Young stellar clusters triggered by a density wave in NGC 2997

Abstract

Bright knots along the arms of grand-design spiral galaxies are frequently seen on near-infrared K-band images. To investigate their nature, low resolution K-band spectra of a string of knots in the southern arm of the grand design, spiral galaxy NGC 2997 were obtained with ISAAC/VLT. Most of the knots show strong Brγ emission while some have H₂ and HeI emission. A few knots show indications of CO absorption. Their spectra and absolute K magnitudes exceeding -12 mag suggest them to be very compact, young stellar clusters with masses up to 5 × 10⁴ M. The knots' azimuthal distance from the K-band spiral correlates well with their Brγ 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 1.6 Myr, 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.