Philipp P. Kronberg

Strong magnetic fields in normal galaxies at high redshift

The origin and growth of magnetic fields in galaxies is still something of an enigma. It is generally assumed that seed fields are amplified over time through the dynamo effect, but there are few constraints on the timescale. It was recently demonstrated that field strengths as traced by rotation measures of distant (and hence ancient) quasars are comparable to those seen today, but it was unclear whether the high fields were in the unusual environments of the quasars themselves or distributed along the lines of sight. Here we report high-resolution spectra that demonstrate that the quasars with strong Mg ii absorption lines are unambiguously associated with larger rotation measures. Because Mg ii absorption occurs in the haloes of normal galaxies along the sightlines to the quasars, this association requires that organized fields of surprisingly high strengths are associated with normal galaxies when the Universe was only about one-third of its present age.

DERIVING THE GLOBAL STRUCTURE OF THE GALACTIC MAGNETIC FIELD FROM FARADAY ROTATION MEASURES OF EXTRAGALACTIC SOURCES

We made use of the two latest sets of rotational measures (RMs) of extra-galactic radio sources, namely the NRAO VLA Sky Survey rotation measures catalog, and a compilation by Kronberg and Newton-McGee, to infer the global structure of the Galactic magnetic field (GMF). We have checked that these two data sets are mutually consistent. Given the existence of clear patterns in all-sky RM distribution we considered GMF models consisting of two components: disk (spiral or ring) and halo. The parameters of these components were determined by fitting different model field geometries to the observed RMs. We found that the model consisting of a symmetric (with respect to the Galactic plane) spiral disk and anti-symmetric halo fits the data best and reproduces the observed distribution of RMs over the sky very well. We confirm that ring disk models are disfavored. Our results favor small pitch angles around ∼ −5 ◦ and an increased vertical scale of electron distribution, in agreement with some recent studies. Based on our fits, we select two benchmark models suitable for studies of cosmic ray propagation, including the ultra-high energies. Subject headings: Galaxy: structure –ISM: magnetic fields– methods: data analysis

A GLOBAL PROBE OF COSMIC MAGNETIC FIELDS TO HIGH REDSHIFTS

Faraday rotation (rotation measure [RM]) probes of magnetic fields in the universe are sensitive to cosmological and evolutionary effects as z increases beyond ~1 because of the scalings of electron density and magnetic fields, and the growth in the number of expected intersections with galaxy-scale intervenors, -->dN/dz. In this new global analysis of an unprecedented large sample of RMs of high-latitude quasars extending out to -->z ~ 3.7, we find that the distribution of RM broadens with redshift in the 20-80 rad m?2 range, despite the ( -->1 + z)?2 wavelength dilution expected in the observed Faraday rotation. Our results indicate that the universe becomes increasingly Faraday-opaque to sources beyond -->z ~ 2; that is, as z increases, progressively fewer sources are found with a small RM in the observers frame. This is in contrast to sources at -->z 1. They suggest that the environments of galaxies were significantly magnetized at high redshifts, with magnetic field strengths that were at least as strong within a few Gyr of the big bang as at the current epoch. We separately investigate a simple unevolving toy model in which the RM is produced by Mg II absorber systems, and find that it can approximately reproduce the observed trend with redshift. An additional possibility is that the intrinsic RM associated with the radio sources was much higher in the past, and we show that this is not a trivial consequence of the higher radio luminosities of the high-redshift sources.

An improved map of the galactic Faraday sky

We aim to summarize the current state of knowledge regarding Galactic Faraday rotation in an all-sky map of the Galactic Faraday depth. For this we have assembled the most extensive catalog of Faraday rotation data of compact extragalactic polarized radio sources to date. In the map-making procedure we used a recently developed algorithm that reconstructs the map and the power spectrum of a statistically isotropic and homogeneous field while taking into account uncertainties in the noise statistics. This procedure is able to identify some rotation angles that are offset by an integer multiple of π. The resulting map can be seen as an improved version of earlier such maps and is made publicly available, along with a map of its uncertainty. For the angular power spectrum we find a power law behavior C� ∝ � −2.17 for a Faraday sky where an overall variance profile as a function of Galactic latitude has been removed, in agreement with earlier work. We show that this is in accordance with a 3D Fourier power spectrum P(k) ∝ k −2.17 of the underlying

