Many aspects of the large-scale structure of the Universe can be described successfully using cosmological models in which $27\ifmmode\pm\else\textpm\fi{}1%$ of the critical mass-energy density consists of cold dark matter (CDM). However, few---if any---of the predictions of CDM models have been successful on scales of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{kpc}$ or less. This lack of success is usually explained by the difficulty of modeling baryonic physics (star formation, supernova and black-hole feedback, etc.). An intriguing alternative to CDM is that the dark matter is an extremely light ($m\ensuremath{\sim}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$) boson having a de Broglie wavelength $\ensuremath{\lambda}\ensuremath{\sim}1\text{ }\text{ }\mathrm{kpc}$, often called fuzzy dark matter (FDM). We describe the arguments from particle physics that motivate FDM, review previous work on its astrophysical signatures, and analyze several unexplored aspects of its behavior. In particular, (i) FDM halos or subhalos smaller than about $1{0}^{7}(m/{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}{)}^{\ensuremath{-}3/2}$ ${M}_{\ensuremath{\bigodot}}$ do not form, and the abundance of halos smaller than a few times $1{0}^{10}(m/{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}{)}^{\ensuremath{-}4/3}$ ${M}_{\ensuremath{\bigodot}}$ is substantially smaller in FDM than in CDM. (ii) FDM halos are comprised of a central core that is a stationary, minimum-energy solution of the Schr\"odinger-Poisson equation, sometimes called a ``soliton,'' surrounded by an envelope that resembles a CDM halo. The soliton can produce a distinct signature in the rotation curves of FDM-dominated systems. (iii) The transition between soliton and envelope is determined by a relaxation process analogous to two-body relaxation in gravitating N-body systems, which proceeds as if the halo were composed of particles with mass $\ensuremath{\sim}\ensuremath{\rho}{\ensuremath{\lambda}}^{3}$ where $\ensuremath{\rho}$ is the halo density. (iv) Relaxation may have substantial effects on the stellar disk and bulge in the inner parts of disk galaxies, but has negligible effect on disk thickening or globular cluster disruption near the solar radius. (v) Relaxation can produce FDM disks but a FDM disk in the solar neighborhood must have a half-thickness of at least $\ensuremath{\sim}300(m/{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}{)}^{\ensuremath{-}2/3}\text{ }\text{ }\mathrm{pc}$ and a midplane density less than $0.2(m/{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}{)}^{2/3}$ times the baryonic disk density. (vi) Solitonic FDM subhalos evaporate by tunneling through the tidal radius and this limits the minimum subhalo mass inside $\ensuremath{\sim}30\text{ }\text{ }\mathrm{kpc}$ of the Milky Way to a few times $1{0}^{8}(m/{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}{)}^{\ensuremath{-}3/2}$ ${M}_{\ensuremath{\bigodot}}$. (vii) If the dark matter in the Fornax dwarf galaxy is composed of CDM, most of the globular clusters observed in that galaxy should have long ago spiraled to its center, and this problem is resolved if the dark matter is FDM. (viii) FDM delays galaxy formation relative to CDM but its galaxy-formation history is consistent with current observations of high-redshift galaxies and the late reionization observed by Planck. If the dark matter is composed of FDM, most observations favor a particle mass $\ensuremath{\gtrsim}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$ and the most significant observational consequences occur if the mass is in the range $1--10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$. There is tension with observations of the Lyman-$\ensuremath{\alpha}$ forest, which favor $m\ensuremath{\gtrsim}10--20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$ and we discuss whether more sophisticated models of reionization may resolve this tension.

