Synthetic observations of spiral arm tracers of a simulated Milky Way
  analog
  Context: The Faraday rotation measure (RM) is often used to study the
magnetic field strength and orientation within the ionized medium of the Milky
Way. Observations indicate a RM in the spiral arms that exceeds the commonly
assumed range. This raises the question of under what conditions spiral arms
create such strong RM. Aims: We investigate the effect of spiral arms on
Galactic RMs through shock compression of the interstellar medium (ISM). It has
recently been suggested that the Sagittarius spiral arm creates a strong peak
in RM where the line of sight (LOS) is tangent to the arm, and that enhanced RM
follows along an intersecting LOS. We seek to understand the physical
conditions that give rise to this effect and the role of viewing geometry.
Methods: We apply a MHD simulation of the multi-phase ISM in a Milky Way type
spiral galaxy disk in combination with radiative transfer to evaluate different
tracers of spiral arm structures. For observers embedded in the disk, dust
intensity, synchrotron emission and the kinematics of molecular gas
observations are derived to identify spiral arm tangents. RMs are calculated
through the disk and evaluated for different observer positions. The observer's
perspective is related to the parameters of the local bubble surrounding the
observer. Results: We reproduce a scattering of tangent points for different
tracers of about $6^\circ$ per spiral arm similar to the Milky Way. As for the
RM, the model shows that compression of the ISM and associated amplification of
the magnetic field in spiral arms enhances RM by a few hundred rad $m^{-2}$ on
top of the mean contribution of the disk. The arm-inter-arm contrast in RM
along the LOS is approximately 10 in the inner Galaxy, fading to ~2 in the
outer Galaxy. We identify a shark-fin like pattern in the RM Milky Way
observations as well as the synthetic data that is characteristic for spiral
arms.
