Why do galaxies stop forming stars?

It has been for nearly a century that massive galaxies in the local Universe tend to fall into two classes, either smooth elliptical shapes with little ongoing star formation or spiral disk structures which continue to form stars. Over the past 100 years we have gathered more and more data to quantify the differences between these two different types of galaxies, but why some galaxies are “red and dead” while others are star-forming disks, remains something of a mystery. While the latest generation of cosmological simulations obtains approximately correct fractions of quenched galaxies, they do so in many different ways, making it difficult to pin down the mechanisms responsible. I will take a look at this question from the perspective of galaxies caught in the act of rapidly quenching their star formation, which are easily detectable in large spectroscopic or multiwavelength galaxy surveys out to high redshift. I will demonstrate how these transitional galaxies can be used to constrain pathways to quenching, and highlight the things that don’t quite work as expected in the current generation of hydrodynamic simulations.
Vivienne Wild (St. Andrews University, UK)