Ultralight scalars as cosmological dark matter

1 Introduction 2 Models: the QCD axion; the strong CP problem; PQWW, KSVZ, DFSZ; anomalies, instantons and the potential; couplings; axions in string theory 3 Production and IC's: SSB and non-perturbative physics; the axion field during inflation and PQ SSB; cosmological populations - decay of parent, topological defects, thermal production, vacuum realignment 4 The Cosmological Field: action; background evolution; misalignment for QCD axion and ALPs; cosmological perturbation theory - ic's, early time treatment, axion sound speed and Jeans scale, transfer functions and WDM; the Schrodinger picture; simualting axions; BEC 5 CMB and LSS: Primary anisotropies; matter power; combined constraints; Isocurvature and inflation 6 Galaxy Formation; halo mass function; high-z and the EOR; density profiles; the CDM small-scale crises 7 Accelerated expansion: the cc problem; axion inflation (natural and monodromy) 8 Gravitational interactions with black holes and pulsars 9 Non-gravitational interactions: stellar astrophysics; LSW; vacuum birefringence; axion forces; direct detection with ADMX and CASPEr; Axion decays; dark radiation; astrophysical magnetic fields; cosmological birefringence 10 Conclusions A Theta vacua of gauge theories B EFT for cosmologists C Friedmann equations D Cosmological fluids E Bayes Theorem and priors F Degeneracies and sampling G Sheth-Tormen HMF

Axion Cosmology

This paper describes the open-source code Enzo, which uses block-structured adaptive mesh refinement to provide high spatial and temporal resolution for modeling astrophysical fluid flows. The code is Cartesian, can be run in one, two, and three dimensions, and supports a wide variety of physics including hydrodynamics, ideal and non-ideal magnetohydrodynamics, N-body dynamics (and, more broadly, self-gravity of fluids and particles), primordial gas chemistry, optically thin radiative cooling of primordial and metal-enriched plasmas (as well as some optically-thick cooling models), radiation transport, cosmological expansion, and models for star formation and feedback in a cosmological context. In addition to explaining the algorithms implemented, we present solutions for a wide range of test problems, demonstrate the code's parallel performance, and discuss the Enzo collaboration's code development methodology.

/pdf/enzo-an-adaptive-mesh-refinement-code-for-astrophysics-2cgbpdu97q.pdf

Enzo: An Adaptive Mesh Refinement Code for Astrophysics

Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulae, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.

The microphysics of collisionless shock waves

If a significant fraction of the dark matter in the Universe is made of an ultra-light scalar field, named fuzzy dark matter (FDM) with a mass of the order of 10^−22 − 10^−21 eV, then its de Broglie wavelength is large enough to impact the physics of large-scale structure formation. In particular, the associated cut-off in the linear matter power spectrum modifies the structure of the intergalactic medium (IGM) at the scales probed by the Lyman-α forest of distant quasars. We study this effect by making use of dedicated cosmological simulations which take into account the hydrodynamics of the IGM. We explore heuristically the amplitude of quantum pressure for the FDM masses considered here and conclude that quantum effects should not modify significantly the non-linear evolution of matter density at the scales relevant to the measured Lyman-α flux power, and for m_a ≳ 10^ − 22 eV. We derive a scaling law between m_a and the mass of the well-studied thermal warm dark matter model that is best adapted to the Lyman-α forest data, and differs significantly from the one inferred by a simple linear extrapolation. By comparing FDM simulations with the Lyman-α flux power spectra determined from the BOSS survey, and marginalizing over relevant nuisance parameters, we exclude FDM masses in the range 10^ − 22 ≲ m_a < 2.3 × 10^ − 21 eV at 95  per cent CL. Adding higher resolution Lyman-α spectra extends the exclusion range up to 2.9 × 10^−21 eV. This provides a significant constraint on FDM models tailored to solve the ‘small-scale problems’ of ΛCDM.

Constraining the mass of light bosonic dark matter using SDSS Lyman-α forest

Self-gravitating bosonic fields can support stable and localised field configurations. For real fields, these solutions oscillate in time and are known as oscillatons. The density profile is static, and is soliton. Such solitons should be ubiquitous in models of axion dark matter, with the soliton characteristic mass and size depending on some inverse power of the axion mass. Stable configurations of non-relativistic axions are studied numerically using the Schrodinger-Poisson system. This method, and the resulting soliton density profiles, are reviewed. Using a scaling symmetry and the uncertainty principle, the core size of the soliton can be related to the central density and axion mass, $m_a$, in a universal way. Solitons have a constant central density due to pressure-support, unlike the cuspy profile of cold dark matter (CDM). One consequence of this fact is that solitons composed of ultra-light axions (ULAs) may resolve the `cusp-core' problem of CDM. In DM halos, thermodynamics will lead to a CDM-like Navarro-Frenk-White profile at large radii, with a central soliton core at small radii. Using Monte-Carlo techniques to explore the possible density profiles of this form, a fit to stellar-kinematical data of dwarf spheroidal galaxies is performed. In order for ULAs to resolve the cusp-core problem (without recourse to baryon feedback or other astrophysical effects) the axion mass must satisfy $m_a<1.1\times 10^{-22}\text{ eV}$ at 95\% C.L. On the other hand, ULAs with $m_a\lesssim 1\times 10^{-22}\text{ eV}$ are in some tension with cosmological structure formation. An axion solution to the cusp-core problem thus makes novel predictions for future measurements of the epoch of reionisation. On the other hand, this can be seen as evidence that structure formation could soon impose a \emph{Catch 22} on axion/scalar field DM, similar to the case of warm DM.

