Dust enshrouded star-forming activity in Arp 299

Abstract

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 hot 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 (A V < ∼ 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 [Ne]/[Ne] 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 (A V ∼ 60 mag) compact source, consistent with the mid-infrared signature of an active galactic nucleus (AGN), and in agreement with recent X-ray findings.

Dynamical Masses in Luminous Infrared Galaxies

“…This conclusion is supported by midinfrared observations such as Gallais et al (2004) in which A V is found to be on order tens rather than hundreds.…”

“…Thus, we are able to choose galaxies with very compact nuclear a Kim et al (1995). b Gallais et al (2004). c Sanders et al (1988).…”

Obscured Activity: AGN, Quasars, Starbursts and ULIGs Observed by the Infrared Space Observatory

“…ISOCAM's spatial scale was sufficient to map the morphologies of local interacting active systems (see for a review) and agree with results from ground-based high resolution images that (optically-faint or invisible) compact sources located in deeply obscured nuclear regions or interaction interfaces dominate the midinfrared emission (e.g. Soifer et al, 2000;Soifer et al, 2001;Charmandaris et al, 2002;Gallais et al, 2004;Siebenmorgen et al, 2004b). For some sources, diffuse emission between these 'hot-spots' can also contribute significantly to the total mid-infrared emission (e.g.…”

“…the Antennae (Mirabel et al, 1998) Fig. 2; UGC12915,12914 ; NGC 985 (Appleton et al, 2002); Arp 299 (Gallais et al, 2004)). The mid-infrared morphologies depend upon the mass ratio of the progenitor galaxies and the merger stage.…”

“…For example, ISOCAM observations of the VV 114 galaxy show that 60% of the mid-infrared flux arises from a diffuse component that is several kpc in size . Nearly forty percent of the 7 and 15µm emission detected in the early-stage merger (nuclei separation 5kpc) galaxy pair Arp 299 (NGC 3690 and IC 694) is diffuse and originates from the interacting disks (Gallais et al, 2004). In more advanced mergers, shocks and gravitational instabilities encourage the onset of star formation.…”