A direct test of density wave theory in a grand-design spiral galaxy

摘要

The exact nature of the arms of spiral galaxies is still an openquestion1. It has been widely assumed that spiral arms in galaxieswith two distinct symmetrical arms are the products ofdensity waves that propagate around the disk, with the spiralarms being visibly enhanced by the star formation thatis triggered as the passing wave compresses gas in the galaxydisk1–3. Such a persistent wave would propagate with anapproximately constant angular speed, its pattern speed Ω_P.The quasi-stationary density wave theory can be tested bymeasuring this quantity and showing that it does not vary withradius in the galaxy. Unfortunately, this measurement is difficultbecause Ω_P is only indirectly connected to observablessuch as the stellar rotation speed. Here, we use the detailedinformation on stellar populations of the grand-design spiralgalaxy UGC 3825, extracted from spectral mapping, to measurethe offset between young stars of a known age and thespiral arm in which they formed, allowing a direct measurementof Ω_P at a range of radii. The offset in this galaxy is foundto be as expected for a pattern speed that varies little withradius, indicating consistency with a quasi-stationary densitywave, and lending credence to this new method.