Focus Meeting 8 - Abstracts


Origins of cosmic magnetism

Subramanian, Kandaswamy

The universe is magnetised from stars to the large-scale coherentmagnetic fields detected in galaxies and galaxy clusters, andeven perhaps the intergalactic medium in voids. The standardpicture for the origin of fields in all astrophysical systems involvesturbulent dynamo amplification of a weak seed magnetic field. In suchdynamos the kinetic energy of motions get converted to magnetic energy.We review the basic idea behind such dynamos and the main challenges theyencounter. While it is relatively easy for magnetic energy to grow,explaining the observed degree of coherence of cosmic magnetic fieldsgenerated by turbulent dynamos, remains challenging. We outline potentialresolution of these challenges, ending with a new paradigm for rapidunified growth of both large and small scale fields in galaxies.  

Magnetism in the SKA Era

Mao, Sui Ann

The unprecedented sensitivity, angular resolution and broadband coverage of Square Kilometre Array polarimetric observations will allow us to address many long-standing mysteries in cosmic magnetism science. In this talk, I will present new and ambitious science topics that SKA polarization data will enable in the next decade. Several new concepts and their required observations in areas of Galactic, extragalactic astrophysics, as well as cosmology will be highlighted. 

What does ALMA tell us about extragalactic magnetic fields?

Nagai, Hiroshi

The Atacama Large Millimeter/submillimeter Array (ALMA) is the most sensitive mm/submm telescope in the world and opening a new window for exploring the universe from nearby to high-z. High precision polarimetry with ALMA is a key capability to studying extragalactic astorophysical phenomena in connection with magnetic fields.Although only handful results from ALMA polarimetry toward extragalactic sources have been published so far, ALMA has already revealed unique and cutting-edge sciences. ALMA observations can probe relatively young electrons of nonthermal radio sources in optically thin regime, and thus study the sites of electron acceleration in energetic events, such as the AGN jet production region (Marti-Vidal et al. 2015; Nagai et al. 2016) and hotspots/shocks (Orienti et al. 2017), and their connections with magnetic fields. ALMA also studies the properties of magnetized plasma in the close vicinity of black hole using Faraday rotation. In near future, there is a possibility of direct imaging of the black hole shadow as well as the accretion flow and jet base together with magnetic field information by VLBI observatoins including ALMA.I will review recent results from ALMA polarimetry and my ongoing work for the study of black hole accretion. I will also give a general idea that ALMA polarimetry will provide insights for extragalactic magnetic field in coming years.

Techniques and algorithmic advances in the SKA era

Vacca, Valentina

One of the key science goals of the new generation of radio telescopes is to shed light on the origin and evolution of cosmological magnetic fields. Magnetic fields presently observed in the Universe are thought to be the result of a pre-existing seed magnetic field amplified during processes of structure formation. According to the proposed scenarios, the seed magnetic field either has a primordial origin or is generated later, in stars and/or pro-galaxies, and then injected in the surrounding medium by galactic winds and/or outflow. The overall picture is not yet clear since our knowledge of magnetic fields in the large scale structure of the Universe is poor. Magnetic fields have been observed up to galaxy clusters scales, where a detailed investigation is limited only to a few systems. Magneto-hydro-dynamical simulations indicate their presence even beyond galaxy cluster, but a firm observational evidence is still missing. A better understanding of magnetic fields in galaxy clusters and their discovery and investigation on larger scales, i.e., along the filaments and the voids of the cosmic web, would permit to put valuable constraints on the magnetic field origin and to investigate mechanism of amplification to the strengths presently observed. Magnetic fields in galaxy clusters and in the large-scale structure of the Universe can be studied in high detail by means of radio observations in total intensity and polarization of diffuse and extended synchrotron sources and of the Faraday effect on background radio galaxies. Because of their weakness, the study of these magnetic fields is extremely challenging and requires high quality observations and advanced techniques of analysis. During this talk, I will give an overview on the innovative approaches developed in the last years to investigate magnetic fields in the large-scale structure of the Universe.

