The evolution of cosmic ray electrons in the cosmic web: Seeding by active galactic nuclei, star formation, and shocks

¹ Dipartimento di Fisica e Astronomia, Universitá di Bologna, Via Gobetti 93/2, 40122 Bologna, Italy
² INAF-Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
³ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia Str. 8, Lviv 79005, Ukraine
⁵ School of Natural Sciences and Medicine, Ilia State University, 3-5 Cholokashvili St., 0194 Tbilisi, Georgia

^⋆ Corresponding author; franco.vazza2@unibo.it

Received: 29 July 2024
Accepted: 6 February 2025

Abstract

A number of processes in the Universe are known to convert a fraction of gas kinetic energy into the acceleration of relativistic electrons, making them observable at radio wavelengths or contributing to a dormant reservoir of low-energy cosmic rays in cosmic structures. We present a new suite of cosmological simulations, with simple galaxy formation models calibrated to work at a specific spatial resolution. This simulations have been tailored to support studies of all the most important processes of injection of relativistic electrons in evolving large-sale structures: accretion and merger shocks, feedback from active galactic nuclei (AGNs), and winds from star-forming regions. We also followed the injection of magnetic fields by AGNs and star formation and computed the observational signatures of these mechanisms. We find that the injection of cosmic ray electrons by shocks is the most optimal volume-filling process and that it also dominates the energy density of fossil relativistic electrons in halos. The combination of the seeding mechanisms studied in this work, regardless of the uncertainties related to physical or numerical uncertainties, is more than enough to fuel large-scale radio emissions with a large amount of seed fossil electrons. We derived an approximated formula to predict the number of fossil cosmic ray electrons injected by z = 0 by the total activity of shocks and AGNs, as well as star formation in the volume of halos. By looking at the maximum possible contribution to the magnetisation of the cosmic web by all our simulated sources, we conclude that galaxy formation-related processes alone cannot explain the values of Faraday rotation for background-polarised sources recently detected using LOFAR.