Pattern speed of main spiral arms in NGC 2997

Context. Bright knots along arms are observed in K-band images of many grand-design spiral galaxies. They are identified as young starforming complexes using K-band spectra which show strong Br$_\gamma$ emission. Their alignment with spiral arms and ages <10 Myr suggest that they are associated with a starforming front linked to a density wave in the galaxies. Aims. Ages may be estimated for the youngest starforming complexes using NIR broad band colors and Br$_\gamma$. A different angular speed of the density wave and material would lead to an azimuthal age gradient of newly formed objects. We aim to detect this gradient. Methods. Deep JHK-Br$_\gamma$ photometry of the southern arm of the grand-design spiral galaxy NGC 2997 was obtained by ISAAC/VLT. All sources in the field brighter than $K = 19$ mag were located. Color-color diagrams were used to identify young stellar complexes among the extended sources. Ages can be estimated for the youngest complexes and correlated with azimuthal distances from the spiral arms defined by the K-band intensity variation. Results. The extended sources with MK < -12 mag display a diffuse appearance and are more concentrated inside the arm region than fainter ones, which are compact and uniformly distributed in the disk. The NIR colors of the bright diffuse objects are consistent with them being young starforming complexes with ages <10 Myr and reddened by up to 8 mag of visual extinction. They show a color gradient as a function of their azimuthal distance from the spiral arms. Interpreting this gradient as an age variation, the pattern speed $\Omega_{\rm p}$ = 16 km s-1 kpc-1 of the main spiral was derived assuming circular motion. Conclusions. The alignment and color gradient of the bright, diffuse complexes strongly support a density wave scenario for NGC 2997. Only the brightest complexes with MK < -12 mag show a well aligned structure along the arm, suggesting that a strong compression in the gas due to the spiral potential is required to form these most massive aggregates, while smaller starforming regions are formed more randomly in the disk. The sharp transition between the two groups at MK = -12 mag may be associated with expulsion of gas when the first supernovae explode in the complex