Capabilities of next generation telescopes for cosmic magnetism

Stil, Jeroen

Observations of synchrotron emission at radio wavelengths and Zeeman splitting of radio spectral lines account for most of our knowledge on cosmic magnetic fields, especially extragalactic magnetic fields. Newly upgraded telescopes and new facilities significantly improve bandwidth, angular resolution and survey speed. These improvements are changing the landscape for cosmic magnetism. How precisely? Different facilities support distinct science cases depending on their operating frequency range, in particular survey science. We review the individual strengths of these surveys, and how the combination can be bigger than the sum of the parts.

Small-scale dynamo as a mechanizm for excitation of extragalactic magnetic fields

Sokoloff, Dmitry

There are observational confirmation for existence of substantial magnetic fields in extragalactic medium including cosmological voids. Origin of such cosmological magnetic fields is a topic of intensive discussions. A viewpoint presented in this talk is that the extragalactic magnetic fields can be excited by a dynamo mechanizm. The point is however that this mechanizm hardly can be a conventional mean-field dynamo based on a joint action of rotation and mirror-asymmetric turbulence. Indeed, extragalactic medium hardly have a substantial rotation. There is however another kind of dynamo known as small-scale or turbulent dynamo/ This mechanism do not requires any rotation or mirror-asymmetry. The only what is required for small-scale dynamo action is a moderate conductivity and turbulence. We present estimations for expected magnetic field strengthes and spatial scales of magnetic fields excited by cosmological small-scale dynamo and compare the predictions with that one for magnetic fields excited by phase transitions in the Early Universe.

Energy equipartition between cosmic rays and magnetic fields

Seta, Amit

The interpretation of synchrotron intensity data requires knowledge of the cosmic ray number density, which is often assumed to be in energy equipartition (or otherwise tightly correlated) with the magnetic field energy density. We examine the energy equipartition assumption between cosmic rays and magnetic fields using both: test--particle simulations (important to capture the effect of magnetic field structure) and MHD simulations by considering cosmic rays as an additional diffusive fluid (important to consider the effects of gas pressure). We find no spatial correlation between the cosmic rays and magnetic field energy densities at turbulent scales. Moreover, the cosmic ray number density and magnetic field strength are statistically independent. Nevertheless, small-scale cosmic ray structures are abundant at low energies. These are particles trapped in random magnetic bottles. These conclusions can significantly change the interpretation of synchrotron observations and thus our understanding of strength and structure of magnetic fields in Milkyway and nearby spiral galaxies.

A Simulation Study for Turbulence and Magnetic Fields in Galaxy Clusters

Ryu, Dongsu

Magnetic fields in galaxy clusters, at least in their outskirts, are conjectured to be originated by small-scale dynamo due to turbulence in the intracluster medium. Yet, the turbulence is not well understood, mostly because it differs from those in other astrophysical environments including the interstellar and interplanetary media in a number of aspects. For instance, the turbulence is induced in highly stratified backgrounds due to the cluster gravity, and also it is driven sporadically by major mergers which are the most energetic events during the hierarchical formation of clusters. To get quantitative measures for turbulence and magnetic fields, we performed a series of simulations using a newly developed, high-accurate MHD code, which followed the development of turbulence and the follow-up amplification of magnetic fields in galaxy clusters. In this talk, we present the results, aiming to address the following issues. Can the strength of the magnetic fields observed in the cluster outskirts, ~ 1 microG, be produced by turbulence dynamo during the cosmic history? Can the large-scale coherent magnetic fields observed in some radio relics, for instance, those over ~ Mpc or a larger scale in the so-called Sausage relic, be explained by turbulence dynamo alone.

