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1. The central densities of Milky Way-mass galaxies in cold and self-interacting dark matter models

   https://doi.org/10.1093/mnras/stab2173  

   Sameie, Omid ; Boylan-Kolchin, Michael ; Sanderson, Robyn ; Vargya, Drona ; Hopkins, Philip F ; Wetzel, Andrew ; Bullock, James ; Graus, Andrew ; Robles, Victor H ( August 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present a suite of baryonic cosmological zoom-in simulations of self-interacting dark matter (SIDM) haloes within the ‘Feedback In Realistic Environment’ (FIRE) project. The three simulated haloes have virial masses of $\sim 10^{12}\, \text{M}_\odot$ at z = 0, and we study velocity-independent self-interaction cross sections of 1 and 10 ${\rm cm^2 \, g^{-1}}$. We study star formation rates and the shape of dark matter density profiles of the parent haloes in both cold dark matter (CDM) and SIDM models. Galaxies formed in the SIDM haloes have higher star formation rates at z ≤ 1, resulting in more massive galaxies compared to the CDM simulations. While both CDM and SIDM simulations show diverse shape of the dark matter density profiles, the SIDM haloes can reach higher and more steep central densities within few kpcs compared to the CDM haloes. We identify a correlation between the build-up of the stars within the half-mass radii of the galaxies and the growth in the central dark matter densities. The thermalization process in the SIDM haloes is enhanced in the presence of a dense stellar component. Hence, SIDM haloes with highly concentrated baryonic profiles are predicted to have higher central dark matter densities than the CDM haloes. Overall, the SIDM haloes are more responsive to the presence of a massive baryonic distribution than their CDM counterparts. 

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2. Dissipative dark matter on FIRE – I. Structural and kinematic properties of dwarf galaxies

   https://doi.org/10.1093/mnras/stab2042  

   Shen, Xuejian ; Hopkins, Philip F ; Necib, Lina ; Jiang, Fangzhou ; Boylan-Kolchin, Michael ; Wetzel, Andrew ( August 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present the first set of cosmological baryonic zoom-in simulations of galaxies including dissipative self-interacting dark matter (dSIDM). These simulations utilize the Feedback In Realistic Environments galaxy formation physics, but allow the dark matter to have dissipative self-interactions analogous to standard model forces, parametrized by the self-interaction cross-section per unit mass, (σ/m), and the dimensionless degree of dissipation, 0 < fdiss < 1. We survey this parameter space, including constant and velocity-dependent cross-sections, and focus on structural and kinematic properties of dwarf galaxies with $M_{\rm halo} \sim 10^{10-11}{\, \rm M_\odot }$ and $M_{\ast } \sim 10^{5-8}{\, \rm M_\odot }$. Central density profiles (parametrized as ρ ∝ rα) of simulated dwarfs become cuspy when $(\sigma /m)_{\rm eff} \gtrsim 0.1\, {\rm cm^{2}\, g^{-1}}$ (and fdiss = 0.5 as fiducial). The power-law slopes asymptote to α ≈ −1.5 in low-mass dwarfs independent of cross-section, which arises from a dark matter ‘cooling flow’. Through comparisons with dark matter only simulations, we find the profile in this regime is insensitive to the inclusion of baryons. However, when $(\sigma /m)_{\rm eff} \ll 0.1\, {\rm cm^{2}\, g^{-1}}$, baryonic effects can produce cored density profiles comparable to non-dissipative cold dark matter (CDM) runs but at smaller radii. Simulated galaxies with $(\sigma /m) \gtrsim 10\, {\rm cm^{2}\, g^{-1}}$ and the fiducial fdiss develop significant coherent rotation of dark matter, accompanied by halo deformation, but this is unlike the well-defined thin ‘dark discs’ often attributed to baryon-like dSIDM. The density profiles in this high cross-section model exhibit lower normalizations given the onset of halo deformation. For our surveyed dSIDM parameters, halo masses and galaxy stellar masses do not show appreciable difference from CDM, but dark matter kinematics and halo concentrations/shapes can differ. 

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3. The Milky Way’s halo and subhaloes in self-interacting dark matter

   https://doi.org/10.1093/mnras/stz2345  

   Robles, Victor H. ; Kelley, Tyler ; Bullock, James S. ; Kaplinghat, Manoj ( September 2019 , Monthly Notices of the Royal Astronomical Society)

   We perform high-resolution simulations of an MW-like galaxy in a self-interacting cold dark matter model with elastic cross-section over mass of $1~\rm cm^2\, g^{-1}$ (SIDM) and compare to a model without self-interactions (CDM). We run our simulations with and without a time-dependent embedded potential to capture effects of the baryonic disc and bulge contributions. The CDM and SIDM simulations with the embedded baryonic potential exhibit remarkably similar host halo profiles, subhalo abundances, and radial distributions within the virial radius. The SIDM host halo is denser in the centre than the CDM host and has no discernible core, in sharp contrast to the case without the baryonic potential (core size ${\sim}7 \, \rm kpc$). The most massive subhaloes (with $V_{\mathrm{peak}}\gt 20 \, \rm km\, s^{-1}$) in our SIDM simulations, expected to host the classical satellite galaxies, have density profiles that are less dense than their CDM analogues at radii less than 500 pc but the deviation diminishes for less massive subhaloes. With the baryonic potential included in the CDM and SIDM simulations, the most massive subhaloes do not display the too-big-to-fail problem. However, the least dense among the massive subhaloes in both these simulations tend to have the smallest pericenter values, a trend that is not apparent among the bright MW satellite galaxies.

    

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

   https://doi.org/10.1093/mnras/stac2376  

   Shen, Xuejian ; Brinckmann, Thejs ; Rapetti, David ; Vogelsberger, Mark ; Mantz, Adam ; Zavala, Jesús ; Allen, Steven W. ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   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.

    

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5. The onset of gravothermal core collapse in velocity-dependent self-interacting dark matter subhaloes

   https://doi.org/10.1093/mnras/stab1725  

   Turner, Hannah C ; Lovell, Mark R ; Zavala, Jesús ; Vogelsberger, Mark ( June 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT It has been proposed that gravothermal collapse due to dark matter self-interactions (i.e. self-interacting dark matter, SIDM) can explain the observed diversity of the Milky Way (MW) satellites’ central dynamical masses. We investigate the process behind this hypothesis using an N-body simulation of a MW-analogue halo with velocity-dependent SIDM (vdSIDM) in which the low-velocity self-scattering cross-section, $\sigma _{\rm T}/m_{\rm x}$, reaches 100 cm2 g−1; we dub this model the vd100 model. We compare the results of this simulation to simulations of the same halo that employ different dark models, including cold dark matter (CDM) and other, less extreme SIDM models. The masses of the vd100 haloes are very similar to their CDM counterparts, but the values of their maximum circular velocities, Vmax, are significantly higher. We determine that these high Vmax subhaloes were objects in the mass range [5 × 106, 1 × 108] M⊙ at z = 1 that undergo gravothermal core collapse. These collapsed haloes have density profiles that are described by single power laws down to the resolution limit of the simulation, and the inner slope of this density profile is approximately −3. Resolving the ever decreasing collapsed region is challenging, and tailored simulations will be required to model the runaway instability accurately at scales <1 kpc. 

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