# An upper limit on primordial magnetic fields from ultra-faint dwarf galaxies

## TL;DR

This paper establishes an upper limit on primordial magnetic fields by analyzing the mass and formation redshifts of ultra-faint dwarf galaxies, constraining early universe magnetic field strength.

## Contribution

It introduces a novel method to constrain primordial magnetic fields using the dynamical mass and formation redshifts of ultra-faint dwarf galaxies.

## Key findings

- Upper limit of 0.50±0.086 nG on primordial magnetic field assuming formation redshift 10.
- Upper limit of 0.31±0.04 nG assuming formation redshift 20.
- Constraints improve understanding of magnetic field strength in the early universe.

## Abstract

The presence of primordial magnetic fields increases the minimum halo mass in which star formation is possible at high redshifts. Estimates of the dynamical mass of ultra-faint dwarf galaxies (UFDs) within their half-light radius constrain their virialized halo mass before their infall into the Milky Way. The inferred halo mass and formation redshift of the UFDs place upper bounds on the primordial comoving magnetic field, $B_0$. We derive an upper limit of $0.50\pm 0.086$ ($0.31\pm 0.04$) nG on $B_0~$ assuming the average formation redshift of the UFD host halos is $z_{\rm form}=$ 10 (20), respectively.