Large-Scale Diffuse Intergalactic Magnetic Fields Constraints With the Cherenkov Telescope Array

Barai, Paramita

Magnetic fields of the order of µ-Gauss are observationally detected in galaxies and clusters, which are believed to be created by the amplification of much weaker primordial seed fields. These fields should be carried out by strong galactic outflows, magnetically enriching the InterGalactic Medium (IGM) at larger cosmological distances. However direct observation of magnetic fields in the IGM is scarce. In this talk, I will give a review of how Intergalactic Magnetic Field (IGMF)'s strength and filling factor can be constrained using numerical simulations and gamma-ray observations.We are performing cosmological hydrodynamical simulations containing dark-matter and all relevant physical processes for the baryons. We analyze the simulations following the methodology of Barai (2008, ApJ, 682, L17), and compute the magnetic field in the simulation volume IGM. Our objectives are: to explore the evolution of IGMFs through cosmic times, and to compute the contributions of galactic outflows (driven by SN versus AGN) in the advection of magnetic fields from the galaxies to the IGM.On the observational side, high-energy TeV photons emitted by distant blazars can interact with the cosmic extragalactic infrared/optical background light, producing electron-positron pairs, and initiating electromagnetic cascades in the IGM. The charged component of these cascades is deflected and delayed by IGMFs, thereby reducing the observed point-like TeV flux, and creating an extended image in the GeV energy range, which can potentially be detected with gamma-ray telescopes (Fermi-LAT, HESS, CTA). Studies [e.g., Neronov & Vovk (2010, Science, 328, 73); Dolag et al. (2011, ApJ, 727, L4)] have put lower limits on the IGMF strength of the order of 10-16 - 10-15 G, and filling factors of 60%. I will overview such work; and describe the constraints on the IGMF which the Cherenkov Telescope Array sensitivity is expected to give(Acharya et al. 2017, CTA consortium science paper).

Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies.

Basu, Aritra

Cosmic evolution of the large-scale magnetic field in star-forming galaxiesremains an open question in observational astronomy. A major tool to probemagnetic fields in distant galaxies would be through statistical measurement ofFaraday rotation measure (RM) towards quasar absorption line systems, which aretracers of galaxies in the high redshift Universe. The distribution of RM oftwo quasar samples, with and without absorption line systems, are compared tostatistically infer the properties of magnetic fields in the intervening galaxypopulation. To extract as much information on the properties of coherentmagnetic fields, we present analytical and empirical form of the probabilitydistribution function of RM when random lines of sight are shot through asample of galaxies with random inclination, impact parameter and azimuthalangle, and assuming that the magnetic field is confined to the disc withaxisymmetric spiral geometry. Interestingly, the dispersion in RM produced bythe large-scale fields in the intervening galaxies is comparable in magnitudeto that observed in previous studies. We find that the width of RMdistribution is directly related to the mean coherent field strength of theintervening galaxy population provided the dispersion within the sample is low.Finally, we discuss sample selection criteria that are crucial for cleaninterpretation of the observations. Selecting high metallicity DLAs as theintervener is the best choice to study magnetic field amplification driven bygalactic dynamo action.

(Magnetised) Camel in the (intra-group) eye of a needle: the genuinely regular magnetic field inside the Stephan's Quintet

Nikiel-Wroczynski, Blazej

Magnetic fields are generally regarded omnipresent in the cosmic space, from stars to clusters of galaxies (and beyond). Wherever found, they are suspected to play a profound role in shaping their environment. A few classes of celestial objects, however, have been studied more scarcely in this regard than the others, and one of them are the galaxy groups. This is due to the technical difficulties of registering the exceptionally weak, extended radio emission that carries the information about the magnetic field inside them to the observer. Even more problematic is the detection of polarised signal (thus, anisotropic magnetic fields): it is much weaker, and additional effects – like beam depolarisation in case of angularly small targets – are present. Homogenous, regular magnetic fields are the most demanding ones to be observed, and so far no efforts to detect them have ended up with a success.I would like to present the recent results from spectropolarimetric observations of the Stephan’s Quintet, the best known galaxy group, made with the WSRT interferometer. For the first time, genuinely regular magnetic field inside a galaxy group – manifesting as a peak in the Faraday dispersion function of sources inside, or behind the Quintet – have been detected. Owing to the unknown pathlength through the magnetised bubble, only the product of the regular field stregth and depth can be calculated; however, it is clear, that the detected field has a few-microgauss strength, and depth of around 10 kpc – or even more. What is the most interesting finding is its extent: at least 60 on 40 kpc. Thus, the energy of the regular magnetic field contained within this system is of the same order, as in magnetised structures found in galaxy clusters – with their host being significantly smaller. The result achieved clearly shows that the detection of magnetic fields in compact galaxy groups is possible, and they might play significant role in shaping the life of their parent systems.

