Probing the dust-enshrounded regions of the interacting galaxy system Arp 299: A near-infrared study

We present high spectral resolution (λ/δλ ≅ 10³) Paβ(1.28 μm) and Brγ(2.17 μm), 3.29 μm dust feature, and near-infrared broadband images of the Arp 299 = NGC 3690/IC 694 galaxy system. The emission is found to be concentrated at the positions of three active regions, known as sources A, B, and C. From our Brγ/Paβ flux ratio we find the visual extinction toward the ionized gas in A, B, and C is ∼6, 6, and 2 magnitudes, respectively, assuming case B recombination and a foreground screen geometry for the obscuring material. Our observations can be explained entirely by a starburst model for the Arp 299 system. The putative active galactic nucleus (AGN) in source A does not dominate the properties of this source. We see no evidence of broad recombination lines. In addition, the ratio of the 3.29 μm dust feature to total luminosity, a tracer of starburst activity, is consistent in each source with that seen in M82 and other starburst galaxies. Also, our imaging observations reveal that the dust feature emission is concentrated in the nucleus of source A, contrary to the extended annular distribution of the feature emission seen surrounding the nucleus of the more distant Seyfert galaxy NGC 7469. In this galaxy, the absence of the feature emission in the nucleus has been attributed to the destruction of the dust carriers in the hard radiation field surrounding the AGN. Our observations suggest that all of the active regions in Arp 299 are characterized by starburst episodes. The observed CO indices and Bry equivalent widths imply that source B is older than source A and source C is the youngest star-forming region. Although these regions are probably complex physical systems not necessarily characterized by a single coeval population of stars, we have compared our observations with an instantaneous starburst model with a Salpeter initial mass function. Using this simple model, we find starburst ages of ∼6 × 10⁶, 8 × 10⁶, and 4 × 10⁶ for sources A, B, and C, respectively.