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One of the main reasons that the debate over the nature of peaked-spectrum sources has not been resolved is because the absorption mechanism responsible for the spectral peak still remains ambiguous. By absorption mechanism, we imply a physical process that causes a medium to be optically transparent at certain radio frequencies and optically opaque (optically thick) at other frequencies. The transition between the optically thin and optically thick regimes in a radio spectrum is referred to as the turnover.

Synchrotron self-absorption (SSA) by the relativistic electrons internal to the emitting source is one commonly proposed model for the turnover in peaked-spectrum sources (Williams,1963;Shklovsky,1965;Kellermann,1966). The turnover due to SSA occurs because the source cannot have a brightness temperature that exceeds the plasma tem- perature of the non-thermal electrons. Alternatively, the frequency at which the turnover occurs in SSA theory can be thought of as the frequency at which relativistic electrons and emitted synchrotron photons have a high chance of scattering (Tingay & de Kool,2003). SSA theory is very successful in explaining the relationship between the source size and turnover frequency (O’Dea,1998), and produces magnetic field strengths consistent with equipartition (Orienti & Dallacasa,2008). Hence, SSA is expected in young sources. SSA theory also makes a firm prediction of a slope of 2.5 below the turnover, independent of the morphology of the emitting medium (Williams,1963;Shklovsky,1965). However, SSA theory can not explain why the brightness temperature is far too low to absorb the radio emission from the counter-jet structures in such sources as PKS 1607+26 and 3C 84 (Shaffer, Kellermann & Cornwell,1999;Walker et al.,2000). Additionally, the emission measure of ionised gas implied by the narrow line emission from CSS sources appears inconsistent with SSA theory (van Breugel, Miley & Heckman,1984).

Free-free absorption (FFA) via an external ionised screen of dense plasma is another commonly proposed absorption mechanism to explain the turnover in the spectrum of peaked-spectrum sources (Kellermann,1966). FFA occurs when a radio photon is absorbed by a free electron that is in the presence of a positive ion (Longair,2011). While FFA from an external homogeneous medium cannot reproduce the relation between linear size and turnover frequencies of peaked-spectrum sources (Tingay & de Kool,2003), variations of FFA theory can. For example, one of the most successful FFA theories is that proposed by

Bicknell, Dopita & O’Dea(1997). In this model, the jets of a source are interacting with an inhomogeneous nuclear medium, with the spectral turnover directly related to the optical depth distribution of the absorbing media. As the jets propagate into the inhomogeneous medium, the radiative bow shock conditionally excites the interstellar medium (ISM). If the ISM clouds are dense enough, FFA can occur. This is highlighted in Figure1.9. The inhomogeneous FFA model ofBicknell, Dopita & O’Dea(1997) is able to recreate the observed correlation between the turnover frequencies and linear sizes, and many spectral studies of individual sources have indicated it is the dominant absorption mechanism (e.g.

Peck, Taylor & Conway,1999;Kameno et al.,2000; Marr, Taylor & Crawford,2001;

Tingay & de Kool,2003;Orienti & Dallacasa,2008;Marr et al.,2014;Tingay et al.,2015;

24 kyr log( n/ cm 3) 49 kyr 73 kyr

98 kyr 122 kyr 146 kyr

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Figure 1.9: Example of the change in densityn as the jet of a radio source, shown in blue, propagates into a dense, inhomogeneous medium, shown in red and orange. The elapsed time since the AGN activity was initiated is indicated in the top right-hand corner of each frame. Figure reproduced fromWagner & Bicknell(2011).

Additional absorption mechanisms, such as the Razin effect (Razin,1957;Kellermann,

1966) and induced Compton scattering (Kuncic, Bicknell & Dopita,1998), have also been applied to explain the turnover of peaked-spectrum sources. However, both of these models have failed to produce the aforementioned scaling relations (O’Dea, 1998). Note that analytic treatment of both SSA and FFA theories is provided in Section2.4of Chapter2.

One of the reasons the degeneracy between SSA and FFA models has remained unresolved is because past investigations lacked comprehensive spectral coverage below the turnover, where the distinction between the different absorption models becomes pronounced. This is especially pertinent for the large peaked-spectrum samples since such catalogues can only be assembled from wide-field surveys, which usually only surveyed the sky at one frequency. Provided the spectra are well sampled below the turnover, it is possible to differentiate between FFA and SSA models through comprehensive statistical fitting of peaked-spectrum spectra (Tingay & de Kool,2003;Callingham et al.,2015). A simplified example of the model differentiation made possible by low radio frequency data is presented in Figure1.10. Therefore, with such telescopes as the MWA and LOFAR now operational, astronomers have unprecedented frequency coverage below the turnover of peaked-spectrum sources, allowing for the most comprehensive spectral comparison of the

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Figure 1.10: A simplified example of the spectra of a source when the turnover is a product of internal FFA or homogeneous SSA, shown in red and purple, respectively. The extremities of the frequency coverage of the MWA are indicated by the dashed black lines, highlighting how useful MWA data is in differentiating between competing absorption theories for sources that have spectral peak frequencies_{& 230 MHz.}

different absorption models on larger data sets then ever before possible.

We also highlight several other methods outside spectral modelling that can be used to differentiate between FFA and SSA in peaked-spectrum sources. For example, being able to measure the circular polarisation above and below the spectral turnover of peaked- spectrum sources provides one of the most unambiguous methods to identify whether SSA or FFA is the dominant absorption mechanism (Melrose,1971). This is because SSA predicts the degree of circular polarisation reverses its sign on either side of the spectral turnover (Jones & Odell,1977). However, measuring the circular polarisation of peaked- spectrum sources is difficult due to low integrated polarisation and high rotation measures of the population (Rudnick & Jones,1982). All previous circular polarisation studies of peaked-spectrum sources have lacked observations significantly below the turnover for an unambiguous detection of the sign reversal (e.g.Rayner, Norris & Sault,2000;Cotton et al.,2003).

Additionally, it is also possible to differentiate between FFA and SSA from the bright- ness temperature of different features in a peaked-spectrum source. For example, FFA been

Figure 1.11: A photo of a 4_{×4 bow-tie dipole MWA tile that is located in the core} of the MWA. The white box offset from the wired mesh groundscreen is the analogue beamformer.

suggested to exist in 3C 84 (Walker, Romney & Benson,1994), because the counter-jet in 3C 84 is significantly fainter than the forward jet. However, this method requires sensitive, sub-arcsecond resolution to be performed below the turnover frequency of a peaked- spectrum source, greatly limiting its applicability. Therefore, the new low frequency, large fractional bandwidth era ushered in by the MWA and LOFAR represents a significant steep forward in identifying the dominant absorption mechanism in the peaked-spectrum source population.