Polarization in the ELAIS-N1 LOFAR Deep Field
Doctoral thesis, 2024
The physical characteristics of cosmic magnetic fields are encoded in the polarization properties of extragalactic radio sources. Linearly polarized radiation undergoes Faraday rotation as it crosses a magneto-ionic medium, and the rotation of the polarization angle of the signal is equal to its wavelength squared times the Rotation Measure (RM). The RM is in turn proportional to the line-of-sight integral of the product of the magnetic field component along the line of sight and the density of thermal electrons.
Mapping the values of the RM across the sky provides a means to constrain cosmic magnetic fields. For this purpose, statistical studies of the properties of polarized radio sources spread over cosmological distances are essential.
Low radio-frequency observations provide better precision on the inferred RM values than higher-frequency ones. On the other hand, low radio-frequency observations are more affected by depolarization, which affects the detection rate of polarized sources. The population of faint polarized extragalactic sources at low radio-frequency is still mostly unknown. In this context, the LOw Frequency ARray (LOFAR) plays an important role because of the sensitivity, angular resolution and precision on the inferred RM values that can be achieved through low-frequency broad-band polarimetry, allowing us to study the polarized radio emission at around 150~MHz in unprecedented detail.
In my work I have developed a new method to combine polarimetric observations made with slightly different frequency configurations and have then applied this method to LOFAR 115-177 MHz observations of the European Large Area ISO Survey-North 1 (ELAIS-N1) deep field, one of the deepest LOFAR Two-Metre Sky Survey (LoTSS) deep fields imaged so far. Within this field an area of 25 square degrees was imaged at 6'' of resolution. By combining 19 datasets each of eight-hour-long duration, I detected 33 polarized components, which corresponds to an average number density of 1.3 polarized component per square degree. This represents the deepest and highest-resolution polarization study at 150 MHz to date.
I compared the results with other RM catalogs, quantified the depolarization properties of sources detected also at 1.4 GHz, and modeled the source counts in polarization from the counts in total flux density. The detected polarized sources were fully characterized and analyzed, and the extragalactic environment was investigated.
This work addresses both the technical and theoretical challenges in observing and interpreting low-frequency polarimetric data. It marks a significant step in solving some of the complex issues modern radio astronomy faces, especially those related to processing the large amounts of data generated by new-generation radio interferometers.
Mapping the values of the RM across the sky provides a means to constrain cosmic magnetic fields. For this purpose, statistical studies of the properties of polarized radio sources spread over cosmological distances are essential.
Low radio-frequency observations provide better precision on the inferred RM values than higher-frequency ones. On the other hand, low radio-frequency observations are more affected by depolarization, which affects the detection rate of polarized sources. The population of faint polarized extragalactic sources at low radio-frequency is still mostly unknown. In this context, the LOw Frequency ARray (LOFAR) plays an important role because of the sensitivity, angular resolution and precision on the inferred RM values that can be achieved through low-frequency broad-band polarimetry, allowing us to study the polarized radio emission at around 150~MHz in unprecedented detail.
In my work I have developed a new method to combine polarimetric observations made with slightly different frequency configurations and have then applied this method to LOFAR 115-177 MHz observations of the European Large Area ISO Survey-North 1 (ELAIS-N1) deep field, one of the deepest LOFAR Two-Metre Sky Survey (LoTSS) deep fields imaged so far. Within this field an area of 25 square degrees was imaged at 6'' of resolution. By combining 19 datasets each of eight-hour-long duration, I detected 33 polarized components, which corresponds to an average number density of 1.3 polarized component per square degree. This represents the deepest and highest-resolution polarization study at 150 MHz to date.
I compared the results with other RM catalogs, quantified the depolarization properties of sources detected also at 1.4 GHz, and modeled the source counts in polarization from the counts in total flux density. The detected polarized sources were fully characterized and analyzed, and the extragalactic environment was investigated.
