Constraining Magnetic Fields in the Circumgalactic Medium
  We study the properties of magnetic fields in the circumgalactic medium (CGM)
of $z<1$ galaxies by correlating Faraday rotation measures of $\sim 1,000$
high-redshift radio sources with the foreground galaxy number density estimated
from the DESI Legacy Imaging Surveys. This method enables us to extract signals
of rotation measures contributed by intervening gas around multiple galaxies.
Our results show that there is no detectable correlation between the
distribution of rotation measures and the number of foreground galaxies,
contrary to several previous results. Utilizing the non-detection signals, we
estimate $3\sigma$ upper limits to the rotation measures from the CGM of $\sim
20 \rm \ rad/m^{2}$ within~50 kpc and $\sim 10 \rm \ rad / m^{2}$ at
separations of $100$ kpc. By adopting a column density distribution of ionized
gas obtained from absorption line measurements, we further estimate the
strengths of coherent magnetic fields parallel to the line of sight of $<\rm 2
\ \mu G$ in the CGM. We show that the estimated upper limits of rotation
measures and magnetic field strengths are sufficient to constrain outputs of
recent galaxy magneto-hydrodynamic simulations. Finally, we discuss possible
causes for the inconsistency between our results and previous works.
