to have its structure changed. We show that the neutron star mass decreases going towards the galactic centre, and that dark matter accumulation beyond a critical value collapses the neutron star into a black hole. Calculations cover cases varying the dark matter particle mass, self-interaction strength, and ratio between the pressure of dark matter and ordinary matter. This allow us to constrain the interaction cross section, σdm, between nucleons and dark matter particles, as well as the dark matter self-interaction cross section.

Matter power spectrum of light freeze-in dark matter: With or without self-interaction

We study the free-streaming effect in a light freeze-in dark matter model. Naturally in the dark sector one can find dark matter related coupling, and such coupling may induce dark matter self-scattering. In case that such scattering is subdominant, the dark matter partition function is not thermal but determined by the freeze-in process, yet its high momentum side is generally also Boltzmann suppressed. We show that the matter power spectrum is very similar to a warm dark matter one in shape. When matched to the current WDM bound, a 24 keV freeze-in dark matter is ruled out at 2σ confidence level. In case that the dark matter self-scattering is strong and decouples at a very late time, by a new numerical calculation we show that the early stage Brownian motion indeed protects the power spectrum against free-streaming suppression. However, such an effect cannot be characterized by a free-streaming length alone; we find that the self-scattering decoupling time is another necessary parameter. The currently interested dark matter self-interaction cross section ∼cm2/g is just marginal for such protection to be effective.

Testing self-interacting dark matter with galaxy warps

Self-interacting dark matter (SIDM) is an able alternative to collisionless dark matter. If dark matter does have self-interactions, we would expect this to cause a separation between the collisionless stars and the dark matter halo of a galaxy as it falls through a dark matter medium. For stars arranged in a disk, this would generate a U-shaped warp. The magnitude of this warping depends on the SIDM cross section, type of self-interaction, relative velocity of galaxy and background, halo structure, and density of the dark matter medium. In this paper, we set constraints on long-range (light mediator) dark matter self-interaction by means of this signal. We begin by measuring U-shaped warps in $3,213$ edge-on disk galaxies within the Sloan Digital Sky Survey. We then forward-model the expected warp from SIDM on a galaxy-by-galaxy basis by combining models of halo structure, density and velocity field reconstructions, and models for the dark matter interactions. We find no evidence for a contribution to the warps from SIDM. Our constraints are highly dependent on the uncertain velocities of our galaxies: for a normalized Rutherford-like cross section we find $\tilde{\sigma}/m_{\rm{DM}} \lesssim 3\times 10^{-13}~\rm{cm}^2/\rm{g}$ at fixed velocity $v = 300~\rm{km/s}$ -- a bound that scales roughly linearly with increasing $v$. In the appendix we translate these bounds into limits on the momentum transfer cross section, finding $\sigma_T(300~\rm{km/s})/m_{\rm{DM}} \lesssim 0.1~\rm{cm}^2/\rm{g}$. [abridged]

Observable tests of self-interacting dark matter in galaxy clusters: cosmological simulations with SIDM and baryons

ABSTRACT We present bahamas-SIDM, the first large-volume, $(400 \, h^{-1} \mathrm{\, Mpc})^{3}$, cosmological simulations including both self-interacting dark matter (SIDM) and baryonic physics. These simulations are important for two primary reasons: (1) they include the effects of baryons on the dark matter distribution and (2) the baryon particles can be used to make mock observables that can be compared directly with observations. As is well known, SIDM haloes are systematically less dense in their centres, and rounder, than CDM haloes. Here, we find that that these changes are not reflected in the distribution of gas or stars within galaxy clusters, or in their X-ray luminosities. However, gravitational lensing observables can discriminate between DM models, and we present a menu of tests that future surveys could use to measure the SIDM interaction strength. We ray-trace our simulated galaxy clusters to produce strong lensing maps. Including baryons boosts the lensing strength of clusters that produce no critical curves in SIDM-only simulations. Comparing the Einstein radii of our simulated clusters with those observed in the CLASH survey, we find that at velocities around $1000 \mathrm{\, km \, s^{-1}}$ an SIDM cross-section of $\sigma /m \gtrsim 1 \, \mathrm{cm^2 \, g^{-1}}$ is likely incompatible with observed cluster lensing.

Observable tests of self-interacting dark matter in galaxy clusters: BCG wobbles in a constant density core

ABSTRACT Models of cold dark matter (CDM) predict that the distribution of dark matter in galaxy clusters should be cuspy, centrally concentrated. Constant density cores would be strong evidence for beyond CDM physics, such as self-interacting dark matter (SIDM). An observable consequence would be oscillations of the brightest cluster galaxy (BCG) in otherwise relaxed galaxy clusters. Offset BCGs have indeed been observed – but only interpreted via a simplified, analytic model of oscillations. We compare these observations to the BAryons and HAloes of MAssive Sysmtes (BAHAMAS)–SIDM suite of cosmological simulations, which include SIDM and a fully hydrodynamical treatment of star formation and feedback. We predict that the median offset of BCGs increases with the SIDM cross-section, cluster mass, and the amount of stellar mass within 10 kpc, while CDM exhibits no trend in mass. Interpolating between the simulated cross-sections, we find that the observations (of 10 clusters) are consistent with CDM at the ∼1.5σ level, and prefer cross-section σ/m &lt; 0.12(0.39) cm2 g−1 at 68 per cent (95 per cent) confidence level. This is on the verge of ruling out velocity-independent dark matter self-interactions as the solution to discrepancies between the predicted and observed behaviour of dwarf galaxies, and will be improved by larger surveys by Euclid or Super-pressure Balloon-borne Imaging Telescope (SuperBIT).

Detecting dark matter cores in galaxy clusters with strong lensing

We test the ability of strong lensing data to constrain the size of a central core in the dark matter haloes of galaxy clusters, using Abell 611 as a prototype. Using simulated data, we show that modelling a cluster halo with ellipticity in the gravitational potential can bias the inferred mass and concentration, which may bias the inferred central density when weak lensing or X-ray data are added. We also highlight the possibility for spurious constraints on the core size if the radial density profile is different from the assumed model. These systematics can be ameliorated if central images are present in the data. Applying our methodology to Abell 611 and imposing a reasonable prior on the stellar mass-to-light ratio restricts the core size to be less than about 4 kpc, with a minimum reduced χ2 of 0.28, and with RMS error of 0|${^{\prime\prime}_{.}}$|12 in image positions. Such small cores imply a constraint on the dark matter self-interaction cross section of the order of |$0.1\ \mathrm{cm^2\, g}^{-1}$| at relative velocities of about 1500 km s−1. In addition, our results imply a bottom-heavy (super-Salpeter) initial mass function for the central galaxy in the Abell 611.