THE EXTREME STAR FORMATION ACTIVITY OF Arp 299 REVEALED BY SPITZER IRS SPECTRAL MAPPING

We present Spitzer/IRS spectral mapping observations of the luminous infrared galaxy Arp 299 (IC 694 + NGC 3690) covering the central similar to 45 '' similar to 9 kpc. The integrated mid-IR spectrum of Arp 299 is similar to that of local starbursts despite its strongly interacting nature and high-IR luminosity, L-IR similar to 6 x 10(11) L-circle dot. This is explained because the star formation (probed by, e. g., high [Ne III] 15.56 mu m/[Ne II] 12.81 mu m line ratios) is spread across at least 6-8 kpc. Moreover, a large fraction of this star formation is taking place in young regions of moderate mid-IR optical depths such as the C+C' complex in the overlap region between the two galaxies and in HII regions in the disks of the galaxies. It is only source A, the nuclear region of IC 694, which shows the typical mid-IR characteristics of ultraluminous infrared galaxies (ULIRGs; L-IR > 10(12) L-circle dot), that is, very compact (less than 1 kpc) and dust-enshrouded star formation resulting in a deep silicate feature and moderate equivalent widths of the polycyclic aromatic hydrocarbons. The nuclear region of NGC 3690, known as source B1, hosts a low-luminosity active galactic nucleus (AGN) and is surrounded by regions of star formation. Although the high-excitation [Ne v] 14.32 mu m line typical of AGN is not detected in B1, its upper limit is consistent with the value expected from the X-ray luminosity. The AGN emission is detected in the form of a strong hot-dust component that accounts for 80%-90% of the 6 mu m luminosity of B1. The similarity between the Arp 299 integrated mid-IR spectrum and those of high-z ULIRGs suggests that Arp 299 may represent a local example, albeit with lower IR luminosity and possibly higher metallicity, of the star formation processes occurring at high-z.