Studying Galactic interstellar turbulence through fluctuations in synchrotron emission

First LOFAR Galactic foreground detection

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: iacobelli@strw.leidenuniv.nl
² Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Radboud University Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
⁶ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Jagiellonian University, ul. Orla 171, 30244 Kraków, Poland
⁸ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
⁹ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
¹⁰ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹¹ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
¹² Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
¹³ Space Telescope Science Institute, 3700 San MartinDrive, Baltimore, MD 21218, USA
¹⁴ SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁵ ARC Centre of Excellence for All-sky astrophysics (CAASTRO), Sydney Institute of Astronomy, University of Sydney, 2006 Sydney, Australia
¹⁶ Harvard-Smithsonian centre for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁹ University of Groningen, Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
²⁰ Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, A.C.T. 2611 Weston, Australia
²¹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²² Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²³ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
²⁴ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, LPC2E UMR 7328 CNRS, 45071 Orléans Cedex 02, France
²⁵ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²⁶ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Ave Charles André, 69561 Saint Genis Laval Cedex, France
²⁷ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
²⁸ Centre for Radio Astronomy Techniques & Technologies (RATT), Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
²⁹ Jodrell Bank centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
³⁰ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
³¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³² Astro Space centre of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³³ SKA South Africa, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
³⁴ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³⁵ Universitá di Bologna – INAF ALMA regional centre, via P. Gobetti 101, 40129 Bologna, Italy

Received: 4 June 2013
Accepted: 17 July 2013

Abstract

Aims. The characteristic outer scale of turbulence (i.e. the scale at which the dominant source of turbulence injects energy to the interstellar medium) and the ratio of the random to ordered components of the magnetic field are key parameters to characterise magnetic turbulence in the interstellar gas, which affects the propagation of cosmic rays within the Galaxy. We provide new constraints to those two parameters.

Methods. We use the LOw Frequency ARray (LOFAR) to image the diffuse continuum emission in the Fan region at (l,b) ~ (137.0°, +7.0°) at 80′′ × 70′′ resolution in the range [146, 174] MHz. We detect multi-scale fluctuations in the Galactic synchrotron emission and compute their power spectrum. Applying theoretical estimates and derivations from the literature for the first time, we derive the outer scale of turbulence and the ratio of random to ordered magnetic field from the characteristics of these fluctuations.

Results. We obtain the deepest image of the Fan region to date and find diffuse continuum emission within the primary beam. The power spectrum displays a power law behaviour for scales between 100 and 8 arcmin with a slope α = −1.84 ± 0.19. We find an upper limit of ~20 pc for the outer scale of the magnetic interstellar turbulence toward the Fan region, which is in agreement with previous estimates in literature. We also find a variation of the ratio of random to ordered field as a function of Galactic coordinates, supporting different turbulent regimes.

Conclusions. We present the first LOFAR detection and imaging of the Galactic diffuse synchrotron emission around 160 MHz from the highly polarized Fan region. The power spectrum of the foreground synchrotron fluctuations is approximately a power law with a slope α ≈ −1.84 up to angular multipoles of ≲1300, corresponding to an angular scale of ~8 arcmin. We use power spectra fluctuations from LOFAR as well as earlier GMRT and WSRT observations to constrain the outer scale of turbulence (L_out) of the Galactic synchrotron foreground, finding a range of plausible values of 10−20 pc. Then, we use this information to deduce lower limits of the ratio of ordered to random magnetic field strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT and GMRT fields considered respectively. Both these constraints are in agreement with previous estimates.