Estimation of pattern speed using very young, stellar clusters

In the 60's, Prof. Stroemgren proposed to use space velocities and ages of moderately young stars to compute their places of formation in order to study the spiral structure in the Galaxy. We have extended this idea to very young stellar clusters in nearby disk galaxies. Near-infrared (NIR) K-band images of grand-design spiral galaxies often show bright knots along their spiral arms. Such knots in NGC 2997 have been identified as massive stellar clusters with ages of less than 10 Myr using JHK photometry and K-band spectra. Ages of these clusters can be estimated from JHK photometry. Their azimuthal distances from the spiral arms, as measured in the K-band, correlates with their ages suggesting that the pattern speed of an underlying density wave can be derived. This method is tested on the grand-design spiral NGC 2997 using VLT data.