Isolating local from foreground Faraday contributions

Rudnick, Lawrence

In order to study the interactions of radio galaxies with their thermal environments or to use background polarized sources as probes of foreground magnetized plasmas, it is necessary to separate the contributions from those components.  Little information is currently available on how the individual properties of radio galaxies, such as their size or morphology, can be used to assess contributions local to the source.  In this talk, we will explore the relationship between the physical size of radio galaxies and their Faraday properties, using several different samples.  We find that larger sources are more highly polarized, contrary to the expectations of depolarization from unrelated foreground screens, so that such effects will have to be controlled in any studies using background polarization probes.  This work is supported, in part, by NSF grant AST17-14205 to the University of Minnesota.

Propagation of Cosmic-Rays and Gamma-Rays in the turbulent Intergalactic Medium

de Gouveia Dal Pino, Elisabete

The propagation of cosmic-rays and  high-energy gamma-rays may induce electromagnetic cascades in the intergalactic medium (IGM), generating electron-positron pairs, whose opening angle is sensitive to the strength of intervening magnetic ?elds. This effect has been used to constrain the strength and coherence length of the turbulent intergalactic magnetic fields (IGMFs) which supposedly permeate the cosmic web up to the scales of the voids. Nevertheless, plasma instabilities arising due to interactions of the ionised component of the intergalactic medium with the electron-positron pairs may a?ect the development of electromagnetic cascades, potentially rendering this method ineffective.Here we present results of cosmic- and gamma-ray propagation in the IGM aiming at  improving current constraints on properties of the IGMF. To this end, we have performed three-dimensional magnetohydrodynamical simulations of the turbulent IGM and studied the development of electromagnetic cascades in this background, exploring the interaction of test particles with the environment and computing the associated fluxes of cosmic- and gamma-rays.

The Role of Magnetic Fields in Evolution of Galaxies

Tabatabaei, Fatemeh

The origin and evolution of cosmic magnetic fields as well as the influence of the magnetic fields on the evolution of galaxies are unknown. Scaling relations can provide important insight on the way the magnetic field is maintained in galaxies. I present the scaling relations between the strength of the magnetic fields and other physical properties both locally and globally from two samples of nearby galaxies and discuss possible deviations in galaxy centers. These regions are found to be controlled by the nonthermal pressure from the magnetic fields, cosmic rays, and turbulence. Our recent study in the central kpc of NGC1097- a prototypical galaxy undergoing quenching- shows that the star formation efficiency drops significantly with the magnetic field strength. Hence, a progressive built up of the magnetic field can result in high-mass stars forming inefficiently and further help the formation of the big bulges of low-mass stars seen in quenched galaxies.

A fresh look on magnetic fields in halos of spiral galaxies

Dettmar, Ralf-Juergen

Recent numerical models of the multiphase ISM underline the importance of cosmic rays and magnetic fields for the physics of the ISM in disc galaxies. Observations of properties of the ISM in galactic halos constrain models of the expected exchange of matter between the star-forming disc and the environment (circumgalactic medium, CGM). We will present new observational evidence from radio-continuum polarization studies of edge-on galaxies on magnetic field strength and structure as well as cosmic ray propagation in galactic halos. The new findings result from the CHANG-ES (Continuum HAlos in Nearby Galaxies - an EVLA Survey; PI J. Irwin) project which has observed 35 edge-on galaxies with the Karl G. Jansky Very Large Array (JVLA) in two frequency bands (L- and C-band) and in three array configurations (D, C, B). This survey benefits significantly from the new multi-channel capability of the upgraded facility. From the total power maps, a "mean" radio-continuum halo has been derived and the polarization information provides information on the magnetic field structure in the halos. The findings will be discussed in the context of the disk-halo interaction of the interstellar medium. In addition, we will also briefly demonstrate how recent LOFAR observations of edge-on galaxies further constrain the extent of magnetic fields in galactic halos.

