The Off‐Nuclear Starbursts in NGC 4038/4039 (The Antennae Galaxies)

Imaging of the Antennae galaxies (NGC 4038/4039) with the Infrared ArrayCamera (IRAC) aboard the Spitzer Space Telescope reveals large concentrationsof star forming activity away from both nuclei of the two merging galaxies.These images confirm earlier findings based on ISO data with lower angularresolution. The short wavelength emission shows numerous compact sourcesidentified as stellar clusters. At the longer wavelengths, bright, moreamorphous and filamentary features correlate well with the known distributionsof denser gas, warm dust, and HII regions. There are also fainter, more diffusecomponents at all wavelengths that permeate the entire region and extend intothe two tidal tails. Non-stellar dust emission dominates the 5.8 and 8.0 micronimages, accounting for as much as 79% of the light at 5.8 micron and 95% at 8micron, averaged over the entire galaxy. Assuming that the non-stellar emissiontraces star formation, the IRAC data provide a view into the total underlyingstar forming activities unaffected by obscuration. Using the flux ratio ofnon-stellar to stellar emission as a guide, we map the local star formationrate in the Antennae and compare that to similar measurements in both normaland infrared-luminous galaxies. This rate in the active regions is found to beas high as those seen in starburst and some ultra-luminous infrared galaxies on``per unit mass'' basis. The two galactic centers actually have lower starforming rates than the off-nuclear regions despite the presence of abundantdense gas and dust, suggesting that the latter is a necessary but notsufficient condition for on-going star formation.Comment: 13 pages, 3 figures, to appear in the ApJ Supplement, September 2004 (Spitzer Special Issue