arXiv subjects

Torsten A. Ensslin

Publications and source records attributed to Torsten A. Ensslin.

At least 37 records · Page 2Linked to original sources

arXiv2008

Optimal ISW detection and joint likelihood for cosmological parameter estimation

We analyse the local variance effect in the standard method for detecting the integrated Sachs-Wolfe effect (ISW) via cross-correlating the cosmic microwave background (CMB) with the large-scale structure (LSS). Local variance is defined as the systematic noise in the ISW detection that originates in the realisation of the matter distribution in the observed Universe. We show that the local variance contributes about 11 per cent to the total variance in the standard method, if a perfect and complete LSS survey up to z ~ 2 is assumed. Due to local variance, the estimated detection significance and cosmological parameter constraints in the standard method are biased. In this work, we present an optimal method of how to reduce the local variance effect in the ISW detection by working conditional on the LSS-data. The variance of the optimal method, and hence the signal-to-noise ratio, depends on the actual realisation of the matter distribution in the observed Universe. We show that for an ideal galaxy survey, the average signal-to-noise ratio is enhanced by about 7 per cent in the optimal method, as compared to the standard method. Furthermore, in the optimal method there is no need to estimate the covariance matrix by Monte Carlo simulations as in the standard method, which saves time and increases the accuracy. Finally, we derive the correct joint likelihood function for cosmological parameters given CMB- and LSS-data within the linear LSS formation regime, which includes a small coupling of the two datasets due to the ISW effect.

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arXiv2008

Simulating cosmic rays in clusters of galaxies - II. A unified scheme for radio halos and relics with predictions of the gamma-ray emission

The thermal plasma of galaxy clusters lost most of its information on how structure formation proceeded as a result of dissipative processes. In contrast, non-equilibrium distributions of cosmic rays (CR) preserve the information about their injection and transport processes and provide thus a unique window of current and past structure formation processes. This information can be unveiled by observations of non-thermal radiative processes, including radio synchrotron, hard X-ray, and gamma-ray emission. To explore this, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of about two orders of magnitudes, and follow self-consistent CR physics on top of the radiative hydrodynamics. We model CR electrons that are accelerated at cosmological structure formation shocks and those that are produced in hadronic interactions of CRs with ambient gas protons. We find that CR protons trace the time integrated non-equilibrium activities of clusters while shock-accelerated CR electrons probe current accretion and merging shock waves. The resulting inhomogeneous synchrotron emission matches the properties of observed radio relics. We propose a unified model for the generation of radio halos. Giant radio halos are dominated in the centre by secondary synchrotron emission with a transition to the synchrotron radiation emitted from shock-accelerated electrons in the cluster periphery. This model is able to explain the observed correlation of mergers with radio halos, the larger peripheral variation of the spectral index, and the large scatter in the scaling relation between cluster mass and synchrotron emission. Future low-frequency radio telescopes (LOFAR, GMRT, MWA, LWA) are expected to probe the accretion shocks of clusters. [abridged]

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arXiv2007

Large scale magnetic field of the Milky Way from WMAP3 data

We report on initial results from a project to constrain the large-scale and turbulent magnetic fields of the Milky Way galaxy, which eventually will incorporate all of the relevant observational data. In this paper we fit popular large scale magnetic field models to WMAP3 polarization maps. We find that the polarization data can constrain certain model parameters but does not uniquely determine the best-fit parameters. We also find that the polarization data alone cannot distinguish between model symmetries, e.g., the existence of field reversals. We show how future UHECR data can break this degeneracy.

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arXiv2007

Cosmic ray physics in calculations of cosmological structure formation

Cosmic rays (CRs) play a decisive role within our own Galaxy. They provide partial pressure support against gravity, they trace past energetic events such as supernovae, and they reveal the underlying structure of the baryonic matter distribution through their interactions. To study the impact of CRs on galaxy and cosmic structure formation and evolution, we develop an approximative framework for treating dynamical and radiative effects of CRs in cosmological simulations. Our guiding principle is to try to find a balance between capturing as many physical properties of CR populations as possible while at the same time requiring as little extra computational resources as possible. We approximate the CR spectrum of each fluid element by a single power-law, with spatially and temporally varying normalisation, low-energy cut-off, and spectral index. Principles of conservation of particle number, energy, and pressure are then used to derive evolution equations for the basic variables describing the CR spectrum, both due to adiabatic and non-adiabatic processes. The processes considered include compression and rarefaction, CR injection via shocks in supernova remnants, injection in structure formation shock waves, in-situ re-acceleration of CRs, CR spatial diffusion, CR energy losses due to Coulomb interactions, ionisation losses, Bremsstrahlung losses, and, finally, hadronic interactions with the background gas, including the associated gamma-ray and radio emission due to subsequent pion decay. We show that the formalism reproduces CR energy densities, pressure, and cooling rates with an accuracy of ~10% in steady state conditions where CR injection balances cooling. Our framework is therefore well suited to be included into numerical simulation schemes of galaxy and structure formation. (abridged)

