Self-Interacting Dark Matter Subhalos in the Milky Way's Tides

We study evolution of self-interacting dark matter (SIDM) subhalos in the Milky Way (MW) tidal field. The interaction between the subhalos and the MW's tides lead to more diverse dark matter distribution in the inner region, compared to their cold dark matter counterparts. We test this scenario with two MW satellite galaxies, Draco and Fornax, opposite extremes in the inner dark matter content, and find that they can be accommodated within the SIDM model proposed to explain the diverse rotation curves of spiral galaxies in the field.Comment: 6 pages, 3 figures. Updated figures and text. Accepted for publication in PR

The impact of baryonic discs on the shapes and profiles of self-interacting dark matter halos

We employ isolated N-body simulations to study the response of self-interacting dark matter (SIDM) halos in the presence of the baryonic potentials. Dark matter self-interactions lead to kinematic thermalization in the inner halo, resulting in a tight correlation between the dark matter and baryon distributions. A deep baryonic potential shortens the phase of SIDM core expansion and triggers core contraction. This effect can be further enhanced by a large self-scattering cross section. We find the final SIDM density profile is sensitive to the baryonic concentration and the strength of dark matter self-interactions. Assuming a spherical initial halo, we also study evolution of the SIDM halo shape together with the density profile. The halo shape at later epochs deviates from spherical symmetry due to the influence of the non-spherical disc potential, and its significance depends on the baryonic contribution to the total gravitational potential, relative to the dark matter one. In addition, we construct a multi-component model for the Milky Way, including an SIDM halo, a stellar disc and a bulge, and show it is consistent with observations from stellar kinematics and streams.Comment: 10 pages, 8 figures, submitted to MNRAS, accepted for publication in MNRA

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

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 $z\leq1$, 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.Comment: 10 pages, 5 figures. Submitted to MNRAS. Comments are welcome

Formation of proto-globular cluster candidates in cosmological simulations of dwarf galaxies at z>4z>4

We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies $(M_{\rm vir} \approx 10^{10}\, {\rm M}_\odot$ at $z=0$) as part of the "Feedback in Realistic Environment" (FIRE) project. Compact ($r_{1/2}<30$ pc), relatively massive ($0.5 \times 10^5 \lesssim M_{\star}/{\rm M}_\odot \lesssim 5\times10^5$), self-bound stellar clusters form at $11\gtrsim z \gtrsim 5$ in progenitors with $M_{\rm vir} \approx 10^9\,{\rm M}_\odot$. Cluster formation is triggered when at least $10^7\,{\rm M}_\odot$ of dense, turbulent gas reaches $\Sigma_{\rm gas} \approx 10^4\, {\rm M}_\odot\, {\rm pc}^{-2}$ as a result of the compressive effects of supernova feedback or from cloud-cloud collisions. The clusters can survive for $2-3\,{\rm Gyr}$; absent numerical effects, they would likely survive substantially longer, perhaps to $z=0$. The longest-lived clusters are those that form at significant distance -- several hundreds of pc -- from their host galaxy. We therefore predict that globular clusters forming in progenitors of present-day dwarf galaxies will be offset from any pre-existing stars within their host dark matter halos as opposed to deeply embedded within a well-defined galaxy. Properties of the nascent clusters are consistent with observations of some of the faintest and most compact high-redshift sources in \textit{Hubble Space Telescope} lensing fields and are at the edge of what will be detectable as point sources in deep imaging of non-lensed fields with the \textit{James Webb Space Telescope}. By contrast, the star clusters' host galaxies will remain undetectable.Comment: 14 pages, 5 figures, submitted to MNRA

