Offset between stellar spiral arms and gas arms of the Milky Way
  Spiral arms shown by different components may not be spatially coincident,
which can constrain formation mechanisms of spiral structure in a galaxy. We
reassess the spiral arm tangency directions in the Milky Way through
identifying the bump features in the longitude plots of survey data for
infrared stars, radio recombination lines (RRLs), star formation sites, CO,
high density regions in clouds, and HI. The bump peaks are taken as indications
for arm tangencies, which are close to the real density peaks near the spiral
arm tangency point but often have $\sim$ 1$^\circ$ offset to the interior of
spiral arms. The arm tangencies identified from the longitudes plots for RRLs,
HII regions, methanol masers, CO, high density gas regions, and HI gas appear
nearly the same Galactic longitude, and therefore there is no obvious offset
for spiral arms traced by different gas components. However, we find obvious
displacements of 1.3$^\circ-$ 5.8$^\circ$ between gaseous bump peaks from the
directions of the maximum density of old stars near the tangencies of the
Scutum-Centaurus Arm, the northern part of the Near 3 kpc Arm, and maybe also
the Sagittarius Arm. The offsets between the density peaks of gas and old stars
for spiral arms are comparable with the arm widths, which is consistent with
expectations for quasi-stationary density wave in our Galaxy.
