University of Groningen
Non-thermal emission and magnetic fields in nearby galaxies
Seethapuram Sridhar, Sarrvesh
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Publication date: 2018
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Seethapuram Sridhar, S. (2018). Non-thermal emission and magnetic fields in nearby galaxies: A low-frequency radio continuum perspective. University of Groningen.
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Non-thermal emission and
magnetic fields in nearby galaxies
A low-frequency radio continuum perspective
PhD thesis
to obtain the degree of PhD at the
University of Groningen
on the authority of the
Rector Magnificus Prof. E. Sterken
and in accordance with
the decision by the College of Deans.
This thesis will be defended in public on
Monday 29 October 2018 at 12.45 hours
by
Sarrvesh Seethapuram Sridhar
born on 15 September 1989
in Chennai, Tamil Nadu, India
Supervisor
Prof. J. M. van der Hulst
Co-supervisor
Dr. George H. Heald
Assessment Committee
Prof. A. Scaife
Prof. J. A. Irwin
Prof. P. Barthel
To N. B. who wanted to be here,
Cover – Front: A typical LOFAR High Band Antenna (HBA) image processed using the facet-based direction-dependent calibration scheme. Back cover: Image of LOFAR Low Band Antenna (LBA) dipoles by Hans Hordijk.
Research included in this thesis was performed at the following scientific institutions:
Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV, Groningen, the Netherlands.
ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990AA, Dwingeloo, the Netherlands.
CSIRO Astronomy and Space Science (CASS), 26 Dick Perry Ave, Kensington, WA 6101, Australia
This research was supported by funding from ASTRON, Nederlandse Onder-zoekschool Voor Astronomie (NOVA), and the Kapteyn Institute. Multiple trips to conferences and work visits were also funded by the Leids Kerkhoven Bosscha Fonds (LKBF). Additional travel funding to Perth, Australia was provided by CSIRO Astronomy and Space Science.
Thesis printed by: GVO drukkers & vormgevers B.V. ISBN: 978–94–034–1148–4
Contents
1 Prologue 1
1.1 Historical overview . . . 2
1.2 Radio continuum emission from galaxies . . . 2
1.2.1 Thermal radio emission . . . 3
1.2.2 Non-thermal radio emission . . . 4
1.2.3 Synchrotron emission, Faraday rotation and magnetic fields 5 1.2.4 Nearby galaxies at low radio frequencies . . . 8
1.3 Radio telescopes used in this thesis . . . 8
1.3.1 Westerbork Synthesis Radio Telescope . . . 10
1.3.2 The International LOFAR Telescope . . . 13
1.3.3 Challenges of observing at low radio frequencies . . . 16
1.4 Outline of this thesis . . . 17
2 The curious case of NGC 4258: a new low-frequency radio-continuum perspective 21 2.1 Introduction . . . 22
2.2 LOFAR Observation and data reduction . . . 24
2.2.1 Observational setup . . . 24
2.2.2 Pre-processing . . . 25
2.2.3 Calibration . . . 26
2.2.4 Ionospheric RM correction . . . 27
2.2.5 Self-calibration and imaging . . . 28
2.2.6 Flux and astrometry uncertainties . . . 28
2.3 Westerbork observations and data reduction . . . 30
2.4 Results . . . 32
2.4.1 Total intensity maps . . . 32
2.4.2 Other nearby galaxies in the LOFAR field of view . . . 34
2.4.3 Spectral properties of NGC 4258 . . . 34
2.4.4 Thermal fraction and non-thermal spectral index . . . 37
2.4.5 Magnetic field strength . . . 40
2.4.6 Relation with the HI disk . . . 42
2.5 Search for polarized emission . . . 42
2.5.1 Polarized emission at 1.4 GHz . . . 42
2.5.2 Polarized emission at 141.8 MHz . . . 44 v
vi CONTENTS
2.5.3 Stacking polarized emission in the galactic disk . . . 46
2.6 Where are the anomalous arms located? . . . 48
2.7 Summary and conclusions . . . 52
3 Multifrequency radio continuum observations of the Pinwheel galaxy (M 101) 55 3.1 Introduction . . . 56
3.2 WSRT observations and data reduction . . . 59
3.3 LOFAR observation and data reduction . . . 60
3.4 Radio continuum morphology of M 101 . . . 64
3.5 The HI disk and the high-velocity gas complex . . . 65
3.6 Integrated flux densities and radio spectrum . . . 69
3.7 Estimating the thermal contribution . . . 71
3.8 Non-thermal spectral index . . . 74
3.8.1 Radial scale length . . . 78
3.9 Equipartition magnetic field strength . . . 80
3.10 Summary and conclusions . . . 83
4 Resolved low-frequency radio images of nearby dwarf galaxies 85 4.1 Introduction . . . 86
4.2 LOFAR observations and data reduction . . . 87
4.2.1 Observational setup and preprocessing . . . 87
4.2.2 Calibration . . . 89
4.2.3 Final imaging . . . 93
4.3 Total intensity maps . . . 95
4.3.1 NGC 1569 . . . 95
4.3.2 NGC 4214 . . . 99
4.3.3 NGC 2366 . . . 102
4.3.4 DDO 50 . . . 106
4.4 Estimating thermal fraction . . . 107
4.5 Non-thermal spectral index maps . . . 110
4.6 Equipartition magnetic field strength . . . 110
4.7 Search for polarized emission . . . 112
4.7.1 Polarized Galactic foreground . . . 114
4.7.2 Polarized emission from a giant radio galaxy . . . 114
4.8 Discussion . . . 116
4.9 Summary and conclusions . . . 119
5 cuFFS: A GPU-accelerated Rotation Measure Synthesis Code 121 5.1 Introduction . . . 122
5.2 Background . . . 123
5.2.1 RM synthesis: Theory . . . 123
5.2.2 RM synthesis: In practice . . . 125
5.2.3 RM synthesis: Computational costs . . . 126
5.3 GPU implementation of RM Synthesis . . . 128
5.3.1 FITS, HDF5, and HDFITS . . . 131
CONTENTS vii
5.4 Conclusion and future outlook . . . 139
6 Conclusions 141 6.1 Summary of key results . . . 141
6.2 Avenues for future research . . . 143
6.2.1 Broadband polarimetry as a probe of anomalous arms in NGC 4258 . . . 143
6.2.2 Mapping the halos of nearby dwarf galaxies . . . 144
Appendices 147 A Calibrating LOFAR HBA Data 149 A.1 Need for direction-dependent calibration . . . 150
A.2 LOFAR Facet Calibration . . . 153
A.2.1 Direction-independent steps . . . 153
A.2.2 Direction-dependent steps . . . 158
Samenvatting 165
Acknowledgements 170
