Characterising the broadband polarisation properties of extragalactic radio sources at 3 GHz
Characterising the broadband polarisation properties of extragalactic radio sources at 3 GHz
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DissertationDate of Exam
30.04.2026Date of Publication
10.06.2026Advisor
Kramer, MichaelCo-Referee
Porciani, CristianoMetadata
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Abstract
Magnetic fields are ubiquitous in the Universe and are fundamentally linked to galaxy evolution, influencing star-formation, cosmic-ray propagation and feedback mechanisms. The radio frequency regime, at centimetre wavelengths, is a powerful probe of cosmic magnetic fields through the detection of galaxies emitting linearly polarised synchrotron emission. The Faraday rotation measure (RM) of this emission can be used to study the properties of both the emitting source and the intervening magneto-ionic media (e.g. the Galactic interstellar medium). Large, dense samples of polarised extragalactic sources form RM grids that can be used to map the foreground magnetic field of the Milky Way. Over the past few years, the increasing sensitivity of modern interferometers has given new accessibility to the μJy radio sky, increasing the RM sky density. It is therefore key to establish a clear understanding of the faint polarised source population and the foreground effects that shape RM measurements. Broadband spectro-polarimetry provides an enhanced perspective on this, revealing Faraday complexities in the Stokes Q and U spectral behaviour, which trace turbulence or differential Faraday rotation along the line-of-sight. Through the projects presented in this thesis, I aim to address two important factors in improving our interpretation of RM grid experiments, (i) understanding both the observational biases and physical nature of Faraday complexity at low Galactic latitudes, (ii) characterising the extragalactic polarised source population.
Firstly, in analysing the SPASS/ATCA RM catalogue (Schnitzeler et. al., 2019), the most extensive broadband polarisation catalogue in the southern sky, we report a Galactic latitude dependence of Faraday complexity for polarised sources at |b| < 10°, with the degree of complexity increasing towards the Galactic plane. Through higher angular resolution (θ = 15′′) follow-up observations of 95 sources, we find that this trend is primarily driven by contamination from large-scale Galactic polarised emission in the SPASS/ATCA spectra, which we effectively filter out. We find 42% of the observed sources in our sample are Faraday complex, with an increased fraction of Faraday complex sources surrounding the spiral arm tangents and towards the Galactic centre. We constrain the scale of this complexity to < 2.4 pc, consistent with turbulent injection scales in the spiral arms. These results emphasise the importance of broadband spectro-polarimetric observations to fully characterise small-scale and/or turbulent structures in the Galactic magnetic field, and that it is essential to correctly filter contaminating polarised emission when interpreting foreground turbulence.
Secondly, we reprocess the VLA-COSMOS 3 GHz Large Project (Smolčić et. al., 2017), one of the deepest S-band continuum surveys (2.3 μJy beam−1), for polarisation calibration and imaging. Here, we present the deepest polarised source count at 3 GHz to date, and the second deepest overall, returning an RM density of 42 deg−2. We find that these source counts are consistent with those at the more typically-studied 1.4 GHz band, a combined effect of spectral index and depolarisation, which we attribute to differential Faraday rotation in the lobes of radio galaxies. Through the available multi-wavelength catalogues, we identify all polarised sources as radio galaxies (i.e. active galactic nuclei), and confirm that no star-forming galaxies are detected in polarisation. We place an upper limit on the density of polarised star-forming galaxies to be < 0.58 deg−2, implying that surveys much deeper than 2.6 μJy beam−1 will be required to readily probe this population, even at higher frequencies where Faraday depolarisation effects are less pronounced.
Finally, I present a demonstrator project with the recently installed MeerKAT S-band receivers, with a focus on extragalactic wide-field imaging. We present MeerKAT S-band observations of the DEEP2 field, the emptiest radio field in the southern sky. The total intensity source counts are consistent with those from the literature, and also show that only a fraction of integration time is required with MeerKAT for comparable results with legacy S-band surveys. Here, I also present the science goals and survey design for the MeerKAT+ S-band Legacy survey, a 3000 hour, full-Stokes survey of the southern sky at Dec ≤ −40°. This survey will be extremely versatile across Galactic, galaxy evolution, magnetism and transient science, and with an expected 105 polarised source detections, will provide a higher frequency perspective on the RM grid of the southern sky.
