Star formation in grand-design, spiral galaxies. Young, massive clusters in the near-infrared

Abstract

Deep, near-infrared JHK-maps were observed for 10 nearby, grand-design,
spiral galaxies using HAWK-I/VLT to study the distribution of young stellar
clusters in them and thereby determine whether strong spiral perturbations can
influence star formation. Complete, magnitude-limited candidate lists of
star-forming complexes were obtained by searching within the K-band maps. The
properties of the complexes were derived from (H-K)-(J-H) diagrams including
the identification of the youngest complexes (i.e. <7 Myr) and the estimation
of their extinction.
Young stellar clusters with ages <7 Myr have significant internal extinction
in the range of Av=3-7m, while older ones typically have Av<1m. The cluster
luminosity function (CLF) is well-fitted by a power law with an exponent of
around -2 and displays no evidence of a high luminosity cut-off. The brightest
cluster complexes in the disk reach luminosities of Mk = -15.5m or estimated
masses of 10^6 Mo. At radii with a strong, two-armed spiral pattern, the star
formation rate in the arms is higher by a factor of 2-5 than in the inter-arm
regions. The CLF in the arms is also shifted towards brighter Mk by at least
0.4m. We also detect clusters with colors compatible with Large Magellanic
Cloud intermediate age clusters and Milky Way globular clusters. The (J-K)-Mk
diagram of several galaxies shows, for the brightest clusters, a clear
separation between young clusters that are highly attenuated by dust and older
ones with low extinction.
The gap in the (J-K)-Mk diagrams implies that there has been a rapid
expulsion of dust at an age around 7 Myr, possibly triggered by supernovae.
Strong spiral perturbations concentrate the formation of clusters in the arm
regions and shifts their CLF towards brighter magnitudes.