Final Report Summary - SEDMORPH (The origins of galaxy bimodality: Linking mergers, starbursts and feedback in observations and simulations) Understanding how and why galaxies form and evolve is one of the most challenging problems in modern astrophysics. Our own galaxy, the Milky Way, shows order and structure, as do most massive galaxies in our local neighbourhood. Yet when we look to very distant galaxies they are disordered and chaotic. The leading theory for the origin of this transformation invokes gas-rich mergers, which trigger massive starbursts leading to bulge and supermassive black hole growth. The aim of this project was to provide conclusive observational evidence to confirm or refute this fundamental theory of galaxy evolution.Considerable quantities of high quality data are now available for both local and distant galaxies; we developed urgently required new methodology to enable the translation of this data into an improved understanding of galaxy formation. In this project a team of researchers with complementary inter-disciplinary skills developed a suite of new techniques to: (1) classify and visualise the broad band optical-NIR spectral energy distributions (SEDs) of galaxies to identify rare post-starburst galaxies from broad band photometry alone; (2) quantify the morphological disturbance of post-merger galaxies; (3) perform Bayesian bulge-disk decomposition of galaxy images, correctly accounting for degeneracies and uncertainties; (4) improve the comparison between observations and simulations via accurate post-processing of simulations. Our new techniques have provided stringent observational constraints on models, improved robustness of model-data comparison and highlighted areas for further study.The SEDmorph team recently published the current state-of-the-art study of local (z~0) post-starburst galaxies (Pawlik et al. 2018), combining the analysis of morphologies, star formation histories and environments to conclude that there are at least 3 different pathways to the formation of post-starburst signatures at low-redshift, with only one of these being the “classical” blue-to-red transition traditionally ascribed to their evolution. Although the starburst can cause bulge growth and lowering of specific star formation rate, it may well not lead to complete transition of the galaxy. On the other hand, there is substantial evidence for merger triggering, despite the rapid decline in tidal features. Rowlands et al. (2017) highlighted the decreasing role of post-starburst/mergers in the building up of galaxy bimodality with decreasing redshift. Even assuming that 100% of post-starburst galaxies (PSBs) transition to the red sequence, they cannot account for red sequence growth today. At low redshift, post-starbursts and mergers are indeed “an interesting curiosity”. At high redshift, the SEDmorph team developed a new method to classify the broad band optical-NIR spectral energy distributions (SEDs) of galaxies (Wild et al. 2014). We used the method to show that the evolution of the mass functions of post-starburst and quiescent galaxies is consistent with all quiescent galaxies at z>0.5 forming following a rapid truncation of star formation (Wild et al. 2016), in stark contrast to low-redshift results. The extremely compact sizes of z>1.0 post-starburst galaxies indicate that these must have formed following rapid dissipational collapse, either caused by major mergers or rapid multi-component growth in the early gas rich Universe (Almaini, Wild et al. 2017). This work has led to a medium JWST GO proposal to be submitted in April 2018. This suggests that mergers are a key component of galaxy evolution in the high redshift (z>1) Universe. Alongside the stellar populations, it is crucial to understand what is happening to the gas. The SEDmorph team measured the evolution of dust and molecular gas properties in PSB galaxies selected to span an age sequence (Rowlands et al. 2014). We showed that local PSBs harbour significant molecular gas and dust reservoirs and residual star formation, indicating that complete quenching of the starburst due to exhaustion or expulsion of gas has not occurred during this timespan. The team obtained competitive ALMA time to observe high redshift PSBs, and future papers will show that high-redshift PSBs are gas and dust poor (Rowlands et al. in prep), and have clustering amplitudes consistent with being descendants of sub-mm galaxies (Wilkinson et al. in prep). Overall the picture is that high and low redshift post-starburst galaxies are completely different beasts. While all PSBs may be caused by galaxy mergers, only at high-redshift are they likely a dominant channel for building the red sequence. Additional results included a complete census of galaxy components in the local Universe (Mendez-Abreu et al. 2016); a comparison of morphological and kinematic type of S0 galaxies in the local Universe (Mendez-Abreu et al. 2018); a new Bayesian method for bulge-disk decomposition (Argyle et al. in prep.); new code to post-process galaxy hydrodynamic simulations (Zheng et al. in prep.), as well as methods to improve the realism of simulations themselves (Vandenbroucke et al. 2018).
