Letter to the Editor
Star-forming knots and density wave in NGC 2997
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: firstname.lastname@example.org
2 Instituto de Física, Univ. Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil
3 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
Accepted: 2 May 2006
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 Br emission with some also having and H emission. A few knots show indications of absorption. This suggests that the knots are very young stellar clusters with masses up to .
Conclusions.The knots azimuthal distance from the K-band spiral correlates well with their Br 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.
Key words: galaxies: individual: NGC 2997 / galaxies: spiral / galaxies: star clusters / galaxies: structure / infrared: galaxies / techniques: spectroscopic
© ESO, 2006