Firstly, in analysing the SPASS/ATCA RM catalogue (Schnitzeler et. al., 2019), the most extensive broadband polarisation catalogue in the southern sky, we report a Galactic latitude dependence of Faraday complexity for polarised sources at |b| < 10°, with the degree of complexity increasing towards the Galactic plane. Through higher angular resolution (θ = 15′′) follow-up observations of 95 sources, we find that this trend is primarily driven by contamination from large-scale Galactic polarised emission in the SPASS/ATCA spectra, which we effectively filter out. We find 42% of the observed sources in our sample are Faraday complex, with an increased fraction of Faraday complex sources surrounding the spiral arm tangents and towards the Galactic centre. We constrain the scale of this complexity to < 2.4 pc, consistent with turbulent injection scales in the spiral arms. These results emphasise the importance of broadband spectro-polarimetric observations to fully characterise small-scale and/or turbulent structures in the Galactic magnetic field, and that it is essential to correctly filter contaminating polarised emission when interpreting foreground turbulence.
Secondly, we reprocess the VLA-COSMOS 3 GHz Large Project (Smolčić et. al., 2017), one of the deepest S-band continuum surveys (2.3 μJy beam−1), for polarisation calibration and imaging. Here, we present the deepest polarised source count at 3 GHz to date, and the second deepest overall, returning an RM density of 42 deg−2. We find that these source counts are consistent with those at the more typically-studied 1.4 GHz band, a combined effect of spectral index and depolarisation, which we attribute to differential Faraday rotation in the lobes of radio galaxies. Through the available multi-wavelength catalogues, we identify all polarised sources as radio galaxies (i.e. active galactic nuclei), and confirm that no star-forming galaxies are detected in polarisation. We place an upper limit on the density of polarised star-forming galaxies to be < 0.58 deg−2, implying that surveys much deeper than 2.6 μJy beam−1 will be required to readily probe this population, even at higher frequencies where Faraday depolarisation effects are less pronounced.
Finally, I present a demonstrator project with the recently installed MeerKAT S-band receivers, with a focus on extragalactic wide-field imaging. We present MeerKAT S-band observations of the DEEP2 field, the emptiest radio field in the southern sky. The total intensity source counts are consistent with those from the literature, and also show that only a fraction of integration time is required with MeerKAT for comparable results with legacy S-band surveys. Here, I also present the science goals and survey design for the MeerKAT+ S-band Legacy survey, a 3000 hour, full-Stokes survey of the southern sky at Dec ≤ −40°. This survey will be extremely versatile across Galactic, galaxy evolution, magnetism and transient science, and with an expected 105 polarised source detections, will provide a higher frequency perspective on the RM grid of the southern sky.
Subjects
Radio astronomy, galaxies, Milky Way, magnetic fields, polarisation, radio surveys, Faraday rotation
Classification (DDC)
520 Astronomie, KartografieRelated Publications
https://doi.org/10.1051/0004-6361/202348993https://doi.org/10.1093/mnras/stae2754
Ranchod, Shilpa: Characterising the broadband polarisation properties of extragalactic radio sources at 3 GHz. - Bonn, 2026. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-90494
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-90494
@phdthesis{handle:20.500.11811/14194,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-90494,
doi: https://doi.org/10.48565/bonndoc-877,
author = {{Shilpa Ranchod}},
title = {Characterising the broadband polarisation properties of extragalactic radio sources at 3 GHz},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = jun,
note = {Magnetic fields are ubiquitous in the Universe and are fundamentally linked to galaxy evolution, influencing star-formation, cosmic-ray propagation and feedback mechanisms. The radio frequency regime, at centimetre wavelengths, is a powerful probe of cosmic magnetic fields through the detection of galaxies emitting linearly polarised synchrotron emission. The Faraday rotation measure (RM) of this emission can be used to study the properties of both the emitting source and the intervening magneto-ionic media (e.g. the Galactic interstellar medium). Large, dense samples of polarised extragalactic sources form RM grids that can be used to map the foreground magnetic field of the Milky Way. Over the past few years, the increasing sensitivity of modern interferometers has given new accessibility to the μJy radio sky, increasing the RM sky density. It is therefore key to establish a clear understanding of the faint polarised source population and the foreground effects that shape RM measurements. Broadband spectro-polarimetry provides an enhanced perspective on this, revealing Faraday complexities in the Stokes Q and U spectral behaviour, which trace turbulence or differential Faraday rotation along the line-of-sight. Through the projects presented in this thesis, I aim to address two important factors in improving our interpretation of RM grid experiments, (i) understanding both the observational biases and physical nature of Faraday complexity at low Galactic latitudes, (ii) characterising the extragalactic polarised source population.
