Christoph Pfrommer

Galactic winds driven by cosmic-ray streaming

Galactic winds are observed in many spiral galaxies with sizes from dwarfs up to the Milky Way, and they sometimes carry a mass in excess of that of newly formed stars by up to a factor of 10. Multiple driving processes of such winds have been proposed, including thermal pressure due to supernova heating, ultraviolet radiation pressure on dust grains or cosmic ray (CR) pressure. We here study wind formation due to CR physics using a numerical model that accounts for CR acceleration by supernovae, CR thermalization by Coulomb and hadronic interactions, and advective CR transport. In addition, we introduce a novel implementation of CR streaming relative to the rest frame of the gas. Streaming CRs excite Alfven waves on which they scatter, thereby limiting the CRs’ effective bulk velocity. We find that CR streaming drives powerful and sustained winds in galaxies with virial masses . In dwarf galaxies () the winds reach a mass loading factor of ∼5, expel ∼60 per cent of the initial baryonic mass contained inside the halo’s virial radius and suppress the star formation rate by a factor of ∼5. In dwarfs, the winds are spherically symmetric while in larger galaxies the outflows transition to biconical morphologies that are aligned with the disc’s angular momentum axis. We show that damping of Alfven waves excited by streaming CRs provides a means of heating the outflows to temperatures that scale with the square of the escape speed, . In larger haloes (), CR streaming is able to drive fountain flows that excite turbulence, providing another means of heating the halo gas. For halo masses , we predict an observable level of Hα and X-ray emission from the heated halo gas. We conclude that CR-driven winds should be crucial in suppressing and regulating the first epoch of galaxy formation, expelling a large fraction of baryons, and – by extension – aid in shaping the faint end of the galaxy luminosity function. They should then also be responsible for much of the metal enrichment of the intergalactic medium.

Is dark matter with long-range interactions a solution to all small-scale problems of \Lambda CDM cosmology?

The cold dark matter paradigm describes the large-scale structure of the Universe remarkably well. However, there exists some tension with the observed abundances and internal density structures of both field dwarf galaxies and galactic satellites. Here, we demonstrate that a simple class of dark matter models may offer a viable solution to all of these problems simultaneously. Their key phenomenological properties are velocity-dependent self-interactions mediated by a light vector messenger and thermal production with much later kinetic decoupling than in the standard case.

Prospects of detecting gamma-ray emission from galaxy clusters : Cosmic rays and dark matter annihilations

We study the possibility for detecting gamma-ray emission from galaxy clusters. We consider (1) leptophilic models of dark matter (DM) annihilation that include a Sommerfeld enhancement (SFE), (2) ...

CONSTRAINTS ON COSMIC RAYS, MAGNETIC FIELDS, AND DARK MATTER FROM GAMMA-RAY OBSERVATIONS OF THE COMA CLUSTER OF GALAXIES WITH VERITAS AND FERMI

Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. Protons should likewise be accelerated and, on account of weak energy losses, can accumulate, suggesting that clusters may also be sources of very high energy (VHE; E > 100 GeV) gamma-ray emission. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations at GeV energies. No significant gamma-ray emission from the Coma Cluster was detected. Integral flux upper limits at the 99% confidence level were measured to be on the order of (2-5) × 10–8 photons m –2 s –1 (VERITAS, >220 GeV) and ~2 × 10–6 photons m –2 s –1 (Fermi, 1-3 GeV), respectively. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be <16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Assuming that the radio-emitting electrons of the Coma halo result from hadronic CR interactions, the observations imply a lower limit on the central magnetic field in Coma of ~(2-5.5) μG, depending on the radial magnetic field profile and on the gamma-ray spectral index. Since these values are below those inferred by Faraday rotation measurements in Coma (for most of the parameter space), this renders the hadronic model a very plausible explanation of the Coma radio halo. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, σv.

Giant radio relics in galaxy clusters: reacceleration of fossil relativistic electrons?

