2006A&A...453L..25G

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. Bright knots in the southern arm of NGC 2997 were studied to determine their nature and evolutionary state. Low resolution near-infrared K- and J-band spectra of the knots were observed with ISAAC on the VLT. Most of the knots show strong HI Br_γ emission with some also having HeI and H₂ emission. A few knots show indications of ¹²CO absorption. This suggests that the knots are very young stellar clusters with masses up to 5x10⁴M_☉. The knots azimuthal distance from the K-band spiral correlates well with their Br_γ 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.