MODELING THE MAGNETIC FIELD IN THE GALACTIC DISK USING NEW ROTATION MEASURE OBSERVATIONS FROM THE VERY LARGE ARRAY

We have determined 194 Faraday rotation measures (RMs) of polarized extragalactic radio sources using new, multi-channel polarization observations at frequencies around 1.4 GHz from the Very Large Array in the Galactic plane at 17° ≤ l ≤ 63° and 205° ≤ l ≤ 253°. This catalog fills in gaps in the RM coverage of the Galactic plane between the Canadian Galactic Plane Survey and Southern Galactic Plane Survey. Using this catalog we have tested the validity of recently proposed axisymmetric and bisymmetric models of the large-scale (or regular) Galactic magnetic field, and found that of the existing models we tested, an axisymmetric spiral model with reversals occurring in rings (as opposed to along spiral arms) best matched our observations. Building on this, we have performed our own modeling, using RMs from both extragalactic sources and pulsars. By developing independent models for the magnetic field in the outer and inner Galaxy, we conclude that in the inner Galaxy, the magnetic field closely follows the spiral arms, while in the outer Galaxy, the field is consistent with being purely azimuthal. Furthermore, the models contain no reversals in the outer Galaxy, and together seem to suggest the existence of a single reversed region that spirals out from the Galactic center.

A SURVEY OF EXTRAGALACTIC FARADAY ROTATION AT HIGH GALACTIC LATITUDE: THE VERTICAL MAGNETIC FIELD OF THE MILKY WAY TOWARD THE GALACTIC POLES

We present a study of the vertical magnetic field of the Milky Way toward the Galactic poles, determined from observations of Faraday rotation toward more than 1000 polarized extragalactic radio sources at Galactic latitudes |b| 77 ◦ , using the Westerbork Radio Synthesis Telescope and the Australia Telescope Compact Array. We find median rotation measures (RMs) of 0.0 ± 0. 5r ad m −2 and +6.3 ± 0. 7r ad m −2 toward the north and south Galactic poles, respectively, demonstrating that there is no coherent vertical magnetic field in the Milky Way at the Sun’s position. If this is a global property of the Milky Way’s magnetism, then the lack of symmetry across the disk rules out pure dipole or quadrupole geometries for the Galactic magnetic field. The angular fluctuations in RM seen in our data show no preferred scale within the range ≈0. ◦ 1t o≈25 ◦ . The observed standard deviation in RM of ∼ 9r ad m −2 then implies an upper limit of ∼1 μG on the strength of the random magnetic field in the warm ionized medium at high Galactic latitudes.

NEW CONSTRAINTS ON THE GALACTIC HALO MAGNETIC FIELD USING ROTATION MEASURES OF EXTRAGALACTIC SOURCES TOWARDS THE OUTER GALAXY

We present a study of the Milky Way disk and halo magnetic field, determined from observations of Faraday rotation measure (RM) towards 641 polarized extragalactic radio sources in the Galactic longitude range 100 117 , within 30 of the Galactic plane. Forjbj < 15 , we observe a symmetric RM distribution about the Galactic plane. This is consistent with a disk field in the Perseus arm of even parity across the Galactic midplane. In the range 15 <jbj < 30 , we find median rotation measures of -15 4 rad m -2 and -62 5 rad m -2 in the northern and southern Galactic hemispheres, respectively. If the RM distribution is a signature of the large-scale field parallel to the Galactic plane, this suggests that the halo magnetic field toward the outer Galaxy does not reverse direction across the mid-plane. The variation of RM as a function of Galactic latitude in this longitude range is such that RMs become more negative at largerjbj. This is consistent with an azimuthal magnetic field of strength 2 G (7 G) at a height 0.8-2 kpc above (below) the Galactic plane between the local and the Perseus spiral arm. We propose that the Milky Way could possess spiral-like halo magnetic fields similar to those observed in M51. Subject headings: magnetic fields —Faraday rotation—polarization—Galaxy: halo

Discovery of New Faint Radio Emission on 8° to 3' Scales in the Coma Field, and Some Galactic and Extragalactic Implications