Many bright radio relics in the outskirts of galaxy clusters have low inferred Mach numbers, defying expectations from shock acceleration theory and heliospheric observations that the injection efficiency of relativistic particles plummets at low Mach numbers. With a suite of cosmological simulations, we follow the diffusive shock acceleration as well as radiative and Coulomb cooling of cosmic ray electrons during the assembly of a cluster. We find a substantial population of fossil electrons. When reaccelerated at a shock (through diffusive shock acceleration), they are competitive with direct injection at strong shocks and overwhelmingly dominate by many orders of magnitude at weak shocks, Mach < 3, which are the vast majority at the cluster periphery. Their relative importance depends on cooling physics and is robust to the shock acceleration model used. While the abundance of fossils can vary by a factor of ~10, the typical reaccelerated fossil population has radio brightness in excellent agreement with observations. Fossil electrons with 1 < gamma < 100 (10 < gamma < 10^4) provide the main seeds for reacceleration at strong (weak) shocks; we show that these are well-resolved by our simulation. We construct a simple self-similar analytic model which assumes steady recent injection and cooling. It agrees well with our simulations, allowing rapid estimates and physical insight into the shape of the distribution function. We predict that LOFAR should find many more bright steep-spectrum radio relics, which are inconsistent with direct injection. A failure to take fossil cosmic ray electrons into account will lead to erroneous conclusions about the nature of particle acceleration at weak shocks; they arise from well-understood physical processes and cannot be ignored.

A comparison of cosmological codes: properties of thermal gas and shock waves in large-scale structures

Cosmological hydrodynamical simulations are a valuable to ol f r understanding the growth of large scale structure and the observables connect ed wi h this. Yet, comparably little attention has been given to validation studies of the proper ties of shocks and of the resulting thermal gas between different numerical methods – somet hing of immediate importance as gravitational shocks are responsible for generating mos t of the entropy of the large scale structure in the Universe. Here, we present results for the s tatistics of thermal gas and the shock wave properties for a large volume simulated with thre e different cosmological numerical codes: the Eulerian total variations diminishing c ode TVD, the Eulerian piecewise parabolic method-based code ENZO, and the Lagrangian smoot hed-particle hydrodynamics code GADGET. Starting from a shared set of initial condition s, we present convergence tests for a cosmological volume of side-length 100Mpc/h, studying in detail the morphological and statistical properties of the thermal gas as a function o f mass and spatial resolution in all codes. By applying shock finding methods to each code, we meas ur the statistics of shock waves and the related cosmic ray acceleration efficiencies, within the sample of simulations and for the results of the different approaches. We discuss t he regimes of uncertainties and disagreement among codes, with a particular focus on the res ults at the scale of galaxy clusters. Even if the bulk of thermal and shock properties are rea son bly in agreement among the three codes, yet some significant differences exist (esp ecially between Eulerian methods and smoothed particle hydrodynamics). In particular, we re po t: a) differences of huge factors (∼ 10 − 100) in the values of average gas density, temperature, entropy , Mach number and shock thermal energy flux in the most rarefied regions of the si mulations (ρ/ρcr < 1) between grid and SPH methods; b) the hint of an entropy core inside clu sters simulated in grid codes; c) significantly different phase diagrams of shocked cells i n gr d codes compared to SPH; d) sizable differences in the morphologies of accretion shock s between grid and SPH methods.

ON THE CLUSTER PHYSICS OF SUNYAEV-ZEL'DOVICH AND X-RAY SURVEYS. III. MEASUREMENT BIASES AND COSMOLOGICAL EVOLUTION OF GAS AND STELLAR MASS FRACTIONS

