The impact of magnetic fields on cold streams feeding galaxies

Abstract

ABSTRACT High-redshift, massive haloes are observed to have sustained high star formation rates, which require that the amount of cold gas in the halo is continuously replenished. The cooling time-scale for the hot virialized halo gas is too long to provide the source of cold gas. Supersonic, cold streams have been invoked as a mechanism for feeding massive haloes at high redshift and delivering the cold gas required for continued star formation at the rates observed. This mechanism for replenishing the cold gas reservoir is motivated by some cosmological simulations. However, the cold streams are likely to be subject to the supersonic version of the Kelvin–Helmholtz instability (KHI), which eventually leads to stream disruption. Cosmological simulations have yet to obtain the spatial resolution required for understanding the detailed stability properties of cold streams. In this paper, we consider instead an idealized model of magnetized cold streams that we spatially resolve. Using linear theory, we show how magnetic fields with dynamically important field strengths do not inhibit the KHI but rather enhance its growth rate. We perform non-linear simulations of magnetized stream disruption and find that magnetic fields can nevertheless increase stream survival times by suppressing the mixing rate of cold gas with the circumgalactic medium. We find that magnetic fields can allow streams to survive ∼2–8 times longer and, consequently, that streams ∼2–8 times thinner can reach the central galaxy if the magnetic field strength is $\sim 0.3\rm {-}0.8 \, \mu$G.