  The density wave theory predicted some physical offsets among different
tracers of star formation. To test this prediction, here we compiled data on 40
galaxies searched observationally for a physical offset between spiral arm
tracers, and found that 24 of them have a positive offset. In a spiral arm, an
arm tracer in a region with a given temperature may be at a different location
(offset) than an arm tracer in a region with a colder temperature. Some
conditions are found to be necessary or sufficient in order to detect an offset
between two arm tracers. To find the offset of a tracer from another tracer,
one needs a proper linear resolution. Starting in the dust lane and going
across the spiral arm, we seek the observed physical width of the star-forming
zone (offset). In our sample of 24 galaxies with measured offsets, we find
offsets with a median value near 326 pc and a mean near 370 pc. These offsets
are comparable to those found in our Milky Way galaxy, between the cold diffuse
CO 1-0 gas set at 0 pc, and the hot dust near 350 pc. Preliminary statistics
are performed on the angular velocity of the gas and stars and angular velocity
of the spiral pattern. Their observed orbital velocity of 200 km/s at a typical
galactic radius near 4 kpc yields an angular speed of the gas and stars near 60
km/s/kpc. Their deduced angular rotation for the spiral pattern averages 36
km/s/kpc. These observational results are close to the results predicted by the
shock-induced star-forming density wave theory. These mean or median property
values will be useful for finding other galaxies that can support density
waves.
