University of Groningen
Large-scale 21-cm Cosmology with LOFAR and AARTFAAC
Gehlot, Bharat Kumar
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Publication date: 2019
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Gehlot, B. K. (2019). Large-scale 21-cm Cosmology with LOFAR and AARTFAAC. University of Groningen.
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Large-scale 21-cm Cosmology with
LOFAR and AARTFAAC
PhD thesis
to obtain the degree of PhD at the University of Groningen
on the authority of the
Rector Magnificus prof. dr. E. Sterken and in accordance with the decision by the College of Deans.
This thesis will be defended in public on
Friday 08 February 2019 at 12:45 hours
by
Bharat Kumar Gehlot
born on 10 August 1991 in New Delhi, India
Supervisors
Prof. L.V.E. Koopmans Prof. R.A.M.J. Wijers
Assessment Committee
Prof. M.A.W. Verheijen Prof. P. Zarka
Cover - Designed by the author. Foreground: A radio continuum mosaic of the Northern Sky (in Hammer-Aitoff projection) produced with 6 hours of AARTFAAC data overlaid on a sky-map using the HiPS option in Stellarium software. LOFAR “Superterp” is shown in the landscape part. Background: An artist’s impression of a starry sky.
ISBN: 978-94-034-1431-7 (printed version) ISBN: 978-94-034-1430-0 (electronic version)
v
Contents
Table of Contents v
1 Introduction 1
1.1 An overview . . . 2
1.2 A Short Chronology of the Universe . . . 3
1.3 Current status of affairs . . . 5
1.3.1 Thompson scattering of CMB photons . . . 6
1.3.2 High redshift quasar and GRB absorption spectra . . . 7
1.3.3 High redshift galaxy surveys . . . 9
1.4 A new probe: the redshifted 21-cm line . . . 9
1.4.1 Current and planned 21-cm experiments . . . 11
1.4.2 The LOw-Frequency ARray (LOFAR) . . . 14
1.4.3 The AARTFAAC Cosmic Explorer . . . 17
1.5 Observational challenges in 21-cm experiments . . . 19
1.5.1 Foreground emission . . . 19
1.5.2 Ionospheric effects . . . 20
1.5.3 Calibration . . . 21
1.6 This thesis . . . 21
2 Wide-field LOFAR-LBA power-spectra analyses 25 2.1 Introduction . . . 27
2.2 Observations and Data processing . . . 29
2.2.1 LOFAR-LBA system . . . 30
2.2.2 Observations . . . 31
2.2.3 Flagging and averaging . . . 31
2.2.4 Calibration . . . 32
2.2.5 Imaging . . . 36
2.2.6 Source modeling . . . 37
2.3 Differential power Spectrum . . . 38
2.3.1 Excess Noise . . . 40
2.3.2 Effect of calibration cut . . . 41
2.4.1 ‘Pitchfork’ structure in polarised intensity . . . 45
2.5 Ionospheric Scintillation . . . 53
2.6 Conclusions and summary . . . 58
Appendix 2.A Rotation Measure synthesis . . . 61
3 Cosmic Dawn power spectrum limit with LOFAR 65 3.1 Introduction . . . 67
3.2 Observations and preprocessing . . . 69
3.2.1 LOFAR-Low Band Array . . . 69
3.2.2 Observations . . . 70
3.2.3 Data selection and preprocessing . . . 71
3.3 Calibration Scheme . . . 71
3.3.1 Calibrating the 3C220 field . . . 73
3.3.2 Calibrating the NCP field . . . 75
3.4 Noise statistics in LOFAR-LBA . . . 77
3.4.1 Physical Excess Noise . . . 77
3.4.2 Comparison with sub-band level noise . . . 81
3.5 Gaussian Process Regression . . . 82
3.5.1 Methodology . . . 82
3.5.2 Application of GPR to the LOFAR-LBA data . . . 83
3.6 Power Spectra Results . . . 86
3.6.1 The 3C220 field: cylindrical power spectra . . . 87
3.6.2 The NCP field: cylindrical power spectra . . . 89
3.6.3 Comparison with noise power spectra . . . 90
3.6.4 Spherically averaged power spectra . . . 92
3.7 Summary and Outlook . . . 94
3.7.1 Outlook . . . 96
4 AARTFAAC Cosmic Explorer 97 4.1 Introduction . . . 99
4.2 Observations and preprocessing . . . 102
4.2.1 The LOFAR AARTFAAC wide-field imager . . . 102
4.2.2 ACE observational setup and status . . . 104
4.2.3 Data preprocessing . . . 105
4.3 Calibration and Imaging . . . 105
4.3.1 Direction Independent calibration . . . 106
4.3.2 Direction Dependent calibration . . . 106
4.3.3 Imaging . . . 108
4.4 Thermal noise statistics . . . 108
4.5 Power Spectrum Analysis . . . 111
4.5.1 Cylindrically averaged power spectrum estimation . . . 112
4.5.3 Spherically averaged power spectrum estimation . . . . 119
4.6 Summary and future work . . . 123
4.6.1 Future outlook and forecast . . . 123
5 Modelling Degree-Scale foregrounds 125 5.1 Introduction . . . 127
5.2 Observations and preprocessing . . . 129
5.2.1 The AARTFAAC wide-field Imager . . . 129
5.2.2 Observational setup . . . 131
5.2.3 Data preprocessing . . . 132
5.3 Calibration and imaging scheme . . . 133
5.4 Modelling the Diffuse Galactic Emission . . . 135
5.4.1 Removing compact sources . . . 135
5.4.2 Modeling with multiscale CLEAN . . . 137
5.4.3 Modelling with Shapelets . . . 139
5.4.4 Comparing the two diffuse emission modelling methods 139 5.5 The Angular power spectrum . . . 140
5.5.1 Power spectra from diffuse foreground modelling methods142 5.6 Summary and Future work . . . 143
5.6.1 Future Work . . . 144
6 Conclusions and future outlook 145 6.1 Main results . . . 146
6.1.1 A wide-field LOFAR-LBA power-spectrum analysis . . . 146
6.1.2 A first Cosmic-Dawn 21-cm signal power spectrum limit 148 6.1.3 The AARTFAAC Cosmic Explorer . . . 149
6.1.4 Modelling foregrounds on degree scales . . . 150
6.2 Future outlook and concluding remarks . . . 150
Bibliography 153
English summary 159
Nederlandse Samenvatting 165
हदी सारांश 171
