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Mergers, star-formation and the build-up of stellar mass in galaxies

Galaxies grow over time, both via the formation of new stars using cold neutral gas and by the merging of multiple galaxy units into a much larger system. These processes are not mutually exclusive, as the merger event can affect the rate at which new stars are formed via complex processes which can either enhance, suppress or completely quench star-formation. In order to probe the buildup of stellar mass in the Universe (a key parameter in understanding the evolution of galaxies), we must study all of these process over a large redshift baseline and for statistically robust samples of galaxies.

Thanks to surveys such a SDSS and GAMA we are beginning to understand these processes at low redshift (z<0.4) and in the intermediate to high stellar mass regime (>109M). However, we have little robust understanding of the time evolution of merger rates, consistently measured star-formation in large samples, merger-modified star-formation or how environment affects all of these processes. In addition, from nearby galaxy samples and the local group, we are just beginning to build a picture of how large mass ratio galaxy-galaxy interactions affect star-formation at lowest stellar masses (<108.5M), and the likely transition between starvation and ram-pressure stripping quenching in galaxies.


Such low mass mergers may be a critical component of the mass growth and feedback process in galaxies. So far, even GAMA fails to detect these low mass systems outside of the very local Universe. However, in combination WAVES-Wide and WAVES-Deep will probe the factors affecting the build-up of stellar mass in galaxies in both the low mass regime and to high redshift - robustly measuring galaxy merger rates, in-situ star-formation, passive fractions to low stellar masses and merger induced star-formation processes, as well as relating all of these factors to large scale environment. In addition, WAVES-UltraDeep will extend this analysis out to z~1.5, and put the most robust constraints on the merger rate of galaxies over the last 9 billion years.

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Fig. 1: Left-top: Current best estimates of the galaxy major merger rate as a function of redshift, WAVES will provide GAMA-like measurements out to z~1. Right-top: Cosmic star-formation history, WAVES will provide consistent and robust measurements to close to the peak in cosmic star-formation. Bottom: the dominant mass growth mechanism in galaxies star-formation vs mergers, measured from GAMA at z<0.2.