Letter to the Editor

Star-forming knots and density wave in NGC 2997

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: pgrosbol@eso.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

Received: 18 April 2006
Accepted: 2 May 2006

Abstract

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.