Probing Intracluster Gas with Faraday Rotation from Black Hole Jets

Abstract

This thesis presents an analysis of the effectiveness of Faraday Rotation and its associated measures in helping to constrain the properties of observed AGN radio lobes and their surrounding environment. A study of Faraday Rotation through the Rotation Measure (RM) distribution allows for a line-of-sight computation of both the thermal electron number density ($n_e$) and magnetic field ($B$) distributions in the intracluster medium (ICM), and so the RM, in conjunction with other common radio observables, may be used to infer such parameters as the cluster mass or source orientation with respect to the plane of the sky. The role of environment on radio lobe properties is first investigated with a sample of 16 Fanaroff-Riley Type II (FR-II) sources, 6 FR-I sources, and one hybrid morphology radio source (HyMoRS) from the Radio Galaxy Zoo (RGZ) citizen science project. These radio lobes have total linear sizes exceeding $\sim 100$~kpc and redshifts of $z<0.3$. FR-I sources appear to be hosted by more massive galaxies, consistent with previous studies. For the FR-II sample, the degree of asymmetry in radio lobe properties of length and luminosity are compared to asymmetry in the surrounding environment, as quantified through optical galaxy clustering. Radio lobe length is found to be anti-correlated with both galaxy clustering and lobe luminosity for FR-IIs, which is in quantitative agreement with predictions from radio source dynamical models, and suggest that galaxy clustering provides a useful proxy for the ambient gas density distribution encountered by the radio lobes. These findings are used to inform theoretical models which are constructed for both n e and B within clusters of varying mass and for radio sources of varying orientation, placed at $z<0.05$ from which the RM distribution was calculated. It is found that an underlying power-law density distribution leads to significant asymmetry in the magnitude of the RM distribution, whilst B controls the variance in RM along the projected jet axis. Together these two results show that, for a radio source pointing out at some angle to the plane of the sky, the far lobe will preferentially exhibit greater variance and magnitude in RM, and will consequently show greater depolarisation. This result is in broad agreement with those expected by the Laing-Garrington effect, in which linearly polarised light travelling through a greater path length to the observer is incident upon more of the intervening depolarising, magnetised medium. These results are characterised further through the inclusion of the dynamical radio source model Radio AGN in Semi-analytic Environments (RAiSE), which provided more physically-realistic radio source parameters up to an age of 1 Gyr. These models are combined for those of the density and magnetic field distributions, and the resultant RM distribution is once again calculated alongside observables for the total source length D and total source luminosity $L_{\nu }$ for frequency $\nu$. The possible signatures of the Laing-Garrington effect are characterised by two new observables: the standard deviation of the RM (${\sigma }_{\mathrm{R}\mathrm{M}}$), and the change in the variance of the RM (the heteroscedasticity, $H$). A series of random test models are produced and are then matched to the nearest model within the known population based on a ${\chi }^2$ parameter estimation on $L_{\nu }$ and $D$, which is then repeated for all of $L_{\nu }$, $D$, ${\sigma }_{\mathrm{R}\mathrm{M}}$, and $H$. Test sources appear to match more precisely to the nearest known model when the additional RM metrics are included in the ${\chi }^2$-test. The confidence on the fit around some small tolerance for the source orientation $\theta$ is increased by a factor of $1.38\times {10}^4$, whilst that of the cluster dark matter halo mass $M_{200}$ is increased by a factor of $6.16\pm 0.07$. The source age $t_{\mathrm{a}\mathrm{g}\mathrm{e}}$ and jet power $Q$ see increases of factors of $1.48\pm 0.10$ and $1.01\pm 0.06$ respectively. In all cases, the increase in confidence on the match is accompanied by a greater difference between the test and model value of that parameter, although this is expected due to sparse sampling of the parameter spaces within 0.25 dex for $Q$ and $M_{200}$, and 0.5 dex for $t_{\mathrm{a}\mathrm{g}\mathrm{e}}$. Assuming that one of $\theta$ or $M_{200}$ is known \textit{a priori} tends to further improve the model’s ability to match random test cases to those already known. Overall, the inclusion of RM metrics improves constraints on radio source and galaxy cluster properties for the models investigated within this thesis, making the Faraday Rotation from black hole jets a promising method by which to measure intracluster gas.