Star formation in grand-design, spiral galaxies

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. m 5 or estimated masses of 10 6 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. 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.

The occulting galaxy pair UGC 3995

“…Keel & White 2001a, H09) 5 . If the slope of the extinction law is determined over both highly opaque and mostly transparent sections, the extinction law effectively becomes gray (low values of R V , see also Natta & Panagia 1984;Keel & White 2001a;Holwerda et al 2009;Grosbøl & Dottori 2012). Thus, the lower range in extinction curve slopes visible in Fig.…”

Mortality and dust expulsion in early phases of stellar clusters

“…Due to the limited age information available for the old clusters, α and γ could not be separated. Thus, α was fixed to the value derived for young clusters by Grosbøl & Dottori (2012) as listed in Table 1. Only clusters one magnitude brighter than M l K were considered for the fits to avoid any bias due to incompleteness of faint clusters.…”

“…We selected six of the grand-design spirals of the study by Grosbøl & Dottori (2012), for which more than 2000 cluster complexes were identified. The galaxies were observed in the NIR JHK s -bands with HAWK-I at the Very Large Telescope and are listed in Table 1 together with their assumed distances estimated from their systemic velocity relative to the 3K cosmic microwave background using a Hubble constant of 73 km s −1 Mpc −1 .…”

“…The distributions of cluster complexes in ten nearby, granddesign spiral galaxies were studied by Grosbøl & Dottori (2012) using near-infrared (NIR) colors. They found observational evidence for a fast reduction of the extinction in young clusters because their colors were clearly separated from those of the older clusters with lower reddening.…”

The S4G view of stellar mass, mid-IR dust, and evolved, intermediate-age stars in nearby galaxies

“…For a sample of 17 AGB-dominated clusters chosen from regions least susceptible to the effects of dust extinction/reddening, we extract optical-to-mid-IR SEDs using archival SINGS data and new HAWK-I JHK imaging (i.e. Grosbol & Dottori 2011). We find that NIR brightness is coupled to the mid-IR dust emission such that a significant reduction of AGB light of up to 1 mag in K-band follows from extinction by the dust shell formed during this stage (Meidt et al 2011b).…”