Firstly, in analysing the SPASS/ATCA RM catalogue (Schnitzeler et. al., 2019), the most extensive broadband polarisation catalogue in the southern sky, we report a Galactic latitude dependence of Faraday complexity for polarised sources at |b| < 10°, with the degree of complexity increasing towards the Galactic plane. Through higher angular resolution (θ = 15′′) follow-up observations of 95 sources, we find that this trend is primarily driven by contamination from large-scale Galactic polarised emission in the SPASS/ATCA spectra, which we effectively filter out. We find 42% of the observed sources in our sample are Faraday complex, with an increased fraction of Faraday complex sources surrounding the spiral arm tangents and towards the Galactic centre. We constrain the scale of this complexity to < 2.4 pc, consistent with turbulent injection scales in the spiral arms. These results emphasise the importance of broadband spectro-polarimetric observations to fully characterise small-scale and/or turbulent structures in the Galactic magnetic field, and that it is essential to correctly filter contaminating polarised emission when interpreting foreground turbulence.
Secondly, we reprocess the VLA-COSMOS 3 GHz Large Project (Smolčić et. al., 2017), one of the deepest S-band continuum surveys (2.3 μJy beam−1), for polarisation calibration and imaging. Here, we present the deepest polarised source count at 3 GHz to date, and the second deepest overall, returning an RM density of 42 deg−2. We find that these source counts are consistent with those at the more typically-studied 1.4 GHz band, a combined effect of spectral index and depolarisation, which we attribute to differential Faraday rotation in the lobes of radio galaxies. Through the available multi-wavelength catalogues, we identify all polarised sources as radio galaxies (i.e. active galactic nuclei), and confirm that no star-forming galaxies are detected in polarisation. We place an upper limit on the density of polarised star-forming galaxies to be < 0.58 deg−2, implying that surveys much deeper than 2.6 μJy beam−1 will be required to readily probe this population, even at higher frequencies where Faraday depolarisation effects are less pronounced.
