Plenary Lectures and VIP Guests

Dr. Tanja Petrushevska

Abstract:

Massive galaxy clusters act as gravitational telescopes, magnifying background supernovae that would otherwise remain undetectable and, in favourable geometries, producing multiple images of the same explosion [1]. The delays between those images give a measurement of the Hubble constant that is independent of both the local distance ladder and the cosmic microwave background, and therefore bears directly on the tension between them [2]. Magnification also opens access to the high-redshift supernova population, whose rates constrain the cosmic star formation history.

I will present our past and ongoing ground-based efforts to find and study these events. Targeted near-infrared campaigns towards galaxy clusters allowed us to measure volumetric core-collapse supernova rates, found to be in agreement with the star formation history, as well as supernova rates for the cluster galaxies themselves [3,4]. Magnification also makes possible spectroscopy that would otherwise be out of reach, which we have used to test whether the intrinsic properties of Type Ia supernovae evolve with redshift, a potential systematic for dark energy measurements [5,6].

I will then turn to the ongoing VST campaign monitoring galaxies and clusters that host gravitationally lensed quasars at roughly daily cadence. This provides both the temporal resolution needed for time delays and the wide field needed to find supernovae in cluster member galaxies, which give an independent distance to the lens [7]. Finally, I will show some of our ongoing efforts within the Euclid-LSST lensed transient search.

Lensed supernova discoveries are expected to increase significantly in the coming years. Well sampled time delays and the identification of cluster-member supernovae both require years of wide-field monitoring that space facilities cannot provide, so ground-based surveys remain essential alongside JWST.