Letter to the Editor

Tracing the sites of obscured star formation in the Antennae galaxies with Herschel-PACS^*

U. Klaas¹, M. Nielbock¹, M. Haas², O. Krause¹ and J. Schreiber¹

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany e-mail: klaas@mpia.de
² Astronomisches Institut, Ruhr-Universität Bochum (AIRUB), Universitätsstr. 150/NA7, 44801 Bochum, Germany

Received: 31 March 2010
Accepted: 19 April 2010

Abstract

Aims. FIR imaging of interacting galaxies allows locating even hidden sites of star formation and measuring of the relative strength of nuclear and extra-nuclear star formation. We want to resolve the star-forming sites in the nearby system of the Antennae.

Methods. Thanks to the unprecedented sharpness and depth of the PACS camera onboard ESA's Herschel Space Observatory, it is possible for the first time to achieve a complete assessment of individual star-forming knots in the FIR with scan maps at 70, 100, and 160 μm. We used clump extraction photometry and SED diagnostics to derive the properties related to star-forming activity.

Results. The PACS 70, 100, and 160 μm maps trace the knotty structure of the most recent star formation along an arc between the two nuclei in the overlap area. The resolution of the starburst knots and additional multi-wavelength data allow their individual star formation history and state to be analysed. In particular, the brightest knot in the mid-infrared (K1), east of the southern nucleus, exhibits the highest activity by far in terms of dust heating and star formation rate, efficiency, and density. With only 2 kpc in diameter, this area has a 10–1000 μm luminosity, which is as high as that of our Milky Way. It shows the highest deficiency in radio emission in the radio-to-FIR luminosity ratio and a lack of X-ray emission, classifying it as a very young complex. The brightest 100 and 160 μm emission region (K2), which is close to the collision front and consists of 3 knots, also shows a high star formation density and efficiency and lack of X-ray emission in its most obscured part, but an excess in the radio-to-FIR luminosity ratio. This suggests a young stage, too, but different conditions in its interstellar medium. Our results provide important checkpoints for numerical simulations of interacting galaxies when modelling the star formation and stellar feedback.