ALMA2040 White Paper Targets Ordered Magnetic Fields at z up to 5.6

The white paper synthesizes recent ALMA detections of polarized dust emission showing kiloparsec-scale ordered fields already present by z~5.6. It outlines four drivers: constraining galactic dynamos, measuring early field strengths, linking fields to galaxy evolution, and probing dust grain alignment via polarized SEDs. Requirements include extended high-frequency polarization and sub-arcsecond resolution to resolve field ordering in typical high-z disks rather than only the brightest mergers.

Current evidence rests on a handful of luminous targets; statistical samples remain absent because sensitivity limits prevent mapping field coherence across representative galaxy populations. This gap directly affects models of angular momentum transport and feedback, where magnetic fields may regulate star formation efficiency comparably to turbulence or outflows.

Connecting these observations to dark matter debates requires distinguishing primordial seed fields amplified by dynamos from fields generated by structure formation itself. ALMA2040 data could test whether early ordered fields correlate with halo spin or merger history, providing an independent constraint on magnetogenesis scenarios that complement Faraday rotation or CMB limits.

Next steps hinge on funding decisions for ALMA2040 by 2030 and coordinated surveys with SKA and ngVLA to trace field evolution from reionization to the peak of star formation.