Table of Links Abstract and 1 Introduction Abstract and 1 Introduction Faraday Rotation and Faraday Synthesis Dara & Instruments 3.1. CHIME and GMIMS surveys and 3.2. CHIME/GMIMS Low Band North 3.3. DRAO Synthesis Telescope Observations 3.4. Ancillary Data Sources Features of the Tadpole 4.1. Morphology in single-frequency images 4.2. Faraday depths 4.3. Faraday complexity 4.4. QU fitting 4.5. Artifacts The Origin of the Tadpole 5.1. Neutral Hydrogen Structure 5.2. Ionized Hydrogen Structure 5.3. Proper Motions of Candidate Stars 5.4. Faraday depth and electron column Summary and Future Prospects Faraday Rotation and Faraday Synthesis Faraday Rotation and Faraday Synthesis Faraday Rotation and Faraday Synthesis Dara & Instruments 3.1. CHIME and GMIMS surveys and 3.2. CHIME/GMIMS Low Band North 3.3. DRAO Synthesis Telescope Observations 3.4. Ancillary Data Sources Dara & Instruments 3.1. CHIME and GMIMS surveys and 3.2. CHIME/GMIMS Low Band North 3.1. CHIME and GMIMS surveys and 3.2. CHIME/GMIMS Low Band North 3.3. DRAO Synthesis Telescope Observations 3.3. DRAO Synthesis Telescope Observations 3.4. Ancillary Data Sources 3.4. Ancillary Data Sources Features of the Tadpole 4.1. Morphology in single-frequency images 4.2. Faraday depths 4.3. Faraday complexity 4.4. QU fitting 4.5. Artifacts Features of the Tadpole 4.1. Morphology in single-frequency images 4.1. Morphology in single-frequency images 4.2. Faraday depths 4.2. Faraday depths 4.3. Faraday complexity 4.3. Faraday complexity 4.4. QU fitting 4.4. QU fitting 4.5. Artifacts 4.5. Artifacts The Origin of the Tadpole 5.1. Neutral Hydrogen Structure 5.2. Ionized Hydrogen Structure 5.3. Proper Motions of Candidate Stars 5.4. Faraday depth and electron column The Origin of the Tadpole 5.1. Neutral Hydrogen Structure 5.1. Neutral Hydrogen Structure 5.2. Ionized Hydrogen Structure 5.2. Ionized Hydrogen Structure 5.3. Proper Motions of Candidate Stars 5.3. Proper Motions of Candidate Stars 5.4. Faraday depth and electron column 5.4. Faraday depth and electron column Summary and Future Prospects Summary and Future Prospects Summary and Future Prospects APPENDIX APPENDIX A. RESOLVED AND UNRESOLVED FARADAY COMPONENTS IN FARADAY SYNTHESIS A. RESOLVED AND UNRESOLVED FARADAY COMPONENTS IN FARADAY SYNTHESIS B. QU FITTING RESULTS B. QU FITTING RESULTS REFERENCES REFERENCES ABSTRACT 1. INTRODUCTION Investigations of the Galactic interstellar medium (ISM) have revealed the pervasive presence of magnetic fields and ionized gas (Ferri`ere 2001). Observations of radio polarization can probe various scales and phases of the ISM, revealing crucial information about the interplay of magnetic fields with other energy sources. Deriving the three-dimensional configuration of the magnetic field from polarization data can be challenging; nevertheless, recent observations have enabled progress towards a much clearer picture of the evolution of the ISM and the formation of clouds and stars (e.g. Tahani et al. 2022a,b). At wavelengths ∼ 1 − 100 cm, polarized radiation largely arises from synchrotron emission generated by cosmic ray electrons as they spiral around magnetic fields. Polarized radiation beyond the Earth’s atmosphere was first detected by Westerhout et al. (1962) and Wielebinski et al. (1962) and then extensively mapped by Brouw & Spoelstra (1976). Recent surveys of this polarized radiation have Nyquist-sampled wide areas of the sky in different frequency ranges (Wolleben et al. 2019, 2021; Carretti et al. 2019). Linearly polarized electromagnetic waves undergo Faraday rotation as they propagate through a magnetoionic medium. The resulting change in polarization angle informs us of electron density and magnetic field strength and direction. Exploiting this effect, polarized emission from extragalactic sources propagating through the Galaxy has been used to measure the twodimensional distribution of magnetic fields in the Milky Way and nearby galaxies averaged along the line of sight (Brown et al. 2003; Taylor et al. 2009; Mao et al. 2012a; Ordog et al. 2017; Tahani et al. 2018; Van Eck et al. 2021; Hutschenreuter et al. 2022; Thomson et al. 2023). The