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A New Catalog of Faraday Rotation Measures and Redshifts for Extragalactic Radio Sources  [PDF]
Alison M. Hammond,Timothy Robishaw,B. M. Gaensler
Physics , 2012,
Abstract: We present a catalog of Faraday rotation measures (RMs) and redshifts for 4003 extragalactic radio sources detected at 1.4 GHz, derived by identifying optical counterparts and spectroscopic redshifts for linearly polarized radio sources from the NRAO VLA Sky Survey. This catalog is more than an order of magnitude larger than any previous sample of RM vs. redshift, and covers the redshift range 0 < z < 5.3 ; the median redshift of the catalog is z = 0.70, and there are more than 1500 sources at redshifts z > 1. For 3650 of these sources at Galactic latitudes |b| >= 20 degrees, we present a second catalog in which we have corrected for the foreground Faraday rotation of the Milky Way, resulting in an estimate of the residual rotation measure (RRM) that aims to isolate the contribution from extragalactic magnetic fields. We find no significant evolution of RRM with redshift, but observe a strong anti-correlation between RRM and fractional polarization, p, that we argue is the result of beam depolarization from small-scale fluctuations in the foreground magnetic field or electron density. We suggest that the observed variance in RRM and the anti-correlation of RRM with p both require a population of magnetized intervening objects that lie outside the Milky Way but in the foreground to the emitting sources.
Magnetic Fields in the Large Scale Structure from Faraday Rotation measurements  [PDF]
A. Neronov,D. Semikoz,M. Banafsheh
Physics , 2013,
Abstract: We search for magnetic fields outside galaxies and galaxy clusters by investigating redshift evolution of Faraday rotation measures (RM) of extragalactic radio sources. Our analysis reveals a strong evidence for the redshift dependence of the mean of the absolute value of the RM. The evidence is further strengthened if the Galactic contribution to the RM is subtracted. The hypothesis of the absence of the redshift evolution of residual RM is ruled out at 5 sigma level. The observed redshift dependence of RM is consistent with the possibility of the presence of nano-Gauss strength magnetic fields with correlation length shorter than 0.1 Mpc in the weakly overdense elements of the Large Scale Structure traced by the Ly-alpha clouds.
Measurements of the Cosmological Evolution of Magnetic Fields with the Square Kilometre Array  [PDF]
Martin Krause,Paul Alexander,Rosie Bolton,Joern Geisbuesch,David A. Green,Julia Riley
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15489.x
Abstract: We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise of ten. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per field. We also calculate the statistics for background RM measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high redshift clusters constrain the power law index for the magnetic field evolution to better than dm=0.4, if the magnetic field in clusters should follow B ~ (1+z)^m.
New Constraints on the Galactic Halo Magnetic Field using Rotation Measures of Extragalactic Sources Towards the Outer Galaxy  [PDF]
S. A. Mao,N. M. McClure-Griffiths,B. M. Gaensler,J. C. Brown,C. L. van Eck,M. Haverkorn,P. P. Kronberg,J. M. Stil,A. Shukurov,A. R. Taylor
Physics , 2012, DOI: 10.1088/0004-637X/755/1/21
Abstract: 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 degs, within 30 degs of the Galactic plane. For |b| < 15 degs, 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 mid-plane. In the range 15<|b|<30 degs, 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 larger |b|. This is consistent with an azimuthal magnetic field of strength 2 microGauss (7 microGauss) 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.
Deriving global structure of the Galactic Magnetic Field from Faraday Rotation Measures of extragalactic sources  [PDF]
M. S. Pshirkov,P. G. Tinyakov,P. P. Kronberg,K. J. Newton-McGee
Physics , 2011, DOI: 10.1088/0004-637X/738/2/192
Abstract: We made use of the two latest sets of Rotational Measures (RMs) of extra-galactic radio sources, namely the NRAO VLA Sky Survey otation Measures Catalogue, and a compilation by Kronberg&Newton-McGee(2011), to infer the global structure of the Galactic Magnetic Field (GMF). We have checked that these two data sets are consistent with each other. Motivated by clear patterns in the observed distribution of RMs over the sky, we considered GMF models consisting of the 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 degrees and an increased vertical scale of electron distribution, in agreement with some previous studies. Based on our fits, we identify two benchmark models suitable for studies of cosmic ray propagation, including the ultra-high energies.
The rotation measures of high luminosity sources as seen from the NVSS  [PDF]
M. S. Pshirkov,P. G. Tinyakov,F. R. Urban
Physics , 2014, DOI: 10.1093/mnras/stv1273
Abstract: We re-analyse the subset of the Faraday rotation measures data from the NRAO VLA Sky Survey catalogue for which redshift and spectral index information is available, in order to better elucidate the relations between these observables. We split this subset in two according to their radioluminosity, and find that higher power sources have a systematically higher residual rotation measure, once the regular field of the Milky Way is subtracted. This rotation measure stands well above the variances due to the turbulent field of our Galaxy, contrarily to low power sources. The effect is more pronounced as the energy threshold becomes more restrictive. If the two sets are merged one observes an apparent evolution of rotation measure with redshift, but our analysis shows that it can be interpreted as an artifact of the different intrinsic properties of brighter sources that are typically observed at larger distances.