Finally, I present a demonstrator project with the recently installed MeerKAT S-band receivers, with a focus on extragalactic wide-field imaging. We present MeerKAT S-band observations of the DEEP2 field, the emptiest radio field in the southern sky. The total intensity source counts are consistent with those from the literature, and also show that only a fraction of integration time is required with MeerKAT for comparable results with legacy S-band surveys. Here, I also present the science goals and survey design for the MeerKAT+ S-band Legacy survey, a 3000 hour, full-Stokes survey of the southern sky at Dec ≤ −40°. This survey will be extremely versatile across Galactic, galaxy evolution, magnetism and transient science, and with an expected 105 polarised source detections, will provide a higher frequency perspective on the RM grid of the southern sky.},
url = {https://hdl.handle.net/20.500.11811/14194}
}
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-90494,
doi: https://doi.org/10.48565/bonndoc-877,
author = {{Shilpa Ranchod}},
title = {Characterising the broadband polarisation properties of extragalactic radio sources at 3 GHz},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = jun,
note = {Magnetic fields are ubiquitous in the Universe and are fundamentally linked to galaxy evolution, influencing star-formation, cosmic-ray propagation and feedback mechanisms. The radio frequency regime, at centimetre wavelengths, is a powerful probe of cosmic magnetic fields through the detection of galaxies emitting linearly polarised synchrotron emission. The Faraday rotation measure (RM) of this emission can be used to study the properties of both the emitting source and the intervening magneto-ionic media (e.g. the Galactic interstellar medium). Large, dense samples of polarised extragalactic sources form RM grids that can be used to map the foreground magnetic field of the Milky Way. Over the past few years, the increasing sensitivity of modern interferometers has given new accessibility to the μJy radio sky, increasing the RM sky density. It is therefore key to establish a clear understanding of the faint polarised source population and the foreground effects that shape RM measurements. Broadband spectro-polarimetry provides an enhanced perspective on this, revealing Faraday complexities in the Stokes Q and U spectral behaviour, which trace turbulence or differential Faraday rotation along the line-of-sight. Through the projects presented in this thesis, I aim to address two important factors in improving our interpretation of RM grid experiments, (i) understanding both the observational biases and physical nature of Faraday complexity at low Galactic latitudes, (ii) characterising the extragalactic polarised source population.
Firstly, in analysing the SPASS/ATCA RM catalogue (Schnitzeler et. al., 2019), the most extensive broadband polarisation catalogue in the southern sky, we report a Galactic latitude dependence of Faraday complexity for polarised sources at |b| < 10°, with the degree of complexity increasing towards the Galactic plane. Through higher angular resolution (θ = 15′′) follow-up observations of 95 sources, we find that this trend is primarily driven by contamination from large-scale Galactic polarised emission in the SPASS/ATCA spectra, which we effectively filter out. We find 42% of the observed sources in our sample are Faraday complex, with an increased fraction of Faraday complex sources surrounding the spiral arm tangents and towards the Galactic centre. We constrain the scale of this complexity to < 2.4 pc, consistent with turbulent injection scales in the spiral arms. These results emphasise the importance of broadband spectro-polarimetric observations to fully characterise small-scale and/or turbulent structures in the Galactic magnetic field, and that it is essential to correctly filter contaminating polarised emission when interpreting foreground turbulence.
Secondly, we reprocess the VLA-COSMOS 3 GHz Large Project (Smolčić et. al., 2017), one of the deepest S-band continuum surveys (2.3 μJy beam−1), for polarisation calibration and imaging. Here, we present the deepest polarised source count at 3 GHz to date, and the second deepest overall, returning an RM density of 42 deg−2. We find that these source counts are consistent with those at the more typically-studied 1.4 GHz band, a combined effect of spectral index and depolarisation, which we attribute to differential Faraday rotation in the lobes of radio galaxies. Through the available multi-wavelength catalogues, we identify all polarised sources as radio galaxies (i.e. active galactic nuclei), and confirm that no star-forming galaxies are detected in polarisation. We place an upper limit on the density of polarised star-forming galaxies to be < 0.58 deg−2, implying that surveys much deeper than 2.6 μJy beam−1 will be required to readily probe this population, even at higher frequencies where Faraday depolarisation effects are less pronounced.
Finally, I present a demonstrator project with the recently installed MeerKAT S-band receivers, with a focus on extragalactic wide-field imaging. We present MeerKAT S-band observations of the DEEP2 field, the emptiest radio field in the southern sky. The total intensity source counts are consistent with those from the literature, and also show that only a fraction of integration time is required with MeerKAT for comparable results with legacy S-band surveys. Here, I also present the science goals and survey design for the MeerKAT+ S-band Legacy survey, a 3000 hour, full-Stokes survey of the southern sky at Dec ≤ −40°. This survey will be extremely versatile across Galactic, galaxy evolution, magnetism and transient science, and with an expected 105 polarised source detections, will provide a higher frequency perspective on the RM grid of the southern sky.},
url = {https://hdl.handle.net/20.500.11811/14194}
}
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