Halo Formation in Self-Interacting Dark Matter Models

The standard model of cosmology assumes that dark matter (DM) is cold and collisionless. This collisionless Cold Dark Matter (CDM) model has been extremely successful in explaining various observational phenomena on mass scales larger than galaxies. Despite the successes on large scales, the CDM model faces several challenges on smaller scales that have remained unanswered for several years. One of the most intriguing discrepancies between CDM predictions and observations stems from the DM content in dwarf and spiral galaxies. While CDM predicts radially divergent DM density (cusps) at the halo centers, many observations suggest constant DM density cores. Abundance of the observed Milky Way satellites has also been at odds with the predictions from CDM. While observations suggest ≤ 100 luminous Milky Way satellites, CDM N-body simulations predict 1-2 orders of magnitude more subhalos for a Milky Way-like galaxy.As an alternative to CDM, self-interacting dark matter (SIDM) models are proposed as potential solutions to some of these cosmological issues. In this class of DM models, DM-DM scatter- ing leads to a re-distribution of energy at the center of the halo such that after a few dynamical times, the halo reaches in-equilibrium isothermal state. This results in the re-distribution of DM particles, which, in principle, can form constant density cores opposite to the cuspy profiles predicted by the CDM model. The interaction between dark matter particles may be extended to the scenarios where DM scatters off from some other relativistic dark sector particles (e.g. dark photons), leading to suppressions in the matter power spectrum. This results in the depletion of DM halos in the regime corresponding to this cutoff in the matter power spectrum, which pro- vides another channel to address the tension between theoretical predictions and observations.In this dissertation, I study the impact of non-gravitational interactions in the dark sector on the distribution and evolution of DM halos. I find that when entangled with the baryonic physics, SIDM models possess rich phenomenology for the formation of structures in the uni- verse. For the dark matter halos in isolation with low baryonic content, DM-DM interactions lead to a cored DM density profile (e.g. dwarf-sized galaxies), as expected from the isothermal behavior of SIDM. On the other hand, in isolated halos with a more significant contribution of baryons (e.g. closer to the mass scale of the Milky Way), a phase of core-contraction is triggered, leading to a high central density. Furthermore, the environmental effects, such as tidal stripping and tidal shocking, can dramatically change the fate of these objects. While gravitational tides remove mass from outskirts of the satellite halos, the heat transport due to the DM self-interactions results in a faster “core-collapse.” The transition from core-expansion to core-collapse is controlled by the orbit and pre-infall halo parameters.In the DM models where DM experiences interactions with dark photons, the suppression in the matter power spectrum causes a depletion of DM halos in the regime of dwarf galaxies. In this dissertation, I develop an analytical Press-Schechter approach to compute halo abundance for SIDM models in a computationally efficient way that is not possible for the conventional numerical simulations. I calibrate and test this analytical model with cosmological simulations, and show that it is robust over different SIDM models, halo mass regimes, and cosmological time-scales. I use this formalism to put a constraint on the parameter space of DM-dark photon interactions that is consistent with previous measurements

Spreading out and staying sharp – creating diverse rotation curves via baryonic and self-interaction effects

© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Galactic rotation curves are a fundamental constraint for any cosmological model. We use controlled N-body simulations of galaxies to study the gravitational effect of baryons in a scenario with collisionless cold dark matter (CDM) versus one with a self-interacting dark matter (SIDM) component. In particular, we examine the inner profiles of the rotation curves in the velocity range Vmax = [30-250] km s−1, whose diversity has been found to be greater than predicted by the ΛCDM scenario. We find that the scatter in the observed rotation curves exceeds that predicted by dark matter only mass-concentration relations in either the CDM nor SIDM models. Allowing for realistic baryonic content and spatial distributions, however, helps create a large variety of rotation curve shapes, which is in a better agreement with observations in the case of self-interactions due to the characteristic cored profiles being more accommodating to the slowly rising rotation curves than CDM. We find individual fits to model two of the most remarkable outliers of similar Vmax, UGC 5721 and IC 2574 - the former a cusp-like rotation curve and the latter a seemingly 8-kpc-cored profile. This diversity in SIDM arises as permutations of overly concentrated haloes with compact baryonic distributions versus underdense haloes with extended baryonic discs. The SIDM solution is promising and its feasibility ultimately depends on the sampling of the halo mass-concentration relation and its interplay with the baryonic profiles, emphasizing the need for a better understanding of the frequency of extreme outliers present in current observational samples

Shapes of Milky-Way-Mass Galaxies with Self-Interacting Dark Matter

Self-interacting dark matter (SIDM) models offer one way to reconcile inconsistencies between observations and predictions from collisionless cold dark matter (CDM) models on dwarf-galaxy scales. In order to incorporate the effects of both baryonic and SIDM interactions, we study a suite of cosmological-baryonic simulations of Milky-Way (MW)-mass galaxies from the Feedback in Realistic Environments (FIRE-2) project where we vary the SIDM self-interaction cross-section $\sigma/m$. We compare the shape of the main dark matter (DM) halo at redshift $z=0$ predicted by SIDM simulations (at $\sigma/m=0.1$, $1$, and $10$ cm$^2$ g$^{-1}$) with CDM simulations using the same initial conditions. In the presence of baryonic feedback effects, we find that SIDM models do not produce the large differences in the inner structure of MW-mass galaxies predicted by SIDM-only models. However, we do find that the radius where the shape of the total mass distribution begins to differ from that of the stellar mass distribution is dependent on $\sigma/m$. This transition could potentially be used to set limits on the SIDM cross-section in the MW