What is the chance of encountering dust extinction along my line-of-sight looking into another galaxy? That is the motivation behind my STARSMOG, STarlight Absorption Reduction through a Survey of Multiple

Occulting Galaxies

Dust absorption remains the poorest constrained parameter in both Cosmological distances and multiwavelength

studies of galaxy populations. A galaxy's dust distribution can be measured to great accuracy in the

case of an overlapping pair of galaxies, i.e., when a foreground spiral galaxy accidentally overlaps a more

distant, preferably elliptical galaxy. We have identified over hundreds nearby of bona-fide overlapping pairs --well separated in redshift but close on the sky-- especially in the GAMA spectroscopic survey, taking advantage of its high completeness

(98%) on small scales.

The plan is to map the fine-scale (~50pc) dust structure in these occulting galaxies, using HST/WFC3 SNAP

observations. The resulting dust maps will (1) serve as an extinction probability for supernova lightcurve fits in

similar type host galaxies, (2) strongly constrain the role of ISM structure in Spectral Energy Distribution

models of spiral galaxies, and (3) map the level of ISM turbulence (through the spatial power-spectrum).

We ask for SNAP observations with a parent list of 355 targets to ensure a complete and comprehensive

coverage of each foreground galaxy mass, radius and inclination. The resulting extinction maps will serve as a

library for SNIa measurements, galaxy SED modelling and ISM turbulence measurements.