Star-forming knots and density wave in NGC 2997

Context. Many grand design spiral galaxies show strings of bright knots along their arms on near-infrared K-band images. The alignment of such knots suggests a relation to the spiral pattern and possibly to a large-scale, star-forming front associated with a density wave. Aims. Bright knots in the southern arm of NGC 2997 were studied to determine their nature and evolutionary state. Methods. Low resolution near-infrared K- and J-band spectra of the knots were observed with ISAAC on the VLT. Results. Most of the knots show strong Hi Br-gamma. emission with some also having HeI and H-2 emission. A few knots show indications of (CO)-C-12 absorption. This suggests that the knots are very young stellar clusters with masses up to 5 x 10(4) M-circle dot. Conclusions. The knots azimuthal distance from the K- band spiral correlates well with their Br-gamma strength, indicating that they are located inside the co-rotation of the density wave, which triggered them through a large-scale, star-forming front. These relative azimuthal distances suggest an age spread of more than 1.6Myr, which is incompatible with standard models for an instantaneous star burst. This indicates a more complex star-formation history, such as several bursts or continuous formation.