Axion dark matter, solitons and the cusp–core problem

We use kinetic hybrid simulations (kinetic ions-fluid electrons) to characterize the fraction of ions that are accelerated to non-thermal energies at non-relativistic collisionless shocks. We investigate the properties of the shock discontinuity and show that shocks propagating almost along the background magnetic field (quasi-parallel shocks) reform quasi-periodically on ion cyclotron scales. Ions that impinge on the shock when the discontinuity is the steepest are specularly reflected. This is a necessary condition for being injected, but it is not sufficient. Also, by following the trajectories of reflected ions, we calculate the minimum energy needed for injection into diffusive shock acceleration, as a function of the shock inclination. We construct a minimal model that accounts for the ion reflection from quasi-periodic shock barrier, for the fraction of injected ions, and for the ion spectrum throughout the transition from thermal to non-thermal energies. This model captures the physics relevant for ion injection at non-relativistic astrophysical shocks with arbitrary strengths and magnetic inclinations, and represents a crucial ingredient for understanding the diffusive shock acceleration of cosmic rays.

https://iopscience.iop.org/article/10.1088/2041-8205/798/2/L28/pdf

Simulations and theory of ion injection at non-relativistic collisionless shocks

Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_{\odot}$), $18\%\pm3\%$ of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell-forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.

Formation and incidence of shell galaxies in the Illustris simulation

We use high-resolution cosmological hydrodynamic galaxy formation simulations to gain insights into how galaxies lose their cold gas at low redshift as they migrate from the field to the high-density regions of clusters of galaxies. We find that beyond three cluster virial radii, the fraction of gas-rich galaxies is constant, representing the field. Within three cluster-centric radii, the fraction of gas-rich galaxies declines steadily with decreasing radius, reaching <10% near the cluster center. Our results suggest galaxies start to feel the effect of the cluster environment on their gas content well beyond the cluster virial radius. We show that almost all gas-rich galaxies at the cluster virial radius are falling in for the first time at nearly radial orbits. Furthermore, we find that almost no galaxy moving outward at the cluster virial radius is gas-rich (with a gas-to-baryon ratio greater than 1%). These results suggest that galaxies that fall into clusters lose their cold gas within a single radial round-trip.

Gas loss in simulated galaxies as they fall into clusters.

We use kinetic hybrid simulations (kinetic ions - fluid electrons) to characterize the fraction of ions that are accelerated to non-thermal energies at non-relativistic collisionless shocks. We investigate the properties of the shock discontinuity and show that shocks propagating almost along the background magnetic field (quasi-parallel shocks) reform quasi-periodically on ion cyclotron scales. Ions that impinge on the shock when the discontinuity is the steepest are specularly reflected. This is a necessary condition for being injected, but it is not sufficient. Also by following the trajectories of reflected ions, we calculate the minimum energy needed for injection into diffusive shock acceleration, as a function of the shock inclination. We construct a minimal model that accounts for the ion reflection from quasi-periodic shock barrier, for the fraction of injected ions, and for the ion spectrum throughout the transition from thermal to non-thermal energies. This model captures the physics relevant for ion injection at non-relativistic astrophysical shocks with arbitrary strengths and magnetic inclinations, and represents a crucial ingredient for understanding the diffusive shock acceleration of cosmic rays.

Ana-Roxana Pop

Papers

Axion dark matter, solitons and the cusp–core problem

Simulations and theory of ion injection at non-relativistic collisionless shocks

Formation and incidence of shell galaxies in the Illustris simulation

Gas loss in simulated galaxies as they fall into clusters.

Simulations and Theory of Ion Injection at Non-relativistic Collisionless Shocks