New insights on galactic dynamos

Chamandy, Luke

Mean-field turbulent dynamo theory is the leading theory to explain the prevalence of large-scale magnetic fields in spiral galaxies, but its systematic comparison with observations is still incomplete and fragmentary.  We demonstrate that a standard $\alpha\Omega$ dynamo produces pitch angles of the large-scale fields of nearby galaxies that are remarkably consistent with available data. This work is a step toward systematic statistical tests of galactic dynamo theory.  One limitation, however, is that some of the dynamo model parameters are difficult to constrain directly using observations.  Hence, we present detailed estimates for the values of key parameters of supernova-driven interstellar turbulence.  Expressions are formulated to relate the turbulent correlation time, outer scale, and root-mean-square turbulent speed to underlying observables or quasi-observables like gas density, supernova-rate density, sound speed, and disk scale height.  On the other hand, it seems likely that important physical effects have not yet been discovered or recognized to be important.  Particularly intriguing is the possible role of small-scale magnetic fluctuations in the mean-field dynamo mechanism.  It is known that when such fluctations exist in the presence of rotation, a new term arises in the mean electromotive force, and we show that this leads to a partial suppression of dynamo action and a reduction of the growth rate in the kinematic regime.  Moreover this effect tends to become more important as the mean-field nears saturation because small-scale fluctuations are then enhanced by turbulent tangling of the near-equipartition large-scale field.  Thus, one obtains a negative feedback effect that quenches the dynamo and leads to the saturation of the large-scale field.  This saturation mechanism is found to be competitive with the well-known dynamical $\alpha$-quenching mechanism for realistic parameter values.

First Results from Wide Field Extragalactic Polarisation Surveys

Gaensler, Bryan

Over the last decade, we have extracted substantial amount of information on extragalactic magnetic fields from the NVSS and other archival Faraday rotation data. However, these data sets are inhomogeneous, measured over narrow bandwidths, and do not have well-characterised uncertainties. To move past these limitations, the community has been working toward a suite of new wide-field polarisation surveys, based around high sensitivity, high angular resolution and broad bandwidths. These surveys, which aim to provide more than a million polarisation measurements over the sky, are now finally underway. In this presentation, I will present early polarisation results from POSSUM, VLASS and other wide-field surveys, which showcase the transformational potential of these new programs.

Magnetic Fields in the Intergalactic Medium from observations of Fast Radio Bursts

Johnston, Simon

Fast Radio Bursts (FRBs) are millisecond pulses of radio emission at cosmological distances. As the radiation from the FRB travels through the intergalactic medium, it undergoes both dispersion and faraday rotation. The dispersion measures the ionised electron content along the line of sight, and this combined with the rotation measure yields the magnetic field strength. I will report on observations of FRBs using the Parkes telescope and concentrate on the measurements of the rotation measure and the implication for the intergalactic medium magnetic field out to redshifts above z=1.

