Magnetising galaxies with cold inflows

Context. High-redshift (z ∼ 2 ‑ 3) galaxies accrete circumgalactic gas through cold streams. Recent high-resolution magnetohydrodynamic (MHD) simulations of these streams show a significant amplification of the intergalactic magnetic field in the shear layer around them. Aims. For this work we estimated the magnetisation of high-redshift galaxies that would result purely due to the accretion of already magnetised gas from cold streams. Methods. We used the mass inflow rates and saturated magnetic field values from cold stream simulations as input to a simple analytic model that calculates the galactic magnetic field purely from mass accretion. Results. Our model predicts average magnetic field strengths that exceed μG values at z ∼ 2 ‑ 3 for inflow rates above 0.1 M⊙ yr‑1. For high inflow rates, our model results are consistent with the recent detection of a strong magnetic field in z ≳ 2.6 galaxies. Conclusions. Within the assumptions of our simple model, magnetised cold streams emerge as a viable mechanism for seeding a dynamically important galactic magnetic field