Quenching galaxies in MOONRISE

Over 100 years ago Edwin Hubble observed that most galaxies are either spiral or elliptical in shape. Since then we have learnt that spiral galaxies are typically forming new stars, while ellipticals are not. Understanding the pathways by which galaxies stop forming stars ("quench"), as well as what processes determine their different shapes, are crucial outstanding questions at the heart of modern galaxy formation and evolution, impacting a range of astrophysical studies from star formation and interstellar medium to dark matter and dark energy.

Simulations have demonstrated that aggregated statistics such as the number of galaxies as a function of mass (mass function), or the evolving fraction of quenched galaxies with Cosmic time, are unable to uniquely constrain the different possible mechanisms to transform galaxies from star-forming spiral to quenched elliptical. This proposal is about catching galaxies in the act of transformation: this provides a powerful and under-explored discriminant between various possible mechanisms, via measurements of the timescales over which quenching operates, the presence or absence of a preceding starburst, the shape of the galaxies, and changes in metal content. In particular, post-starburst galaxies (PSBs) serve as smoking gun examples of rapid quenching at work, easily identifiable via their distinctive spectral characteristics. In a seminal paper in 2009, the PI Wild noted the potential importance of PSBs to the build-up of the quenched galaxy population, moving such galaxies from being purely an "interesting curiosity" to the mainstream of galaxy evolution. Since then, she has pioneered techniques to identify PSBs in deep multi-wavelength photometric surveys, and track their evolving mass functions to estimate the shifting balance of slow and rapid quenching events over Cosmic time.

However, to conclusively prove the importance of rapid quenching and understand its cause, we need to measure more than just the mass of transitioning galaxies. The forthcoming MOONS instrument on ESOs VLT will allow us to measure the detailed properties of these transitioning galaxies for the first time in large numbers at "Cosmic noon", when galaxies formed most of their stars. As part of the MOONS GTO team, and co-chair of the working group responsible for providing redshifts, spectral lines and galaxy parameters to the entire team, Wild has exclusive access to the 190-night MOONRISE extragalactic legacy survey. An increased sample size of post-starburst galaxies by 2 orders of magnitude compared to current state-of-the-art observations will revolutionise our understanding of why galaxies stop forming stars. Advanced statistical methods, as well as comparison to simulations, will help elucidate the importance of different quenching mechanisms such as the expulsion of gas, heating of circum-galactic gas or tidal stripping, as a function of cosmic time, galaxy mass and local and global environment.