A new wave of transients: supernova cosmology with the Rubin Observatory, 4MOST and Euclid

The work proposed here tackles some of the most important open questions in astrophysics, broadly centred on understanding the properties of the Universe itself. It is closely linked with the STFC Science Roadmap challenges pertaining to the roles and nature of dark matter and dark energy.

When the Rubin Observatory begins operation in early 2025, its Legacy Survey of Space and Time (LSST) will rapidly discover vast numbers of new astronomical transient objects. These are expected to be mostly supernovae (exploding stars) of various types. We propose to begin a new survey, TiDES (the Time Domain Extragalactic Survey), which will ultimately measure spectra of tens of thousands of transient objects discovered by LSST, plus a similar number of host galaxies of transients. The spectra provide two crucial pieces of information for each object: its classification (whether it is a supernova or not, and its type) and its and redshift. This information, along with brightness measurements from LSST, will enable the Type Ia supernovae to be used as "standard candles" for cosmology (standard candles are objects whose brightness is known and whose apparent brightness on the sky can be used as an indicator of their distance). The 5-year TiDES survey will create the largest, homogeneous Type Ia supernova sample by far. Simulations show that with this sample, we will measure the equation of state parameter (the ratio of pressure to density) for dark energy with only 2% uncertainty. This will be the tightest constraint to date and will strongly limit the range of theoretical explanations for the cause of the accelerating expansion of the universe.

To achieve these overall goals, we will initiate the TiDES survey using the new 4MOST instrument, which is currently under construction. One key objective of the proposed work is to commission 4MOST at the VISTA telescope in Chile and verify the science survey mechanism for TiDES. This will enable thousands of spectra of transient objects to be obtained during the grant period, already more than doubling the existing number of Type Ia supernovae for cosmology.

In addition to their use for classification of transients and redshift measurements, the spectra will enable further tests to be made, which are expected to lead to further improvement in cosmological constraints. For example, we aim to make quantitative measurements of features in the spectra of SNe Ia, some of which are known to correlate with peak brightness. Hence, by measuring these features we can further standardise the sample of Type Ia supernovae, improving their power as distance indicators.

A further avenue will be to use exquisite, wide-area imaging in optical and near infrared bands from the ESA Euclid mission, which is due to be launched in July 2023. This will provide a new route to study the host galaxies of the type Ia supernovae. Combined with the optical images from LSST and spectra of host galaxies obtained with 4MOST, measurements of host galaxy properties will be made (such as stellar mass, age, and shape). These improved measurements (compared to previous SN surveys) are expected to lead to reduced biases in measurements of cosmological parameters.