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. 0t oAV � 8.0 mag). Nuclear extinction covers a broad range in AV from 0.4 to 7.7 mag , 69% of the nuclei having AV ≥ 2.0 mag. Extinction in the external regions is substantially lower than in the nuclei with 60% 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