Integral field optical spectroscopy of a representative sample of ULIRGs . II . Two - dimensional kpc - scale extinction structure ⋆

Context. Ultraluminous infrared galaxies are merging systems characterized for containing large amounts of dust and emitting the bulk of their energy in the infrared. Dust affects several of the observed properties derived from optical and near-IR rest-frame data such as the stellar morphology, and the ionized gas distribution. Systematic analysis of the dust distribution in representative samples of ULIRGs are needed to investigate its two-dimensional structure, and to establish its impact in the derivation of fundamental properties such as star formation rates, effective radii, and dynamical masses. Aims. We investigate the two-dimensional kpc-scale structure of the extinction in a representative sample of local ULIRGs using the Hα/Hβ line ratio. Methods. We use optical integral field spectroscopy obtained with the INTEGRAL instrument at the William Herschel Telescope. Complementary optical and near-IR high angular resolution HST images have also been used. Compared to classical optical long-slit analysis, our data provides information about the entire system, and not only in pre-selected orientations. Results. The extinction exhibits a very complex and patchy structure in ULIRGs on kpc scales, from basically transparent regions to others deeply embedded in dust (AV)≃0.0 to AV≃8.0 mag). Nuclear extinction covers a broad range in AV from 0.6 to 6 mag, 69% of the nuclei having AV≥2.0 mag. Extinction in the external regions is substantially lower than in the nuclei with 64% of the ULIRGs in the sample having median AV of less than 2 mag for the entire galaxy. While post-coalescence nuclei tend to cluster around AV values of 2 to 3 mag, pre-coalescence nuclei appear more homogeneously distributed over the entire 0.4 mag ≤AV ≤ 7.7 mag range. For the average extinction (AV≃2.0) derived for the ULIRGs of the sample, the ratio of the de-reddened to observed SFR values is 6. The extinction-corrected, Hα-based SFR ranges from 10 to 300 M⊙ yr−1. For only 28% of the cases the de-reddened SFR is <20 M⊙ yr−1, whereas for the observed SFR this percentage increases to 72%. The IR-based SFR is always higher than the optical-based one, with differences ranging from about 2 to up to 30. The nuclear observed SFR has an average contribution to the total one of 16% for the entire sample. Once corrected for extinction, the average value becomes 31%. Because of mostly extinction effects, the optical (I-band) half-light radius in the sample galaxies is on average a factor 2.3 larger than the corresponding near-IR (H-band) value.