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arXiv2007

Cosmic rays and the primordial gas

One of the most outstanding problems in the gravitational collapse scenario of early structure formation is the cooling of primordial gas to allow for small mass objects to form. As the neutral primordial gas is a poor radiator at temperatures (T\le10^4\unit{K}), molecular hydrogen is needed for further cooling down to temperatures (T\sim100\unit{K}). The formation of molecular hydrogen is catalyzed by the presence of free electrons, which could be provided by the ionization due to an early population of cosmic rays. In order to investigate this possibility we developed a code to study the effects of ionizing cosmic rays on the thermal and chemical evolution of primordial gas. We found that cosmic rays can provide enough free electrons needed for the formation of molecular hydrogen, and therefore can increase the cooling ability of such primordial gas under following conditions: A dissociating photon flux with (F<10^{-18}\unit{erg cm^{-2} Hz^{-1} s^{-1}}), initial temperature of the gas (\sim10^{3}\unit{K}), total gas number densities (n\ge1\unit{cm^{-3}}), Cosmic ray sources with (\dotε_{CR}>10^{-33}\unit{erg cm^{-3} s^{-1}}).

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arXiv2007

Simulating cosmic rays in clusters of galaxies - I. Effects on the Sunyaev-Zel'dovich effect and the X-ray emission

We performed high-resolution simulations of a sample of 14 galaxy clusters that span a mass range from 5 x 10^13 M_solar/h to 2 x 10^15 M_solar/h to study the effects of cosmic rays (CRs) on thermal cluster observables such as X-ray emission and the Sunyaev-Zel'dovich effect. We analyse the CR effects on the intra-cluster medium while simultaneously taking into account the cluster's dynamical state as well as the mass of the cluster. The modelling of the cosmic ray physics includes adiabatic CR transport processes, injection by supernovae and cosmological structure formation shocks, as well as CR thermalization by Coulomb interaction and catastrophic losses by hadronic interactions. While the relative pressure contained in CRs within the virial radius is of the order of 2 per cent in our non-radiative simulations, their contribution rises to 32 per cent in our simulations with dissipative gas physics including radiative cooling, star formation, and supernova feedback. Interestingly, in the radiative simulations the relative CR pressure reaches high values of the order of equipartition with the thermal gas in each cluster galaxy due to the fast thermal cooling of gas which diminishes the thermal pressure support relative to that in CRs. This also leads to a lower effective adiabatic index of the composite gas that increases the compressibility of the intra-cluster medium. This effect slightly increases the central density, thermal pressure and the gas fraction. While the X-ray luminosity in low mass cool core clusters is boosted by up to 40 per cent, the integrated Sunyaev-Zel'dovich effect appears to be remarkably robust and the total flux decrement only slightly reduced by typically 2 per cent. The resolved Sunyaev-Zel'dovich maps, however, show a larger variation with an increased central flux decrement. [abridged]

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arXiv2006

Cosmic ray feedback in hydrodynamical simulations of galaxy and galaxy cluster formation

It is well known that cosmic rays (CRs) contribute significantly to the pressure of the interstellar medium in our own Galaxy, suggesting that they may play an important role in regulating star formation during the formation and evolution of galaxies. We will present a novel numerical treatment of the physics of CRs and its implementation in the parallel smoothed particle hydrodynamics (SPH) code GADGET-2. In our methodology, the non-thermal CR population is treated self-consistently in order to assess its dynamical impact on the thermal gas as well as other implications on cosmological observables. In simulations of galaxy formation, we find that CRs can significantly reduce the star formation efficiencies of small galaxies. This effect becomes progressively stronger towards low mass scales. In cosmological simulations of the formation of dwarf galaxies at high redshift, we find that the total mass-to-light ratio of small halos and the faint-end of the luminosity function are affected. In high resolution simulations of galaxy clusters, we find lower contributions of CR pressure, due to the smaller CR injection efficiencies at low Mach number flow shocks inside halos, and the softer adiabatic index of CRs, which disfavours them when a composite of thermal gas and CRs is adiabatically compressed. Within cool core regions, the CR pressure reaches equipartition with the thermal pressure leading to an enhanced compressibility of the central intra-cluster medium, an effect that increases the central density and pressure of the gas. While the X-ray luminosity in low mass cool core clusters is boosted, the integrated Sunyaev-Zel'dovich effect is only slightly changed. The resolved Sunyaev-Zel'dovich maps, however, show a larger variation with an increased central flux decrement.

