Dust Battery: A Novel Mechanism for Seed Magnetic Field Generation in the Early Universe

Abstract: We propose a novel dust battery mechanism for generating seed magnetic fields in the early universe, in which charged dust grains are radiatively accelerated, inducing strong electric currents that subsequently generate magnetic fields. Our analysis demonstrates that this process is effective even at very low metallicities (approximately $ \sim 10^{-5} Z_\odot$), and capable of producing seed fields with significant amplitudes of $B \sim \rm \mu G$ around luminous sources over timescales of years to Myr and across spatial scales ranging from au to kpc. Crucially, we find that this mechanism is generically $\sim10^8$ times more effective than the radiatively-driven electron battery or Biermann battery in relatively cool gas ($\ll 10^{5}\,$K), including both neutral and ionized gas. Furthermore, our results suggest that, to first order, dissipation effects do not appear to significantly impede this process, and that it can feasibly generate coherent seed fields on macroscopically large ISM scales (much larger than turbulent dissipation scales or electron mean-free-paths in the ISM). These seed fields could then be amplified by subsequent dynamo actions to the observed magnetic fields in galaxies. Additionally, we propose a sub-grid model for integration into cosmological simulations, and the required electric-field expressions for magnetohydrodynamic-particle-in-a-cell (MHD-PIC) simulations that explicitly model dust dynamics. Finally, we explore the broad applicability of this mechanism across different scales and conditions, emphasizing its robustness compared to other known battery mechanisms.

• The paper introduces the dust battery mechanism that generates seed magnetic fields in the early Universe using radiatively accelerated charged dust grains.
• It demonstrates mathematically that the mechanism is eight orders of magnitude more effective than conventional processes like the Biermann battery in cool, partially neutral gas.
• The study suggests incorporating this robust mechanism into cosmological simulations to enhance models of galactic magnetic field evolution.

Dust Battery: A Novel Mechanism for Seed Magnetic Field Generation in the Early Universe

This paper investigates a novel mechanism for the generation of seed magnetic fields in the early Universe, termed the "dust battery." The authors, Soliman et al., propose that charged dust grains, when radiatively accelerated in the vicinity of luminous astrophysical sources, can induce potent electric currents, thereby creating magnetic fields. This process is proposed to be significantly more effective than traditional methods, such as the Biermann battery or radiatively-driven electron batteries, especially in cool and partially neutral gas environments.

Mechanism and Effectiveness

The dust battery mechanism leverages the differential acceleration of charged dust grains versus other plasma components to produce strong local electric fields. This differential motion generates a dust current that induces magnetic fields through charge separation. Notably, this process is effective even at very low metallicities (~10^-5 Z_⊙), creating seed fields with strengths up to microgauss. The authors demonstrate mathematically that this mechanism is approximately eight orders of magnitude more productive than competing processes in cool gas (<10^5 K).

Implications for Early Universe Magnetism

The mechanism posited by the authors suggests that magnetic fields are generated around stars, supernovae, and AGN with coherence scales ranging from astronomical units to kiloparsecs. Such magnetic fields can then be amplified by cosmic dynamos to levels observed in modern galaxies. This suggests that the early Universe could have been magnetized efficiently far earlier than previously considered, possibly accounting for the observed ubiquity of galactic magnetic fields.

Robustness Against Dissipation

Crucially, the authors find that dissipative effects do not significantly hinder the dust battery process. This robustness is attributed to the strong coupling between dust and partially neutral gas, which facilitates the maintenance of non-zero electric field gradients and hence ongoing magnetic field generation. They propose sub-grid models suitable for cosmological simulations that integrate this mechanism, potentially altering our understanding of galactic evolution and magnetic field amplification.

Comparison with Other Mechanisms

The paper places the dust battery mechanism in context with other established magnetic field generation theories. Notably, the authors compare it with the Biermann battery and kinetic instabilities like the Weibel instability. While useful under specific conditions, these mechanisms often rely on hot, highly ionized environments, limiting their efficacy in the predominantly neutral early Universe where seedlings are sparse. In contrast, the dust battery's ability to function in neutral, collisional environments at low metallicities makes it a compelling candidate for widespread early magnetic seed generation.

Potential for Future Research

The study opens several avenues for future inquiry. For one, incorporating the dust battery mechanism into large-scale cosmological simulations could offer new insights into the magnetic fields threading the cosmic web. Moreover, detailed observations and simulations that capture the interaction between radiation, dust, and magnetic fields in primordial conditions may help validate the mechanism's predictions.

In conclusion, this paper provides a detailed, quantitative analysis of a potentially significant mechanism for magnetic field generation in the early Universe. By demonstrating the dust battery's effectiveness under conditions prevalent shortly after cosmic dawn, the authors present a compelling argument for its role in galactic and cosmic magnetic field evolution. This work thus holds profound implications for astrophysics, advancing our understanding of the magnetic Universe from its nascent stages to the complex structures observed today.