This work addresses both the technical and theoretical challenges in observing and interpreting low-frequency polarimetric data. It marks a significant step in solving some of the complex issues modern radio astronomy faces, especially those related to processing the large amounts of data generated by new-generation radio interferometers.
physical data and processes: magnetic fields, physical data and polarization
radio continuum: galaxies
techniques: polarimetric
methods: numerical, observational
galaxies: individual (ELAIS-N1)
Polarization
lecture hall EB, 4th floor, Hörsalsvägen 11
Opponent: Lawrence Rudnick, Minnesota Institute for Astrophysics, Minneapolis, Minnesota, United States
Author
Sara Piras
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
S. Piras, C. Horellou, J. E. Conway, M. Thomasson, S. del Palacio, T. W. Shimwell, S. P. O’Sullivan, E. Carretti, V. Vacca, A. Bonafede, and I. Prandoni, "LOFAR Deep Fields: Probing the sub-mJy regime of polarized extragalactic sources in ELAIS-N1. II. Analysis"
Magnetic fields permeate the Universe across all scales and cosmic epochs. These fields intertwine with cosmic web filaments, influencing the Large Scale Structure of the Universe. Despite their ubiquity and important role in astrophysical processes, our knowledge and understanding of such large-scale magnetic field properties and origins are still limited.
The physical characteristics of cosmic magnetic fields are encoded in the polarization properties of extragalactic radio sources. Deep radio observations at low radio frequencies with the LOw Frequency ARray (LOFAR) are a key resource for probing extragalactic sources and cosmic magnetism across a variety of environments and redshifts.
In this work, we have developed a new method to combine LOFAR polarimetric observations of the European Large Area ISO Survey-North 1 (ELAIS-N1) deep field, covering an area of 25 square degrees at a resolution of 6''. This field is one of the deepest LOFAR Two-Metre Sky Survey (LoTSS) deep fields imaged so far. Our work represents the deepest and highest-resolution polarization study at 150 MHz to date, confidently detecting polarization in 31 radio galaxies that exhibit a variety of morphologies and polarization properties.
This work addresses both the technical and theoretical challenges in observing and interpreting low-frequency polarimetric data. It marks a significant step in solving some of the complex issues that modern radio astronomy faces, particularly those related to processing the large amounts of data generated by new-generation radio interferometers.
The physical characteristics of cosmic magnetic fields are encoded in the polarization properties of extragalactic radio sources. Deep radio observations at low radio frequencies with the LOw Frequency ARray (LOFAR) are a key resource for probing extragalactic sources and cosmic magnetism across a variety of environments and redshifts.
In this work, we have developed a new method to combine LOFAR polarimetric observations of the European Large Area ISO Survey-North 1 (ELAIS-N1) deep field, covering an area of 25 square degrees at a resolution of 6''. This field is one of the deepest LOFAR Two-Metre Sky Survey (LoTSS) deep fields imaged so far. Our work represents the deepest and highest-resolution polarization study at 150 MHz to date, confidently detecting polarization in 31 radio galaxies that exhibit a variety of morphologies and polarization properties.
This work addresses both the technical and theoretical challenges in observing and interpreting low-frequency polarimetric data. It marks a significant step in solving some of the complex issues that modern radio astronomy faces, particularly those related to processing the large amounts of data generated by new-generation radio interferometers.
Onsala space observatory infrastructure
Swedish Research Council (VR) (2017-00648), 2018-01-01 -- 2021-12-31.
Subject Categories
Physical Sciences
Roots
Basic sciences
Infrastructure
C3SE (Chalmers Centre for Computational Science and Engineering)
Onsala Space Observatory
ISBN
978-91-8103-129-4
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5587
Publisher
Chalmers
lecture hall EB, 4th floor, Hörsalsvägen 11
Opponent: Lawrence Rudnick, Minnesota Institute for Astrophysics, Minneapolis, Minnesota, United States