Faraday rotation of the emission of the Galaxy itself is an especially powerful probe of the diffuse ISM because the emitting cosmic rays and the Faraday rotating thermal gas are mixed, where these emission regions illuminate different Faraday rotating regions along the line of sight (e.g. Gaensler et al. 2001; Mao et al. 2012b; Van Eck et al. 2017). Faraday rotation probes the convolution of ionized gas density and the line-of-sight magnetic field; therefore, it is sensitive to two distinct components of the ISM. The majority of the ionized gas in the Milky Way ISM is found in the warm ionized medium (WIM), traced by Hα emission with an ionization fraction ≳ 90% based on observations of the [O I]λ6300 line (Hausen et al. 2002). However, particularly at low frequencies ≲ 1 GHz, Faraday rotation is sensitive to very small columns of free electrons and might trace a warm partially-ionized medium (WPIM) (Heiles & Haverkorn 2012) or the low (few percent; Wolfire et al. 2003; Jenkins 2013) ionization in the warm neutral medium (WNM) (Foster et al. 2013; Van Eck et al. 2017; Bracco et al. 2022). In fact, low-frequency Faraday rotation observations may not be sensitive to the traditional WIM (Haffner et al. 2009) at all because the electron density there is high enough to cause depolarization even with weak magnetic fields (Van Eck et al. 2017). The outline of this paper is as follows. In Section 2, we review Faraday rotation and Faraday synthesis. In Section 3, we describe the Canadian Hydrogen Intensity Mapping Experiment (CHIME) data (Section 3.2), the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope (ST) data (Section 3.3), and published data sets to which we compare the CHIME maps (Section 3.4). We present the observed features of the tadpole in Section 4 and discuss its origin in Section 5. We summarize the paper in Section 6. In Appendix A, we present simulations of the impact of marginally-resolved Faraday complexity on Faraday synthesis observations. In Appendix B, we present our QU fitting results. This paper is available on arxiv under CC BY 4.0 DEED license. This paper is available on arxiv under CC BY 4.0 DEED license. available on arxiv available on arxiv [1] We mostly adopt the terminology described in Table 1 of Sun et al. (2015): “Faraday depth”, “Faraday spectrum”, “Faraday synthesis” (Brentjens & de Bruyn 2005), “rotation measure spread function”, “Faraday clean” (Heald 2009), and “3D Faraday synthesis” (Bell & Enßlin 2012). Authors: (1) Nasser Mohammed, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (2) Anna Ordog, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (3) Rebecca A. Booth, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada; (4) Andrea Bracco, INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy and Laboratoire de Physique de l’Ecole Normale Superieure, ENS, Universit´e PSL, CNRS, Sorbonne Universite, Universite de Paris, F-75005 Paris, France; (5) Jo-Anne C. Brown, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada; (6) Ettore Carretti, INAF-Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy; (7) John M. Dickey, School of Natural Sciences, University of Tasmania, Hobart, Tas 7000 Australia; (8) Simon Foreman, Department of Physics, Arizona State University, Tempe, AZ 85287, USA; (9) Mark Halpern, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (10) Marijke Haverkorn, Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands; (11) Alex S. Hill, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (12) Gary Hinshaw, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (13) Joseph W. Kania, Department of Physics and Astronomy, West Virginia University, P.O. Box 6315, Morgantown, WV 26506, USA and Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA; (14) Roland Kothes, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (15) T.L. Landecker, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (16) Joshua MacEachern, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (17) Kiyoshi W. Masui, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA; (18) Aimee Menard, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (19) Ryan