Star formation in grand-design, spiral galaxies

Young, massive clusters in the near-infrared^{⋆,⋆⋆,⋆⋆⋆}

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
e-mail: dottori@ufrgs.br

Received: 15 September 2011
Accepted: 23 February 2012

Abstract

Aims. Spiral structure is a prominent feature in many disk galaxies and is often outlined by bright, young objects. We study the distribution of young stellar clusters in grand-design spiral galaxies and thereby determine whether strong spiral perturbations can influence star formation.

Methods. Deep, near-infrared JHK-maps were observed for ten nearby, grand-design, spiral galaxies using HAWK-I at the Very Large Telescope. 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.

Results. Young stellar clusters with ages  ≲7 Myr have significant internal extinction in the range of A_V = 3–7^m, while older ones typically have A_V  <  1^m. 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 M_K = –15$\stackrel{\mathrm{m}}{.}$5 or estimated masses of 10⁶ M_⊙. At radii with a strong, two-armed spiral pattern, the star formation rate in the arms is higher by a factor of 2–5 than in the inter-arm regions. The CLF in the arms is also shifted towards brighter M_K by at least 0$\stackrel{\mathrm{m}}{.}$4. We also detect clusters with colors compatible with Large Magellanic Cloud intermediate age clusters and Milky Way globular clusters. The (J − K) – M_K 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.

Conclusions. The gap in the (J − K) – M_K 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.