Estimating extragalactic Faraday rotation  [PDF]
Niels Oppermann,Henrik Junklewitz,Maksim Greiner,Torsten A. En?lin,Takuya Akahori,Ettore Carretti,Bryan M. Gaensler,Ariel Goobar,Lisa Harvey-Smith,Melanie Johnston-Hollitt,Luke Pratley,Dominic H. F. M. Schnitzeler,Jeroen M. Stil,Valentina Vacca
Physics , 2014, DOI: 10.1051/0004-6361/201423995
Abstract: (abridged) Observations of Faraday rotation for extragalactic sources probe magnetic fields both inside and outside the Milky Way. Building on our earlier estimate of the Galactic contribution, we set out to estimate the extragalactic contributions. We discuss the problems involved; in particular, we point out that taking the difference between the observed values and the Galactic foreground reconstruction is not a good estimate for the extragalactic contributions. We point out a degeneracy between the contributions to the observed values due to extragalactic magnetic fields and observational noise and comment on the dangers of over-interpreting an estimate without taking into account its uncertainty information. To overcome these difficulties, we develop an extended reconstruction algorithm based on the assumption that the observational uncertainties are accurately described for a subset of the data, which can overcome the degeneracy with the extragalactic contributions. We present a probabilistic derivation of the algorithm and demonstrate its performance using a simulation, yielding a high quality reconstruction of the Galactic Faraday rotation foreground, a precise estimate of the typical extragalactic contribution, and a well-defined probabilistic description of the extragalactic contribution for each data point. We then apply this reconstruction technique to a catalog of Faraday rotation observations. We vary our assumptions about the data, showing that the dispersion of extragalactic contributions to observed Faraday depths is most likely lower than 7 rad/m^2, in agreement with earlier results, and that the extragalactic contribution to an individual data point is poorly constrained by the data in most cases.
The Redshift Distribution of Extragalactic Radio Sources  [PDF]
C A Jackson,J V Wall
Physics , 1998,
Abstract: Extragalactic radio sources are a unique cosmological probe in that they trace large-scale structure on scales inaccessible to other wavelengths. However as radio survey data is inherently 2D, the redshift distribution, N(z), is necessary to derive spatial information. To obtain this distribution either we measure thousands of radio source redshifts to directly determine N(z) or we derive N(z) from statistical analyses of radio source count and identification data. In this paper we show how the dual-population unification scheme can be incorporated into a rigorous statistical analysis of radio source count data, with the result that our simple parametric evolution and beaming model revises previous estimations of N(z), specifically at low flux densites. This revision is particularly pertinent given that the new generation of radio surveys extend to milli-jansky flux density levels: sampling source densities high enough to reveal spatial structure. In turn, these new radio surveys will provide potent tests which will refine our model.
Using SKA Rotation Measures to Reveal the Mysteries of the Magnetised Universe  [PDF]
Melanie Johnston-Hollitt,Federica Govoni,Rainer Beck,Siamak Dehghan,Luke Pratley,Takuya Akahori,George Heald,Ivan Agudo,Annalisa Bonafede,Ettore Carretti,Tracy Clarke,Sergio Colafrancesco,Torsten En?lin,Luigina Feretti,Bryan Gaensler,Marijke Haverkorn,Sui Ann Mao,Niels Oppermann,Lawrence Rudnick,Anna Scaife,Dominic Schnitzeler,Jeroen Stil,A. Russ Taylor,Valentina Vacca
Physics , 2015,
Abstract: We know that magnetic fields are pervasive across all scales in the Universe and over all of cosmic time and yet our understanding of many of the properties of magnetic fields is still limited. We do not yet know when, where or how the first magnetic fields in the Universe were formed, nor do we fully understand their role in fundamental processes such as galaxy formation or cosmic ray acceleration or how they influence the evolution of astrophysical objects. The greatest challenge to addressing these issues has been a lack of deep, broad bandwidth polarimetric data over large areas of the sky. The Square Kilometre Array will radically improve this situation via an all-sky polarisation survey that delivers both high quality polarisation imaging in combination with observations of 7-14 million extragalactic rotation measures. Here we summarise how this survey will improve our understanding of a range of astrophysical phenomena on scales from individual Galactic objects to the cosmic web.
The VIMOS Public Extragalactic Redshift Survey - Searching for Cosmic Voids  [PDF]
D. Micheletti,A. Iovino,A. J. Hawken,B. R. Granett,M. Bolzonella,A. Cappi,L. Guzzo,U. Abbas,C. Adami,S. Arnouts,J. Bel,D. Bottini,E. Branchini,J. Coupon,O. Cucciati,I. Davidzon,G. De Lucia,S. de la Torre,A. Fritz,P. Franzetti,M. Fumana,B. Garilli,O. Ilbert,J. Krywult,V. Le Brun,O. Le Fèvre,D. Maccagni,K. Ma?ek,F. Marulli,H. J. McCracken,M. Polletta,A. Pollo,C. Schimd,M. Scodeggio,L. A. M. Tasca,R. Tojeiro,D. Vergani,A. Zanichelli,A. Burden,C. Di Porto,A. Marchetti,C. Marinoni,Y. Mellier,T. Moutard,L. Moscardini,R. C. Nichol,J. A. Peacock,W. J. Percival,G. Zamorani
Physics , 2014, DOI: 10.1051/0004-6361/201424107
Abstract: The characterisation of cosmic voids gives unique information about the large-scale distribution of galaxies, their evolution and the cosmological model. We identify and characterise cosmic voids in the VIMOS Public Extragalactic Redshift Survey (VIPERS) at redshift 0.55 < z < 0.9. A new void search method is developed based upon the identification of empty spheres that fit between galaxies. The method can be used to characterise the cosmic voids despite the presence of complex survey boundaries and internal gaps. We investigate the impact of systematic observational effects and validate the method against mock catalogues. We measure the void size distribution and the void-galaxy correlation function. We construct a catalogue of voids in VIPERS. The distribution of voids is found to agree well with the distribution of voids found in mock catalogues. The void-galaxy correlation function shows indications of outflow velocity from the voids.
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