Faraday tomography of the interstellar medium at low radio frequencies

Turić, Luka (2023) Faraday tomography of the interstellar medium at low radio frequencies. Doctoral thesis, Sveučilište u Zagrebu, Prirodoslovno-matematički fakultet.

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Abstract

The direct way to address cosmological questions regarding Cosmic Dawn (CD) and the Epoch of Reionization (EoR) is by observing the neutral hydrogen line at a wavelength of 21 cm (1420 MHz), which allows us to investigate the evolution of neutral hydrogen (HI) throughout the history of the universe (Mesinger, 2019). Today, this cosmological signal is observed at frequencies ranging from 30 to 200 MHz using various radio interferometers such as LOFAR (LOw Frequency ARray, van Haarlem et al., 2013), and soon, SKA (Square Kilometer Array, Koopmans et al., 2015). Detecting the cosmological signal is not straightforward due to foreground radiation at low radio frequencies, which, in terms of fluctuations, is orders of magnitude stronger than the cosmological signal itself. The dominant foreground radiation comes from our Galaxy as synchrotron radiation, produced by the spiralling motion of ultrarelativistic charged particles, mainly electrons, along the magnetic field lines. It dominates at frequencies below 10 GHz (Pacholczyk, 1970; Rybicki & Lightman, 1986). It is intrinsically linearly polarized, with a polarization degree of about 70% (Le Roux, 1961). The intensity of synchrotron radiation depends on a density of ultrarelativistic cosmic electrons, the strength of the magnetic field component perpendicular to the line of sight, and the exponent in the energy distribution of ultrarelativistic cosmic electrons (Mesinger, 2019). The spectral index of the radiation is different at lower and higher frequencies due to the ageing of the energy spectrum of cosmic electrons. As cosmic electrons traverse the interstellar medium (ISM), they lose energy through interactions with interstellar matter, magnetic fields, and radiation. The energy loss through synchrotron radiation is more significant for higher-energy particles because the radiation power is proportional to the square of the electron’s kinetic energy. In addition to variations in the spectral index across the entire sky, variations in the brightness temperature (intensity defined by the Rayleigh-Jeans law) of Galactic synchrotron radiation reflect spatial fluctuations in the density of cosmic electrons and the strength of the magnetic field in the ISM.

Item Type:	Thesis (Doctoral thesis)
Uncontrolled Keywords:	interstellar medium; neutral hydrogen; dust; magnetic field; synchrotron radiation; Faraday rotation; RM synthesis; depolarization canals; RHT; polarization horizon
Subjects:	NATURAL SCIENCES > Physics > Astronomy and Astrophysics
Divisions:	Division of Experimental Physics
Projects:	Project title Project leader Project code Project type Razumijevanje zračenja na niskim radio frekvencijama: ususret novoj generaciji radio teleskopa-LowFreqCRO Vibor Jelić; Vernesa Smolčić; Željka Marija Bošnjak; Bruno Šlaus IP-2018-01-2889 HRZZ
Depositing User:	Vibor Jelić
Date Deposited:	21 Feb 2024 09:01
URI:	http://fulir.irb.hr/id/eprint/8609

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