R. Ransom, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada and Department of Physics and Astronomy, Okanagan College, Kelowna, BC V1Y 4X8, Canada; (20) Wolfgang Reich, Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany;(21) Patricia Reich, 16 (22) J. Richard Shaw, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada (23) Seth R. Siegel, Perimeter Institute for Theoretical Physics, 31 Caroline Street N, Waterloo, ON N25 2YL, Canada, Department of Physics, McGill University, 3600 rue University, Montreal, QC H3A 2T8, Canada, and Trottier Space Institute, McGill University, 3550 rue University, Montreal, QC H3A 2A7, Canada; (24) Mehrnoosh Tahani, Banting and KIPAC Fellowships: Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA; (25) Alec J. M. Thomson, ATNF, CSIRO Space & Astronomy, Bentley, WA, Australia; (26) Tristan Pinsonneault-Marotte, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (27) Haochen Wang, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA; (28) Jennifer L. West, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (29) Maik Wolleben, Skaha Remote Sensing Ltd., 3165 Juniper Drive, Naramata, BC V0H 1N0, Canada. Authors: Authors: (1) Nasser Mohammed, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (2) Anna Ordog, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (3) Rebecca A. Booth, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada; (4) Andrea Bracco, INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy and Laboratoire de Physique de l’Ecole Normale Superieure, ENS, Universit´e PSL, CNRS, Sorbonne Universite, Universite de Paris, F-75005 Paris, France; (5) Jo-Anne C. Brown, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada; (6) Ettore Carretti, INAF-Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy; (7) John M. Dickey, School of Natural Sciences, University of Tasmania, Hobart, Tas 7000 Australia; (8) Simon Foreman, Department of Physics, Arizona State University, Tempe, AZ 85287, USA; (9) Mark Halpern, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (10) Marijke Haverkorn, Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands; (11) Alex S. Hill, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (12) Gary Hinshaw, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (13) Joseph W. Kania, Department of Physics and Astronomy, West Virginia University, P.O. Box 6315, Morgantown, WV 26506, USA and Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA; (14) Roland Kothes, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (15) T.L. Landecker, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (16) Joshua MacEachern, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (17) Kiyoshi W. Masui, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA; (18) Aimee Menard, Department of Computer Science, Math, Physics, & Statistics, University of British Columbia, Okanagan Campus, Kelowna, BC V1V 1V7, Canada and Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (19) Ryan R. Ransom, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada and Department of Physics and Astronomy, Okanagan College, Kelowna, BC V1Y 4X8, Canada; (20) Wolfgang Reich, Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany;(21) Patricia Reich, 16 (22) J. Richard Shaw, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada (23) Seth R. Siegel, Perimeter Institute for Theoretical Physics, 31 Caroline Street N, Waterloo, ON N25 2YL, Canada, Department of Physics, McGill University, 3600 rue University, Montreal, QC H3A 2T8, Canada, and Trottier Space Institute, McGill University, 3550 rue University, Montreal, QC H3A 2A7, Canada; (24) Mehrnoosh Tahani, Banting and KIPAC Fellowships: Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA; (25) Alec J. M. Thomson, ATNF, CSIRO Space & Astronomy, Bentley, WA, Australia; (26) Tristan Pinsonneault-Marotte, Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada; (27) Haochen Wang, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA; (28) Jennifer L. West, Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada; (29) Maik Wolleben, Skaha Remote Sensing Ltd., 3165 Juniper Drive, Naramata, BC V0H 1N0, Canada.
