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Protogalactic environments are typically identified using quasar absorptionlines, and these galactic building blocks can manifest as Damped Lyman-AlphaAbsorbers (DLAs) and Lyman Limit Systems (LLSs). We use radio observations ofFaraday effects to test whether DLAs and LLSs host a magnetised medium, bycombining DLA and LLS detections throughout the literature with 1.4 GHzpolarization data from the NRAO VLA Sky Survey (NVSS). We obtain a control, aDLA, and a LLS sample consisting of 114, 19, and 27 lines-of-sight respectively- all of which are polarized at $\ge8\sigma$ to ensure Rician bias isnegligible. Using a Bayesian framework, we are unable to detect either coherentor random magnetic fields in DLAs: the regular coherent magnetic fields withinthe DLAs must be $\le2.8$ $\mu$G, and the lack of depolarization is consistentwith the weakly magnetised gas in DLAs being non-turbulent and quiescent.However, we find mild suggestive evidence that LLSs have coherent magneticfields: after controlling for the redshift-distribution of our data, we find a71.5% probability that LLSs have a higher RM than a control sample. We alsofind strong evidence that LLSs host random magnetic fields, with a 95.5%probability that LLS lines-of-sight have lower polarized fractions than acontrol sample. The regular coherent magnetic fields within the LLSs must be$\le2.4$ $\mu$G, and the magnetised gas must be highly turbulent with a typicalscale on the order of $\approx5$-20 pc, which is similar to that of the MilkyWay. This is consistent with the standard dynamo pedagogy, whereby magneticfields in protogalaxies increase in coherence and strength as a function ofcosmic time. Our results are consistent with a hierarchical galaxy formationscenario, with the DLAs, LLSs, and strong magnesium II (MgII) systems exploringthree different stages of magnetic field evolution in galaxies.

astrophysical journal/the astrophysical journal

Observed Faraday Effects in Damped Lyman-Alpha Absorbers and Lyman Limit  Systems: The Magnetised Environment of Galactic Building Blocks at Redshift=2