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

Abstract

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 (RV) and normalization (AV). The normalization agrees with the HST attenuation map and the slope is lower than the Milky Way relation (RV < 3.1), which is likely linked to the spatial sampling of the disc. We speculate that the values of RV point to either coherent interstellar medium structures in the disc larger than usual (∼9 kpc) or higher starting values of RV, indicative of recent processing of the dust. The foreground galaxy is a low stellar mass spiral (M* ∼ 3 × 109 M⊙) with a high dust content (Mdust ∼ 0.5 × 106 M⊙). 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.