Star Formation in Grand-design, Spiral Galaxies: Young, Massive Clusters in the NIR, Mortality and Dust Expulsion

Friday, 13 September 2013 9 a.m. — 10 a.m. MST

AURA Lecture Hall

NOIRLab South Colloquia
HORACIO DOTTORI (Instituto de Física, UFRGS, Porto Alegre, Brasil and Gemini South Visiting Astronomer)

Deep, near-infrared JHK-maps were observed for ten nearby, grand-design, spiral galaxies using HAWK-I at the VLT. Complete, magnitude-limited candidate lists of star-forming complexes were obtained by searching within the K-band maps. The (J-H) vs. (H-K) diagram allows to identify stellar clusters younger than 7 Myr, to estimate their extinction and to study their spatial distribution with respect to spiral patterns. The youngest clusters have significant internal extinction Av ~ 3-7 mag or larger, while olders ones have typically AV  < 1 mag. 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 = -15.5 mag or estimated masses of 106 Msun. At radii with a strong two-armed spiral pattern the star formation rate in the arms is higher by a factor 2-5 than in the inter-arm regions. The CLF in the arms is also shifted towards brighter MK by at least 0.4 mag than in the inter-arm regions. For the brightest clusters, a gap separate the youngest clusters from the oldest one in the (J-K) vs. MK diagram. This property is compatible with a rapid dust expulsion at the age of 7 Myr.

We also performed 2-MASS photometry for a sample of 48 Local Group resolved young clusters with previous age determinations. Here, we also verify the presence of the gap in (J-K) vs. MK diagram.

Finally, Monte Carlo simulations of cluster populations were performed using the power-law distribution function Integrated colors were computed from Starburst99.