The Extreme Star Formation Activity of Arp299 Revealed by Spitzer IRS
  Spectral Mapping
  We present Spitzer/IRS spectral mapping observations of the luminous infrared
galaxy (LIRG) Arp299 (IC694 + NGC3690) covering the central 45arcsec ~ 9kpc.
The integrated mid-IR spectrum of Arp299 is similar to that of local starbursts
despite its strongly interacting nature and high infrared luminosity, L_IR ~
6x10^11 Lsun. This is explained because the star formation (probed by e.g. high
[NeIII]15.56micron/[NeII]micron line ratios) is spread across at least 6-8kpc.
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 IC694, that shows the
typical mid-IR characteristics of ultraluminous infrared galaxies (ULIRGs, L_IR
> 10^12 Lsun), that is, very compact (less than 1kpc) and dust-enshrouded star
formation resulting in a deep silicate feature and moderate equivalent widths
of the PAHs. The nuclear region of NGC3690, known as source B1, hosts a
low-luminosity AGN and is surrounded by regions of star formation. Although the
high excitation [NeV]14.32micron 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 6micron luminosity of B1. The similarity between the
Arp299 integrated mid-IR spectrum and those of high-z ULIRGs suggests that
Arp299 may represent a local example, albeit with lower IR luminosity and
possibly higher metallicity, of the star-formation processes occurring at
high-z.