Gas masses tightly correlate with the virial masses of galaxy clusters, allowing for a precise determination of cosmological parameters by means of X-ray surveys. However, the gas mass fractions (f gas) at the virial radius (R 200) derived from recent Suzaku observations are considerably larger than the cosmic mean, calling into question the accuracy of cosmological parameters. Here, we use a large suite of cosmological hydrodynamical simulations to study measurement biases of f gas. We employ different variants of simulated physics, including radiative gas physics, star formation, and thermal feedback by active galactic nuclei, which we show is able to arrest overcooling and to result in constant stellar mass fractions for redshifts z < 1. Computing the mass profiles in 48 angular cones, we find anisotropic gas and total mass distributions that imply an angular variance of f gas at the level of 30%. This anisotropy originates from the recent formation epoch of clusters and from the strong internal baryon-to-dark-matter density bias. In the most extreme cones, f gas can be biased high by a factor of two at R 200 in massive clusters (M 200 ~ 1015 M ☉), thereby providing an explanation for high f gas measurements by Suzaku. While projection lowers this factor, there are other measurement biases that may (partially) compensate. At R 200, f gas is biased high by 20% when assuming hydrostatic equilibrium masses, i.e., neglecting the kinetic pressure, and by another ~10%-20% due to the presence of density clumping. At larger radii, both measurement biases increase dramatically. While the cluster sample variance of the true f gas decreases to a level of 5% at R 200, the sample variance that includes both measurement biases remains fairly constant at the level of 10%-20%. The constant redshift evolution of f gas within R 500 for massive clusters is encouraging for using gas masses to derive cosmological parameters, provided the measurement biases can be controlled.

The Lyman α forest in a blazar-heated Universe

It has been realized only recently that TeV emission from blazars can significantly heat the intergalactic medium (IGM) by pair-producing high-energy electrons and positrons, which in turn excite vigorous plasma instabilities, leading to a local dissipation of the pairs’ kinetic energy. In this work, we use cosmological hydrodynamical simulations to model the impact of this blazar heating on the Lyman α forest at intermediate redshifts (z∼ 2–3). We find that blazar heating produces an inverted temperature–density relation in the IGM and naturally resolves many of the problems present in previous simulations of the forest that included photoionization heating alone. In particular, our simulations with blazar heating simultaneously reproduce the observed effective optical depth and temperature as a function of redshift, the observed probability distribution functions (PDFs) of the transmitted flux, and the observed flux power spectra, over the full redshift range 2 < z < 3 analysed here. Additionally, by deblending the absorption features of Lyman α spectra into a sum of thermally broadened individual lines, we find superb agreement with the observed lower cut-off of the linewidth distribution and abundances of neutral hydrogen column densities per unit redshift. Using the most recent constraints on the cosmic ultraviolet (UV) background, this excellent agreement with observations does not require rescaling the amplitude of the UV background – a procedure that was routinely used in the past to match the observed level of transmitted flux. We also show that our blazar-heated model matches the data better than standard simulations even when such a rescaling is allowed. This concordance between Lyman α data and simulation results, which are based on the most recent cosmological parameters, also suggests that the inclusion of blazar heating alleviates previous tensions on constraints for σ8 derived from Lyman α measurements and other cosmological data. Finally, we show that blazar heating dramatically alters the volume-weighted temperature PDF, implying an important change in the strengths of structure formation shocks (and thereby possibly particle acceleration in these shocks). The density PDF is also modified, suggesting that blazar heating may have interesting effects on structure formation, particularly on the smallest galaxies.

ETHOS—an effective theory of structure formation: From dark particle physics to the matter distribution of the Universe

We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in almost any microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of nonrelativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. Models with similar effective parameters in ETHOS but with different dark particle physics would nevertheless result in similar dark matter distributions. We present a general method to map an ultraviolet complete or effective field theory of low-energy dark matter physics into parameters that affect the linear matter power spectrum and carry out this mapping for several representative particle models. We further propose a simple but useful choice for characterizing the dark matter self-interaction transfer cross section that parametrizes self-scattering in structure formation simulations. Taken together, these effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.

ETHOS – an effective theory of structure formation: dark matter physics as a possible explanation of the small-scale CDM problems

We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of 2.8 × 10^4 M_⊙ and a softening length of 72.4 pc. We demonstrate that all alternative models have only a negligible impact on large-scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small-scale primordial damping in the power spectrum and late-time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even oversolve the too-big-to-fail problem; thus ruling them out.