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Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action—II. Effects of Turbulence Driving in the ICM and Beam Smoothing
oleh: Aritra Basu, Sharanya Sur
| Format: | Article |
|---|---|
| Diterbitkan: | MDPI AG 2021-09-01 |
Deskripsi
Polarized synchrotron emission from the radio halos of diffuse intracluster medium (ICM) in galaxy clusters are yet to be observed. To investigate the expected polarization in the ICM, we use high resolution (1 kpc) magnetohydrodynamic simulations of fluctuation dynamos, which produces intermittent magnetic field structures, for varying scales of turbulent driving (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub></semantics></math></inline-formula>) to generate synthetic observations of the polarized emission. We focus on how the inferred diffuse polarized emission for different <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub></semantics></math></inline-formula> is affected due to smoothing by a finite telescope resolution. The mean fractional polarization <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow></semantics></math></inline-formula> vary as <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow><mo>∝</mo><msubsup><mi>l</mi><mrow><mi mathvariant="normal">f</mi></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msubsup></mrow></semantics></math></inline-formula> with <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo><mo>></mo><mn>20</mn><mo>%</mo></mrow></semantics></math></inline-formula> for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub><mo>≳</mo><mn>60</mn></mrow></semantics></math></inline-formula> kpc, at frequencies <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ν</mi><mo>></mo><mn>4</mn><mspace width="0.166667em"></mspace><mi>GHz</mi></mrow></semantics></math></inline-formula>. Faraday depolarization at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ν</mi><mo><</mo><mn>3</mn></mrow></semantics></math></inline-formula> GHz leads to deviation from this relation, and in combination with beam depolarization, filamentary polarized structures are completely erased, reducing <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow></semantics></math></inline-formula> to below 5% level at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ν</mi><mo>≲</mo><mn>1</mn></mrow></semantics></math></inline-formula> GHz. Smoothing on scales up to 30 kpc reduces <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow></semantics></math></inline-formula> above 4 GHz by at most a factor of 2 compared to that expected at 1 kpc resolution of the simulations, especially for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub><mo>≳</mo><mn>100</mn></mrow></semantics></math></inline-formula> kpc, while at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ν</mi><mo><</mo><mn>3</mn></mrow></semantics></math></inline-formula> GHz, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow></semantics></math></inline-formula> is reduced by a factor of more than 5 for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub><mo>≳</mo><mn>100</mn></mrow></semantics></math></inline-formula> kpc, and by more than 10 for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>l</mi><mi mathvariant="normal">f</mi></msub><mo>≲</mo><mn>100</mn></mrow></semantics></math></inline-formula> kpc. Our results suggest that observational estimates of, or constrain on, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⟨</mo><mi>p</mi><mo>⟩</mo></mrow></semantics></math></inline-formula> at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ν</mi><mo>≳</mo><mn>4</mn></mrow></semantics></math></inline-formula> GHz could be used as an indicator of the turbulent driving scale in the ICM.