Statistics of young starforming complexes in spiral galaxies using NIR photometry

Abstract

All slightly extended sources were identified on deep K-band maps of 46 spiral galaxies reaching at least K=20.3 mag/arcsec2 at a signal-to-noise level of 3. The galaxies had inclination angles <65 deg and linear resolutions <100 pc with seeing better than 1". The sample includes both barred and normal spirals with a wide spread in types. We also analyzed J- and H-band colors for 4 galaxies for which such images were available. An apparent magnitude limit of K = 19 mag was used for the sources analyzed in order to avoid marginal detections. Furthermore, we derived the source distributions of magnitudes and relative locations with respect to the spiral patterns. Almost 70% (15/22) of the grand-design spiral galaxies show significant concentration of bright K-band knots in their arm regions corresponding to 30% (15/46) of the full sample. Color-color diagrams for the 4 spirals with JHK photometry suggest that a significant fraction of the diffuse sources found in the arms are complexes of young stellar clusters with ages <10 Myr and reddened with several magnitudes of visual extinction. The brightest knots reach an absolute K-band magnitude Mk of -15.5 mag corresponding to stellar clusters or complexes with total masses up to at least 10^5 Mo. Brightest magnitude and number of knots correlate with the total absolute magnitude of the host galaxy. More knots are seen in galaxies with high far-infrared flux and strong two-armed spiral perturbations. The bright knots constitute up to a few percent of the total K-band flux from their parent galaxy and account for a star formation rate of ~1 Mo/yr for the brightest grand-design spiral galaxies.