VLT/VIMOS observations of an occulting galaxy pair: redshifts and effective extinction curve

We present Very Large Telescope/Visible Multiobject Spectrograph Integral Field Unit observations of an occulting galaxy pair previously discovered in Hubble Space Telescope (HST) observations. The foreground galaxy is a low-inclination spiral disc, which causes clear attenuation features seen against the bright bulge and disc of the background galaxy. We find redshifts of z = 0.064 +/- 0.003 and 0.065 for the foreground and background galaxy, respectively. This relatively small difference does not rule out gravitational interaction between the two galaxies. Emission line ratios point to a star-forming, not active galactic nuclei dominated foreground galaxy. We fit the Cardelli, Clayton and Mathis extinction law to the spectra of individual fibres to derive slope (R-V) and normalization (A(V)). The normalization agrees with the HST attenuation map and the slope is lower than the Milky Way relation (R-V < 3.1), which is likely linked to the spatial sampling of the disc. We speculate that the values of R-V point to either coherent interstellar medium structures in the disc larger than usual (similar to 9 kpc) or higher starting values of R-V, indicative of recent processing of the dust. The foreground galaxy is a low stellar mass spiral (M-* similar to 3 x 10(9) M-circle dot) with a high dust content (M-dust similar to 0.5 x 10(6) M-circle dot). The dust disc geometry visible in the HST image would explain the observed spectral energy distribution properties of smaller galaxies: a lower mean dust temperature, a high dust-to-stellar mass ratio but relatively little optical attenuation. Ongoing efforts to find occulting pairs with a small foreground galaxy will show how common this geometry is.