Dust enshrouded star-forming activity in Arp 299

Abstract

We present mid-infrared spectro-imaging (5-16 μm) observations of the infrared luminous interacting system Arp 299 (= Mrk 171 = IC 694+NGC 3690) obtained with the ISOCAM instrument aboard ISO. Our observations show that nearly 40% of the total emission at 7 and 15 μm is diffuse, originating from the interacting disks of the galaxies. Moreover, they indicate the presence of large amounts of hot dust in the main infrared sources of the system and large extinctions toward the nuclei. While the observed spectra have an overall similar shape, mainly composed of Unidentified Infrared Bands (UIB) in the short wavelength domain, a strong continuum at ∼13 μm and a deep silicate absorption band at 10 μm, their differences reveal the varying physical conditions of each component. For each source, the spectral energy distribution (SED) can be reproduced by a linear combination of a UIB "canonical" spectral template and a host dust continuum due to a 230-300 K black body, after independently applying an extinction correction to both of them. We find that the UIB extinction does not vary much throughout the system (Av ≲ 5 mag) suggesting that most UIBs originate from less enshrouded regions. IC 694 appears to dominate the infrared emission of the system and our observations support the interpretation of a deeply embedded nuclear starburst located behind an absorption of about 40 magnitudes. The central region of NGC 3690 displays a hard radiation field characterized by a [NeIII]/[NeII] ratio ≥ 1.8. It also hosts a strong continuum from 5 to 16 μm which can be explained as thermal emission from a deeply embedded (Av ∼ 60 mag) compact source, consistent with the mid-infrared signature of an active galactic nucleus (AGN), and in agreement with recent X-ray findings.