VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve

TL;DR

This study uses VLT/VIMOS IFU observations to analyze an occulting galaxy pair, deriving redshifts, extinction properties, and dust content, revealing insights into dust structure and galaxy interaction.

Contribution

It provides the first detailed extinction law fitting for an occulting galaxy pair, revealing unique dust properties and spatial structures in the foreground galaxy.

Findings

Foreground and background galaxies have similar redshifts, suggesting possible interaction.

The extinction law slope ($R_V$) is lower than the Milky Way value, indicating different dust properties.

The foreground galaxy is a low-mass, dust-rich spiral with a large dust disk geometry.

Abstract

We present VLT/VIMOS IFU observations of an occulting galaxy pair previously discovered in HST observations. The foreground galaxy is a low-inclination spiral disk, which causes clear attenuation features seen against the bright bulge and disk of the background galaxy. We find redshifts of $z=0.064 \pm0.003$ and z=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 AGN-dominated foreground galaxy. We fit the Cardelli, Clayton & Mathis (CCM) 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 disk. We speculate that the values of $R_V$ point to either coherent ISM structures in the disk larger than usual ($\sim9$ kpc) or higher starting values of $R_V$, indicative of recent processing of the dust. The foreground galaxy is a low stellar mass spiral ($M_* \sim 3 \times 10^9 M_\odot$) with a high dust content ($M_{\rm dust} \sim 0.5 \times 10^6 M_\odot$). The dust disk geometry visible in the HST image would explain the observed SED 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 galaxies will show how common this geometry is.