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arXiv2006

Cosmological structure formation shocks and cosmic rays in hydrodynamical simulations

Cosmological shock waves during structure formation not only play a decisive role for the thermalization of gas in virializing structures but also for the acceleration of relativistic cosmic rays (CRs) through diffusive shock acceleration. We discuss a novel numerical treatment of the physics of cosmic rays in combination with a formalism for identifying and measuring the shock strength on-the-fly during a smoothed particle hydrodynamics simulation. In our methodology, the non-thermal CR population is treated self-consistently in order to assess its dynamical impact on the thermal gas as well as other implications on cosmological observables. Using this formalism, we study the history of the thermalization process in high-resolution hydrodynamic simulations of the Lambda cold dark matter model. Collapsed cosmological structures are surrounded by shocks with high Mach numbers up to 1000, but they play only a minor role in the energy balance of thermalization. However, this finding has important consequences for our understanding of the spatial distribution of CRs in the large-scale structure. In high resolution simulations of galaxy clusters, we find a low contribution of the averaged CR pressure, due to the small acceleration efficiency of lower Mach numbers of flow shocks inside halos and the softer adiabatic index of CRs. However, within cool core regions, the CR pressure reaches equipartition with the thermal pressure leading there to a lower effective adiabatic index and thus to an enhanced compressibility of the central intracluster medium. This effect increases the central density and pressure of the cluster and thus the resulting X-ray emission and the central Sunyaev-Zel'dovich flux decrement. The integrated Sunyaev-Zel'dovich effect, however, is only slightly changed.

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arXiv2006

Cosmic ray feedback in hydrodynamical simulations of galaxy formation

It is well known that cosmic rays (CRs) contribute significantly to the pressure of the interstellar medium in our own Galaxy, suggesting that they may play an important role in regulating star formation during the formation and evolution of galaxies. We here discuss a novel numerical treatment of the physics of CRs and its implementation in the parallel smoothed particle hydrodynamics code GADGET-2. In our methodology, the non-thermal CR population of each gaseous fluid element is approximated by a simple power law spectrum in particle momentum, characterized by an amplitude, a cut-off, and a fixed slope. Adiabatic compression, and a number of physical source and sink terms are modelled which modify the CR pressure of each particle. The most important sources considered are injection by supernovae and diffusive shock acceleration, while the primary sinks are thermalization by Coulomb interactions, and catastrophic losses by hadronic interactions. We also include diffusion of CRs. Our scheme allows us to carry out the first cosmological structure formation simulations that self-consistently account for CR physics. In simulations of isolated galaxies, we find that CRs can significantly reduce the star formation efficiencies of small galaxies, with virial velocities below \~80 km/s, an effect that becomes progressively stronger towards low mass scales. In cosmological simulations at high redshift, the total mass-to-light ratio of small halos and the faint-end of the luminosity function are strongly affected. When CR acceleration in shocks is followed as well, up to ~40% of the energy dissipated at structure formation shocks can appear as CR pressure at z~3-6, but this fraction drops to ~10% at low redshifts when the shock distribution becomes increasingly dominated by lower Mach numbers. (abridged)

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arXiv2006

Detecting shock waves in cosmological smoothed particle hydrodynamics simulations

We develop a formalism for the identification and accurate estimation of the strength of structure formation shocks during cosmological smoothed particle hydrodynamics simulations. Shocks not only play a decisive role for the thermalization of gas in virialising structures but also for the acceleration of relativistic cosmic rays (CRs) through diffusive shock acceleration. Our formalism is applicable both to ordinary non-relativistic thermal gas, and to plasmas composed of CRs and thermal gas. To this end, we derive an analytical solution to the one-dimensional Riemann shock tube problem for a composite plasma of CRs and thermal gas. We apply our methods to study the properties of structure formation shocks in high-resolution hydrodynamic simulations of the LCDM model. We find that most of the energy is dissipated in weak internal shocks with Mach numbers M~2 which are predominantly central flow shocks or merger shock waves traversing halo centres. Collapsed cosmological structures are surrounded by external shocks with much higher Mach numbers up to M~1000, but they play only a minor role in the energy balance of thermalization. We show that after the epoch of cosmic reionisation the Mach number distribution is significantly modified by an efficient suppression of strong external shock waves due to the associated increase of the sound speed of the diffuse gas. Invoking a model for CR acceleration in shock waves, we find that the average strength of shock waves responsible for CR energy injection is higher than that for shocks that dominate the thermalization of the gas. When combined with radiative dissipation and star formation, our formalism can also be used to study CR injection by supernova shocks, or to construct models for shock-induced star formation in the interstellar medium. (abridged)

