Statistics on 24 spiral galaxies having different observed arm locations using different arm tracers

Abstract

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.