The redshift evolution of galactic-scale magnetic fields

Abstract

Magnetic fields play an important role in galaxy evolution, however, the redshift evolution of galactic-scale magnetic fields is not well constrained, with only a handful of direct magnetic field strength measurements in distant galaxies. In my thesis, I investigate this evolution using both radio polarimetric observations and synthetic observations made with the IllustrisTNG50 simulation.<br />In the first half of my thesis I present the analysis of broadband (1 &ndash; 8 GHz) spectro-polarimetric Very Large Array observations of two lensing systems (B1600+434 and B0218+357), and the derived magnetic field strength (2 &ndash; 1.8 &mu;G and 1.2 &ndash; 20 &mu;G) of the lensing galaxies at z = 0.414 and z = 0.685. While one of our systems probe to higher redshift than before, our other system allows us to measure the magnetic field of the galaxy&rsquo;s halo. Our results are in agreement with the magnetic field strength and structure of nearby galaxies, and compatible with the dynamo theory: the e-folding time of the large-scale dynamo we derived is consistent with the theoretical value (&tau;_dynamo = 2 x 10⁸ yr). We find that the dynamo has likely already built up a regular field in galaxies at z &sim; 0.7.<br />In the second half of my thesis I compute how the observables of magnetic fields evolve over redshift using 16 500 galaxies at redshifts of 0 &le; z &le; 2, with stellar masses in the range 9 &le; log(M_*/M_&odot;) &le; 12, from the state-of-the-art cosmological magneto-hydrodynamic simulation IllustrisTNG50. We explore two methods: deriving the magnetic field strength of intervening galaxies in front of polarized background quasars and deriving the magnetic field strength of the intergalactic medium (IGM) by utilizing fast radio bursts (FRB). I estimated the rotation measure (RM) contribution of FRB host galaxies and how it changes with redshift, galaxy type, the stellar mass of the galaxies, galaxy inclination, and on an FRB&rsquo;s offset from the center of the galaxy. With these predictions we can isolate the RM of the IGM from the observed RM of FRBs, and derive its magnetic field strength. To constrain an &sigma;_RM,IGM of 2 rad m^&minus;2 with 95% confidence level we need to observe 95 000 FRBs at z = 0.5, while at z = 2 this number is significantly lower: 9 500 FRBs. I also present the calculated RM of the intervening galaxies and how it changes with redshift. The derived probability density functions can be compared to those of observed samples, in order to calculate the magnetic field strength of the intervening galaxies, and if they are more likely to have a large-scale regular field or a random field.<br />Once the Square Kilometer Array starts operating in 2029, we expect the number of both lensing systems and quasars with intervening galaxies to dramatically increase. Our results and way of analysis of the lensing systems demonstrate how this method can be applied to different galaxies. The number of observed polarized FRBs is also steadily increasing, and we would expect to measure the magnetic field strength of the IGM with a 2 rad m^&minus;2 precision in under 10 years.