Galaxy star formation and quenching

Galaxy star formation - stellar mass relation: Evolution and quenching pathways.

Star formation is a crucial aspect of galaxy evolution. Galaxies can be separated into two distinct classes: those that are actively forming stars and are blue in colour, and those that have no appreciable star formation and are red in colour. Galaxies that are blue and star-forming often have disk-like morphologies and form a tight correlation in the star formation rate - stellar-mass plane (referred to as the galaxy main sequence): more massive galaxies are forming stars at a faster rate. The current consensus is that galaxies evolve from the active ‘blue’ category to be “red-and-dead". Many evolutionary pathways have been proposed to explain the transition, and it is not yet clear what the dominant drivers are.

In Leslie et al. 2016, we proposed that AGN-feedback (e.g. from AGN-driven winds, and heating) might have an important role in quenching the SFRs of galaxies. Using local galaxies in the Sloan Digital Sky Survey we classified galaxies according to their optical emission-line ratios and studied their location on the star formation rate - stellar-mass plane. We found that optically-selected AGNs have lower than average SFRs, and that galaxies form a sequence from the ‘blue cloud’ galaxies which are actively forming stars, through a combination of composite, Seyfert, and low-ionization nuclear emission-line region galaxies, ending as ‘red-and-dead’ galaxies. We have continued this work by investigating morphology in combination with AGN activity in the SFR-stellar mass plane in McPartland et al. 2019 and found that bulge growth is an integral part of this quenching pathway.

In Leslie et al. 2020 we measured the evolution of the main-sequence with cosmic-time using over 200,000 galaxies in the COSMOS field. We derive new dust-independent radio-determined star-formation rates from stacking analysis out to redshift z~5. We also studied the main-sequence as a function of galaxy morphology (at z<1.5) and find that galaxies with a larger bulge tend to have lower star formation rates at fixed mass and redshift. No clear trends were found with galaxy environment from 0.5<z<3.