Enhancing the Roman Cosmology Program with Strongly Lensed Supernovae
Wide-Field Science – Large
Louis-Gregory Strolger / Space Telescope Science Institute, PI
One of the primary mission objectives for the Nancy Grace Roman Space Telescope is to investigate the nature of dark energy with a variety of methods. Observations of Type Ia supernovae (SNe Ia) will be one of the principal anchors of the Roman cosmology program, through traditional luminosity distance measurements from the High Latitude Time Domain Survey (HLTDS). The Wide Field Instrument (WFI) can provide another valuable cosmological probe, without altering the mission strategy: time delay cosmography with gravitationally lensed SNe. These rare events manifest when the light from a stellar explosion propagating along different paths is focused by a lensing potential (a galaxy or galaxy cluster), forming multiple images of the SN on the sky. Depending on the relative geometrical and gravitational potential differences of each path, the SN images appear delayed by hours to months (for galaxy-scale lenses) or years (for cluster-scale lenses). These time delays can be used to measure a combination of angular diameter distances that constrain the Hubble constant (H0) and other cosmological parameters, including the dark energy equation of state (e.g., w), in a single step. Constraints on cosmological parameters from lensed SNe are highly complementary and fully independent to those of non-lensed SNe Ia, providing a valuable check on systematics. This proposal will lay the groundwork for including this new probe in the Roman cosmology toolkit, which can provide a <1% H0 measurement, by producing simulations and tools needed for efficient detection and modeling of each new lensing system.
Previous work predicts the planned Roman HLTDS will discover over 40 strongly lensed SNe, but that assumes a robust detection pipeline with high efficiency. We have two avenues for identifying strongly lensed SNe in public Roman data. First, we can simply search all publicly available images for lensed SNe using convolutional neural networks (CNNs) designed to identify the combination of lensed arcs and multiple transient sources. Second, we can maintain a catalog of all known galaxy and cluster-scale lenses (currently >10,000), and continuously search for transients in those locations. This method requires that we continuously update the catalog of known lenses that have been discovered throughout the Roman mission. CNN lens-finding algorithms will therefore be repeatedly run on all publicly available data, and newly identified lenses will be added to the catalog for future observations at the same location. Both CNN architectures will require a large (~50,000 lenses) training set to be reliable at the start of the mission.
The early success of Roman Lensed SN cosmology therefore depends upon a suite of accurate and detailed pixel- and catalog-level simulations of Roman lensed SN observations. The simulations are critical for developing and testing detection/analysis pipelines, developing plans and proposals for external follow-up resources, and optimizing existing tools for the unique capabilities of Roman. This proposal will produce the necessary simulations, which are an extension of (but not included in) those planned for the HLTDS. Using the simulations, we will create a data challenge to identify the optimal static lens finding, lensed SN detection, and time delay measurement algorithms to be used with Roman data products. Adapting existing tools and incorporating the best performing methods from the data challenge, we will produce robust, well-documented, and public detection and analysis pipelines. Strongly lensed SNe have the potential to drastically improve the Roman cosmology program with a probe fully independent of SNe Ia. Roman and the HLTDS is truly the first opportunity to create a gold standard sample of lensed SNe, and this program produces all the components needed to ensure we can effectively leverage the enormous potential for cosmology.