Galactic magnetic fields seeded by ultralight dark photons

TL;DR

Galactic magnetic fields have an origin that remains uncertain across kiloparsec to megaparsec scales. The authors propose ultralight dark photons with mass $m_{\gamma'} \lesssim 10^{-21}$ eV, kinetically mixed with the SM photon via $\epsilon$, as a source of seed magnetic fields generated during proto-galaxy virialization through Lorentz-force-driven charge separation, with the misalignment mechanism providing a coherent dark electromagnetic field. They derive the dark-field amplitudes $E'$ and $B'$ (with $B'$ scaling as $\mathbf{v}E_0$ and a coherence length $\ell_c = \frac{2\pi}{m_{\gamma'}v}$) and show that the seed field strength can be substantial before plasma screening, then compute the observable field after screening, $B$, which is typically around $10^{-21}$ G for representative parameters due to $\Omega_p$ suppression. The seed fields can be amplified by a large-scale galactic dynamo with growth rate $\Gamma$ up to the orbital frequency $\Omega$, potentially rising to the observed $\mu$G levels over ~10 Gyr; the results depend on the ULDP fractional density $f$, the free-electron fraction $X_e$, and $\epsilon$. This work links ULDP dark matter to galactic magnetism and highlights the need for dynamo simulations and future observational probes (e.g., SKA) to test the viability of this seed mechanism.

Abstract

In this work, we show that ultralight dark photons, which couple to the Standard Model photon through kinetic mixing, can potentially source galactic scale magnetic fields. Although these magnetic fields would be too weak to detect at present in galaxies due to plasma screening effects, we show that dark photons can provide the seed magnetic field strength ($10^{-20}$ G) required for dynamo amplification in galaxies. Such dynamo-amplified magnetic fields are consistent with observations of $μ$G strength galactic magnetic fields.

Figures (1)

• Figure 1: Magnetic field amplitudes (red) from ULDPs assuming a $f=0.01$ fraction of DM are in ULDPs, with a kinetic mixing of $\epsilon=0.1$. We have assumed two different free electron fractions of $X_e = 10^{-4}$ for the solid line and $X_e = 10^{-6}$ for the dashed line, and taken halo parameters that match a $10^8~M_\odot$ halo at redshift $1+z=20$. Field values above the faded dotted line indicate ideal values which could be amplified via the dynamo effect into the observed galactic magnetic fields. Also plotted are the associated coherence lengths (blue) for the ULDPs. We have indicated a rough "seed field," sufficiently large to be amplified to presently observed galactic magnetic field strengths, given the results of simulations in Ref. Schober2012.