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.

💡 Research Summary

This paper presents a comprehensive Spitzer/IRS spectral‑mapping study of the luminous infrared galaxy (LIRG) Arp 299, which comprises the interacting pair IC 694 and NGC 3690. The observations cover the central 45 arcsec (≈9 kpc) region in the 5–38 µm range, providing both an integrated spectrum and spatially resolved spectra for the two nuclei, the overlap region (designated C and C′), and the disks of the galaxies.

The integrated mid‑infrared (MIR) spectrum of Arp 299 closely resembles that of local starburst galaxies despite the system’s high infrared luminosity (L_IR ≈ 6 × 10¹¹ L_⊙) and its strongly interacting nature. This similarity is explained by the fact that the bulk of the star formation is not confined to a compact nucleus but is distributed over a scale of 6–8 kpc. High