CMB weak lensing with the primordial magnetic field

Yamazaki, Dai

We illustrate that a primordial magnetic field (PMF) suppresses the cosmic microwave background (CMB) B mode from the weak lensing (WL) effect. The WL effect is dependent on the lensing potential (LP) from the matter perturbations. A distribution of the matter perturbations on the cosmological scales is given by the matter power spectrum (MPS). Therefore, the WL effect on the CMB B mode is affected by the MPS. Considering effects of an ensemble average energy density of the PMF, which we call ''the background PMF'' in this letter, on the MPS correctly, the amplitude of MPS is suppressed on the wavenumber range 0.01 h Mpc-1 < k < 0.2 h Mpc-1. Since the MPS affects the LP and the WL effect on the CMB B mode, the PMF can damp these. The previous approaches to study the WL effect on the CMB B mode with a PMF have only considered vector and tensor modes from a PMF. On the one hand effects of vector and tensor modes from a PMF boost the CMB B mode on the higher multipoles l > 1000 mainly, on the other hand the effects of the background PMF damp the CMB B mode from the WL effect on the all multipole range. The matter density in the Universe determines the WL effects. Therefore, when we constrain the PMF and the matter density parameters from cosmological observational data sets including the CMB B mode, we expect that there is a degeneracy between these parameters. The CMB B mode, also provide important information of the background gravitational wave, the inflation theory, the matter density fluctuations and the structure formations in the cosmological scale through the cosmological parameter search. If we precisely research these topics and correctly constrain the cosmological parameters from the cosmological observation including the CMB B mode, we should consider the background PMF correctly.

Magnetizing the Intergalactic Medium during Reionization

Langer, Mathieu

Increasing evidence suggests that cosmological sheets, filaments and voids may be substantially magnetised. The origin of magnetic fields in the the Intergalactic Medium is currently uncertain. It seems now well known that non-standard extensions to the physics of the Standard Model are capable of providing mechanisms susceptible of magnetising the Universe at large. Much less well known is the fact that standard, classical physics of matter-radiation interactions possesses actually the same potential. I will discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous Intergalactic Medium. Operating in the neighbourhood of ionising sources during the Epoch of Reionization, this mechanism is capable of generating magnetic seeds of relevant strengths on scales comparable to the distance between ionising sources. In addition, summing up the contributions of all ionising sources and taking into account the distribution of gas inhomogeneities, I will show that this mechanism leaves the IGM, at the end of Reionization, with a level of magnetization that might account for the current magnetic fields strengths in the cosmic web.

Magnetic Fields in Galaxies and Clusters of Galaxies in MHD Simulations

Sarazin, Craig

MHD simulations of magnetic fields in galaxies and clusters of galaxies, and their effect on the interstellar and intracluster gas will be discussed.  Simulations of galaxies falling into clusters show that magnetic fields can drape galaxies, suppressing thermal conduction and hydrodynamical instabilities.  These effects, particularly the reduction of thermal evaporation, allow elliptical galaxies to retain small coronae of hot gas, in agreement with X-ray observations.  Without magnetic draping, these coronae would evaporate quickly.  We consider simulations with a variety of magnetic field geometries, and with magnetic fields which are connected or disjoint between galaxies and the cluster.  The field geometry affects the structure of tails, particularly for spiral galaxies;  the MHD results are compared to the observations of “jelly fish” galaxies.  Finally, simulations of the cumulative effect of magnetic fields and stripping in entire clusters and groups will be presented. These simulations include galaxies orbiting in equilibrium clusters, and galaxies which are initially bound to a pair of merging clusters.

Simulations of the polarized sky of the SKA

Loi, Francesca

The advent of the Square Kilometre Array (SKA) will have an unprecedented impact on the study of the cosmic magnetism. An all-sky survey at 1.4 GHz will be proposed for the SKA with the aim to produce an accurate map of the Rotation Measure (RM) based on a enormous number of radio sources spread over cosmological distances. The spatial density of these radio sources will be such that we will obtain a RM grid with several hundreds of sources per squared degree. In order to be prepared to use this RM grid, we need to understand if and how we will be able to invert the information encoded in the grid to reconstruct the strength and the structure of large scale magnetic fields. In this talk, I will present an original numerical approach that is able to produce full-Stokes realistic images of the radio sky that we would observe with the SKA. Among the several uses of this tool, I will show the results concerning the study of magnetic fields in galaxy clusters based on the application of the Faraday RM Synthesis on simulated data.

