Spatial and velocity offsets of Galactic masers from the centers of
  spiral arms
  Some galactic theories of spiral arms predict an offset between different
tracers of star formation. Our goal here is to find such an offset between the
observed location of radio masers and the location of the arms, using a recent
4-arm model fitted to the CO 1-0 gas. Our method here is to compare a recent
global 4-arm spiral model (as fitted to the arm tangents in the observed broad
CO 1-0 gas) with the recent results for the trigonometric distances of radio
masers, for the main arms (Cygnus-Norma, Perseus, Sagittarius-Carina, Scutum,
and Norma). Our results indicate that most radio masers are near the inner arm
edge (toward the Galactic Center) of spiral arms. These masers are offsets from
the model arm (where the broad CO 1-0 molecular region resides) by 0.34 (plus
or minus 0.06) kpc inward. In radial velocity space, the median offset between
masers and the CO-fitted model is around 10 (plus or minus 1) km/s. Based on
the masers being observed here to be radially inward of the broad CO gas in the
Cygnus arm at 15 kpc along the Galactic Meridian, the corotation radius of the
Milky Way disk is more than 15 kpc distant from the Galactic Centre and the
density-wave angular pattern speed is less than 15 km/s/kpc. Arm pitch angle
should be measured using many arm tracers, and located on both side of the
Galactic Meridian, to ensure better precision and avoid a bias pertinent to a
single tracer.