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arXiv2006

Deep 1.4 GHz VLA Observations of the Radio Halo and Relic in Abell 2256

We present deep VLA observations of the merging galaxy cluster Abell 2256. This cluster is known to possess diffuse steep spectrum radio relic emission in the peripheral regions. Our new observations provide the first detailed image of the central diffuse radio halo emission in this cluster. The radio halo extends over more than 800 kpc in the cluster core, while the relic emission covers a region of ~1125 x 520 kpc. A spectral index map of the radio relic shows a spectral steepening from the northwest toward the southeast edge of the emission, with an average spectral index between 1369 MHz and 1703 MHz of -1.2 across the relic. Polarization maps reveal high fractional polarization of up to 45% in the relic region with an average polarization of 20% across the relic region. The observed Faraday rotation measure is consistent with the Galactic estimate and the dispersion in the rotation measure is small, suggesting that there is very little contribution to the rotation measure of the relic from the intracluster medium. We use these Faraday properties of the relic to argue that it is located on the front side of the cluster.

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arXiv2006

Future magnetic fields studies using the Planck Surveyor experiment

The Planck mission will permit measurements of the polarization of the cosmic microwave background and of polarized foregrounds such as our own Galaxy with an unprecedented combination of accuracy and completeness. This will provide information on cosmological and galactic magnetic fields. The latter can be studied in detail via nearly Faraday-rotation free synchrotron and polarized dust emission. Methods are discussed to extract physically relevant information on the magnetic turbulence from Planck data and other measurements.

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arXiv2005

Magnetic Fields in Clusters of Galaxies

A brief overview about our knowledge on galaxy cluster magnetic fields is provided. Emphasize is given to the mutual dependence of our knowledge on relativistic particles in galaxy clusters and the magnetic field strength. Furthermore, we describe efforts to measure magnetic field strengths, characteristic length-scales, and power-spectra with reliable accuracy. An interpretation of these results in terms of non-helical dynamo theory is given. If this interpretation turns out to be correct, the understanding of cluster magnetic fields is directly connected to our understanding of intra-cluster turbulence.

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arXiv2005

A Bayesian view on Faraday rotation maps - Seeing the magnetic power spectra in galaxy clusters

We present a Bayesian maximum likelihood analysis of Faraday rotation measure (RM) maps of extended radio sources to determine magnetic field power spectra in clusters of galaxies. Using this approach, it is possible to determine the uncertainties in the measurements. We apply this approach to the RM map of Hydra A and derive the power spectrum of the cluster magnetic field. For Hydra A, we measure a spectral index of -5/3 over at least one order of magnitude implying Kolmogorov type turbulence. We find a dominant scale of about 3 kpc on which the magnetic power is concentrated, since the magnetic autocorrelation length is lambda_B = 3 +/- 0.5 kpc. Furthermore, we investigate the influences of the assumption about the sampling volume (described by a window function) on the magnetic power spectrum. The central magnetic field strength was determined to be about 7 +/- 2 muG for the most likely geometries.

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arXiv2004

The quest for cosmic ray protons in galaxy clusters

There have been many speculations about the presence of cosmic ray protons (CRps) in galaxy clusters over the past two decades. However, no direct evidence such as the characteristic gamma-ray signature of decaying pions has been found so far. These pions would be a direct tracer of hadronic CRp interactions with the ambient thermal gas also yielding observable synchrotron and inverse Compton emission by additionally produced secondary electrons. The obvious question concerns the type of galaxy clusters most likely to yield a signal: Particularly suited sites should be cluster cooling cores due to their high gas and magnetic energy densities. We studied a nearby sample of clusters evincing cooling cores in order to place stringent limits on the cluster CRp population by using non-detections of EGRET. In this context, we examined the possibility of a hadronic origin of Coma-sized radio halos as well as radio mini-halos. Especially for mini-halos, strong clues are provided by the very plausible small amount of required CRp energy density and a matching radio profile. Introducing the hadronic minimum energy criterion, we show that the energetically favored CRp energy density is constrained to 2% +/- 1% of the thermal energy density in Perseus. We also studied the CRp population within the cooling core region of Virgo using the TeV gamma-ray detection of M 87 by HEGRA. Both the expected radial gamma-ray profile and the required amount of CRp support this hadronic scenario.

