SPITZER-IRS STUDY OF THE ANTENNAE GALAXIES NGC 4038/39

Using the Infrared Spectrograph on the Spitzer Space Telescope, we observed the Antennae galaxies obtaining spectral maps of the entire central region and high signal-to-noise 5-38 mu m spectra of the two galactic nuclei and six infrared-luminous regions. The total infrared luminosity of our six IR peaks plus the two nuclei is L-IR = 3.8 x 10(10) L-circle dot, with their derived star formation rates ranging between 0.2 and 2 M-circle dot yr(-1), with a total of 6.6 M-circle dot yr(-1). None of the typical mid-IR tracers of active galactic nucleus activity is detected in either nucleus of the system, excluding the presence of a dust-enshrouded accretion disk. The hardest and most luminous radiation originates from two compact clusters in the southern part of the overlap region, which also have the highest dust temperatures. Polycyclic aromatic hydrocarbon (PAH) emission and other tracers of softer radiation are spatially extended throughout and beyond the overlap region, but regions with a harder and more intense radiation field show a reduced PAH strength. The strong H-2 emission is rather confined around the nucleus of NGC 4039, where shocks appear to be the dominant excitation mechanism, and the southern part of the overlap region, where it traces the most recent starburst activity. The luminosity ratio between the warm molecular gas (traced by the H-2 lines) and the total far-IR emission is similar to 1.6 x 10(-4), similar to that found in many starbursts and ultraluminous infrared galaxies. The total mass of warm H-2 in the Antennae is 2.5 x 10(7) M-circle dot, with a fraction of warm to total H-2 gas mass of about 0.35%. The average warm H-2 temperature is 302 +/- 26 K and appears anticorrelated with the radiation field hardness, possibly due to an evolution of the photodissociation region (PDR) morphology. The previously reported tight correlation between the H-2 and PAH emission was not found but higher total PAH emission to continuum ratios were found in PDRs with warmer gas.