From NVSS RM Catalogue to Future Polarisation Surveys

Ma, Yik Ki

With RM measured towards 37,543 polarised sources, the NVSS RM Catalogue (Taylor et al. 2009) is widely exploited in studies of the foreground magneto-ionised media. However, due to limitations imposed by observations in survey mode in the narrowband era, the results are inevitably affected by various systematic effects. With new JVLA broadband spectro-polarimetric observations at L-band, we set off to observationally test the robustness of the NVSS RM Catalogue, in anticipation of current and future polarisation surveys such as VLASS, POSSUM, and the eventual SKA. Our pointed observations, in conjunction with simulations, allow us to estimate the impact of off-axis polarisation leakage on the measured RM values. We show that off-axis leakage terms must be properly calibrated in future all-sky polarisation surveys, in order to obtain high fidelity polarisation information from sources down to low fractional polarisation. In addition, we explore the time domain of the polarised sky by utilising broadband data obtained at two epochs (separated by three years) to investigate time variabilities in polarisation, including that in Faraday depths.

Probing the Magnetized Disk-Halo Interaction in M51 via Wideband Polarimetry

Kierdorf, Maja

An excellent laboratory for studying  well ordered magnetic fields is the grand design face-on spiral galaxy M51. Due to wavelength-dependent Faraday depolarization, polarized emission at different radio frequencies gives a picture of the galaxy at different depths: previous observations at L-band (1-2GHz) probes the halo region while at C-band (4-8GHz) the polarized emission comes from the disk region of M51. In this talk, I will present new observations of M51 with the Very Large Array at S-band (2-4GHz), where currently no polarization data exists, to shed new light on the interaction region between disk and halo. The wide frequency coverage and high spatial resolution of about 7 arcsec allow us to probe the disk-halo interaction region to get detailed information on the magnetic field structure in the plane of the sky and on the vertical magnetic field component in this unknown layer.Previous models applied to the polarized emission shows a change from an axisymmetric (m=0, in the disk) to a bisymmetric (m=1, in the halo) magnetic field configuration. I will show our observational results of the transition layer, which is critical for our understanding on the origin of large-scale halo fields and how they are connected to the underlying galactic disk.

The primordial magnetic field in our cosmic backyard

Enßlin, Torsten

We present a reconstruction of the actual 3D structure of magnetic fields, which were seeded by density perturbations during the radiation dominated epoch of the Univerese and later on were evolved by structure formation. To achieve this goal, we rely on three dimensional initial density fields inferred from the 2M++ galaxy compilation via the Bayesian BORG algorithm. Using those, we estimate the magnetogenesis by the so called Harrison mechanism.  This effect produces magnetic fields exploiting the different photon drag on electrons and ions in vortical motions, which are exited due to second order perturbation effects in the Early Universe. Subsequently we study the evolution of these seed fields through the non-linear cosmic structure formation by virtue of a MHD simulation to obtain a 3D estimate for the structure of this primordial magnetic field component today. At recombination we obtain a reliable lower limit on the large scale magnetic field strength around 10?²³ G, with a power spectrum peaking at about 2 Mpc?¹ h in comoving scales. At present we expect this evolved primordial field to have strengthts above ˜ 10?²7 G and ˜ 10?²? G in clusters of galaxies and voids, respectively. We also calculate the corresponding Faraday rotation measure map and show the magnetic field morphology and strength for specific objects of the Local Universe.

Magnetic fields in simulations of ram pressure stripped galaxies

Ramos-Martinez, Mariana

The removal of the interstellar medium (ISM) of disk galaxies through ram pressure stripping (RPS) has been extensively studied in numerous simulations. Nevertheless, the role of magnetic fields (MF) on the gas dynamics in this process has been hardly studied, although the MF influence on the large-scale disk structure is well established. With this in mind, we present 3D magnetohydrodynamic (MHD) simulations of disk galaxies subject to RPS, under the wind-tunnel approximation, with different disk inclination angles to study the evolution of the galactic MF and its impact in the gas stripping. The main effect of including a galactic MF is a flared disk. From our simulations, we observe that when the intracluster medium (ICM) wind hits a galaxy, the gas in the disk is compressed, leading  to an enhancement of the MF intensity in the upstream side. Since the gas is bound by the MF, we observe that in face-on and nearly face-on (slightly inclined) disks, the gas is swept by the wind in ring-like structures and drags the field lines with it, so that they show a more symmetrical configuration, similar to the initial one. In the case of highly inclined and edge-on disks, the configuration of the field is asymmetric compared to the initial one, the field lines are compressed in the upstream side and more extended towards the downstream side. Since the magnetic field is dragged with the swept gas of the disks, the ICM is magnetized in the downstream side up to ~0.1µG, where nearly face-on disks contribute the most, given that the tail developed by the swept gas is more extended than in highly inclined models.