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arXiv2004

Unveiling the composition of radio plasma bubbles in galaxy clusters with the Sunyaev-Zel'dovich effect

The Chandra X-ray Observatory is finding a large number of cavities in the X-ray emitting intra-cluster medium which often coincide with the lobes of the central radio galaxy. We propose high-resolution Sunyaev-Zel'dovich (SZ) observations in order to infer the still unknown dynamically dominating component of the radio plasma bubbles. This work calculates the thermal and relativistic SZ emission of different compositions of these plasma bubbles while simultaneously allowing for the cluster's kinetic SZ effect. As examples, we present simulations of an Atacama Large Millimeter Array (ALMA) observation and of a Green Bank Telescope (GBT) observation of the cores of the Perseus cluster and Abell 2052. We predict a 5 sigma detection of the southern radio bubble of Perseus in a few hours with the GBT and ALMA while assuming a relativistic electron population within the bubble. In Abell 2052, a similar detection would require a few ten hours with either telescope, the longer exposures mainly being the result of the higher redshift and the lower central temperature of this cluster. Future high-sensitivity multi-frequency SZ observations will be able to infer the energy spectrum of the dynamically dominating electron population in order to measure its temperature or spectral characteristics. This knowledge can yield indirect indications for an underlying radio jet model.

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arXiv2004

Constraining the population of cosmic ray protons in cooling flow clusters with gamma-ray and radio observations: Are radio mini-halos of hadronic origin?

We wish to constrain the cosmic-ray proton (CRp) population in galaxy clusters. By hadronic interactions with the thermal gas of the intra-cluster medium (ICM), the CRp produce gamma-rays for which we develop an analytic formalism to deduce their spectral distribution. Assuming the CRp-to-thermal energy density ratio X_CRp and the CRp spectral index to be spatially constant, we derive an analytic relation between the gamma-ray and bolometric X-ray fluxes, F_gamma and F_X. Based on our relation, we compile a sample of suitable clusters which are promising candidates for future detection of gamma-rays resulting from hadronic CRp interactions. Comparing to EGRET upper limits, we constrain the CRp population in the cooling flow clusters Perseus and Virgo to X_CRp < 20%. Assuming a plausible value for the CRp diffusion coefficient kappa, we find the central CRp injection luminosity of M 87 to be limited to 10^43 erg s^-1 kappa/(10^29 cm^2 s^-1). The synchrotron emission from secondary electrons generated in CRp hadronic interactions allows even tighter limits to be placed on the CRp population using radio observations. We obtain excellent agreement between the observed and theoretical radio brightness profiles for Perseus, but not for Coma without a radially increasing CRp-to-thermal energy density profile. Since the CRp and magnetic energy densities necessary to reproduce the observed radio flux are very plausible, we propose synchrotron emission from secondary electrons as an attractive explanation of the radio mini-halos found in cooling flow clusters. This model can be tested with future sensitive gamma-ray observations of the accompanying pi0-decays. We identify Perseus (A 426), Virgo, Ophiuchus, and Coma (A 1656) as the most promising candidate clusters for such observations.

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arXiv2004

Estimating galaxy cluster magnetic fields by the classical and hadronic minimum energy criterion

We wish to estimate magnetic field strengths of radio emitting galaxy clusters by minimising the non-thermal energy density contained in cosmic ray electrons (CRe), protons (CRp), and magnetic fields. The classical minimum energy estimate can be constructed independently of the origin of the radio synchrotron emitting CRe yielding thus an absolute minimum of the non-thermal energy density. Provided the observed synchrotron emission is generated by a CRe population originating from hadronic interactions of CRp with the ambient thermal gas of the intra-cluster medium, the parameter space of the classical scenario can be tightened by means of the hadronic minimum energy criterion. For both approaches, we derive the theoretically expected tolerance regions for the inferred minimum energy densities. Application to the radio halo of the Coma cluster and the radio mini-halo of the Perseus cluster yields equipartition between cosmic rays and magnetic fields within the expected tolerance regions. In the hadronic scenario, the inferred central magnetic field strength ranges from 2.4 muG (Coma) to 8.8 muG (Perseus), while the optimal CRp energy density is constrained to 2% +/- 1% of the thermal energy density (Perseus). We discuss the possibility of a hadronic origin of the Coma radio halo while current observations favour such a scenario for the Perseus radio mini-halo. Combining future expected detections of radio synchrotron, hard X-ray inverse Compton, and hadronically induced gamma-ray emission should allow an estimate of volume averaged cluster magnetic fields and provide information about their dynamical state.

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