We present a deep, 8 ◦ diameter, 0.4 GHz radio image using a first time combination of the NAIC Arecibo 305-m telescope in Puerto Rico, and the wide-angle interferometer at the Dominion Radio Astrophysical Observatory at Penticton, Canada. Our observations are centered on the Coma Cluster of galaxies in the “Great Wall” of galaxies near the North Galactic Pole. The complementary nature of these two instruments enables us to produce a distortion-free image that is sensitive to radiation on scales from 8 ◦ down to that of an individual galaxy halo at the 100 Mpc distance of the Great Wall. Newly revealed patches of distributed radio “glow” are seen well above the detection limit. One prominent such area coincides with groupings of radio galaxies near the Coma cluster, and indicates intergalactic IGM magnetic fields in the range 0.2 to 0.4 µG on scales of up to ∼ 4Mpc. Other patches of diffuse emission, not previously explored at these high latitudes on arcminute scales, probably contain Galactic “cirrus”. A striking anticorrelation is found between low-level diffuse radio glow and some regions of enhanced optical galaxy surface density, suggesting that cosmological Large Scale Structure (LSS), normally defined by the baryonic (or dark) matter density, is not uniquely traced by faint continuum radio glow. Rather, intergalactic diffuse synchrotron radiation represents IGM magnetic and Cosmic ray energy density, instead of matter density. The diffuse, arcminute-level structures over a large region of sky are potentially important pathfinders to CMB foreground radiation on high multipole scales.

Giant Radio Galaxies and Cosmic-Ray Acceleration

Giant radio galaxies (GRGs) are prime and unique laboratories for constraining the plasma processes that accelerate relativistic electrons within large intergalactic volumes. The evidence for short radiative loss times rules out certain scenarios for energy transport within their very large dimensions. This, combined with their high energy content, large ordered magnetic field structures, the absence of strong large-scale shocks, and very low upper limits on their internal thermal plasma densities, points to a direct and efficient conversion of force-free magnetic field to particle energy. This is underlined by the evidence in GRGs that their internal Alfven speeds are higher than the lobe expansion speeds. We discuss these constraints in the context of models in which the central black hole energy is initially extracted as electromagnetic Poynting flux that injects large amounts of magnetic flux into the lobes. Recent advances in the theory of collisionless magnetic reconnection make this a favored mechanism to explain the particle acceleration in these systems. The energy reservoir is likely to be force-free fields, which is independently consistent with recent models of initial electromagnetic energy transfer from the parent galaxys supermassive black hole. Such a scenario has wide-ranging astrophysical consequences: it implies that space-distributed magnetic reconnection or some other highly efficient field-to-particle energy conversion process likely dominates in all extended extragalactic radio sources.

Strong magnetic fields and large rotation measures in protogalaxies from supernova seeding

We present a model for the seeding and evolution of magnetic fi elds in protogalaxies. Super- nova (SN) explosions during the assembly of a protogalaxy self-consistently provide magnetic seed fields, which are subsequently amplified by compression , shear flows and random mo- tions. Our model explains the origin of strong magnetic field s ofG amplitude within the first starforming protogalactic structures shortly after the fir st stars have formed. We implement the model into the MHD version of the cosmological N-body / SPH simulation code GADGET and we couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky Way-like galactic halo for- mation using a standardCDM cosmology and analyse the strength and distribution of the subsequent evolving magnetic field. Within starforming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shape magnetic fie ld at a rate of �10 9 G Gyr 1 . Subsequently, the magnetic field strength increases expone ntially on timescales of a few ten million years within the innermost regions of the halo. Furt hermore, turbulent diffusion, shocks and gas motions transport the magnetic field towards t he halo outskirts. At redshift z�0, the entire galactic halo is magnetized and the field amplit ude is of the order of a fewG in the center of the halo and �10 9 G at the virial radius. Additionally, we analyse the intrinsic rotation measure (R M) of the forming galactic halo over redshift. The mean halo intrinsic RM peaks between redshifts z�4 and z�2 and reaches abso- lute values around 1000 rad m 2 . While the halo virializes towards redshift z�0, the intrinsic RM values decline to a mean value below 10 rad m 2 . At high redshifts, the distribution of individual starforming and thus magnetized regions is widespread. This leads to a widespread distribution of large intrinsic RM values. In our model for the evolution of galactic magnetic fields, th e seed magnetic field amplitude and distribution is no longer a free parameter, but determin ed self-consistently by the star formation process occuring during the formation of cosmic structures. Thus, it provides a solution to the seed field problem.