Intracluster magnetic field: primordial field or dynamo?

Beresnyak, Andrey

The intracluster medium have magnetic fields with strength of a few µG and the correlation scale of a few tens of kpc, the largest scales of the magnetic field so far observed in the Universe. Can this magnetic field be used as a test for the primordial magnetic field? I would argue that if the initial cluster field was modified by the nonlinear dynamo, the process would be insensitive to the value of the initial field. My model relies on statistics of turbulence from simulations of cluster formation in a fully cosmological context as well as nonlinear dynamo theory. Although the initial field is not a parameter of this model, it predicts length scale and magnitude of the magnetic field which are compatible with observations.

The Toothbrush Radio Relic and Magnetic Fields in the Intracluster Medium

Hoeft, Matthias

We precent recent deep observations of Toothbrush radio relic with the Karl Jansky Very Large Array (VLA). Using all four VLA configurations we obtained an unprecedented detailed view of the relic (Rajpurohit et al. 2018). The images revealed that the emission is rather filamentary. We conclude that wider filaments are caused by projection. In contrast, narrow filaments likely reflect the magnetic field distribution. The relic is significantly polarised above a few GHz and quickly depolarises towards lower frequencies. We present a detailed analysis of the polarisation and depolarisation of the Toothbrush and discuss the Rotation Measure spectra. The Toothbrush radio relic provides a unique insight into the magnetisation of the intra-cluster medium. 


Mckinven, Ryan

The intergalactic medium (IGM) is expected to be permeated with an intergalactic magnetic field (IGMF). We apply Faraday Rotation Measure Synthesis (FRMS) for the first time to real radio-polarimetric data taken from the GALFA Continuum Transit Survey (GALFACTS) and explore various considerations that must be made when selecting sources for extracting the IGMF. The ratio of the intrinsic to observed rotation measure (RM) of the foreground emission is probed using extragalactic sources. This parameter ($X_{RM}$) is estimated over different regions of the sky and is used to rescale the foreground RM contribution experienced by extragalactic source emission. This allows for an extragalactic RM contribution to be isolated for each source. The spread in distribution of this extragalactic RM contribution provides an upper bound constraint on the RM contribution of the IGMF. More broadly, this research illustrates FRMS’s utility in constraining the IGMF. The considerations explored here should prove useful for future researchers devising similar experiments with larger bandwidths of next generation telescopes (SKA) and their precursor/pathfinders (e.g. LOFAR, ASKAP).   

All Hands On Deck: Understanding Astrophysical Jets from Supermassive Black Holes via Observation, Theory, Simulation and Laboratory Experiment

Li, Hui

We present 3D relativistic and non-relativistic MHD simulations of AGN jets in galaxy cluster environments to study the morphology, energetics and polarization signatures of such jets and lobes.  In particular, we investigate the following issues: 1. On the global jet/lobe scales, what magnetic field configurations are constrained by observations in radio and X-rays in order to explain the morphology and polarization properties? 2. On the global jet/lobe scales, what kind of jets (hydromagnetic or magnetic tower) can survive the “weather” of the background ICM? 3. On the pc-kpc scales, will the current-driven kink instability destroy the jets? What are the likely processes that impact the energy conversion from magnetic fields to kinetic flow and/or relativistic particles?  4. How laboratory magnetized plasma jet experiments are helping us to understand the jet stability and energy conversion?  We present observational and numerical modeling results that argue for a relatively small amount of energy dissipation along the jets until most of the jet energy is deposited in lobes and their interaction with the background ICM. Future prospects of progress in simulations will be discussed as well.

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