2008A&A...490...87G

Very young stellar clusters and cluster complexes may be embedded in dust lanes along spiral arms in disk galaxies and escape detection in visual bands. Observations in the near-infrared K-band offer an almost unbiased view of such clusters or complexes due to the small attenuation by dust at this wavelength. The objective is to determine their population size, absolute K-band magnitude distribution above the limiting magnitude imposed by the data, and location relative to the spiral pattern in disk galaxies. All slightly extended sources were identified on deep K-band maps of 46 spiral galaxies reaching at least K=20.3mag arcsec^–2 at a signal-to-noise level of 3. The galaxies had inclination angles <65° and linear resolutions <100pc 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=19mag 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 <10Myr and reddened with several magnitudes of visual extinction. The brightest knots reach an absolute K-band magnitude M_K of -15.5mag corresponding to stellar clusters or complexes with total masses up to at least 10⁵M_☉. 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 ∼1M_☉/yr for the brightest grand-design spiral galaxies.