Decomposing the growth mechanisms of galaxies over the last 10 billion years

Abstract

Determining how galaxies accumulate stellar mass is paramount to understanding the Universe. Two primary mechanisms drive this process: star-formation (SF) & mergers. Our understanding of star formation, and to some degree the processes that influence the baryon cycle (environment, gas supply, feedback, etc), are either relatively well constrained or will develop significantly over the coming decades via upcoming facilities (i.e. through their imprint on galaxy properties measured with deep multi-wavelength and spectroscopic data). However, the same can not be said for mergers. It is telling that we indirectly know hierarchical assembly through mergers is one of the most crucial processes that shape our Universe, but the robust observational measurement of mergers is almost non-existent outside of the local Universe - let alone how these mergers impact galaxy properties. This is not likely to significantly change in the coming decades as existing or approved facilities/surveys are inadequate in charactering mergers in the distant Universe. Motivated by this, we discuss an ambitious study to first explore mergers, and then the co-dependent astrophysical process that govern the accumulation of stellar mass over the last ~10billion years, and highlight the essential need for a 10m+ class multi-object spectroscopic facility.

Figures (2)

• Figure 1: Overview of the methodology used to identify the key drivers of stellar mass growth in galaxies. Left: Galaxies grow via star-formation and mergers, which rate is regulated by many astrophysical processes (e.g., AGN/SNe feedback, environmental quenching etc). Middle: By combining a deep redshift survey with synergistic studies from e.g. SKA we can measure the stellar mass growth and gas depletion rates over a time interval, $\Delta$t, decomposed into distinct astrophysical mechanisms. Right: If all processes are accounted for, the sum of these should account for the evolution in the SMF and GMF. Credit galaxy images: NASA
• Figure 2: Left: Robust measurements of even the major-merger M* close-pair merger fraction are almost non-existent outside of the local Universe. Middle: We can only extend this to minor mergers at z$\sim$0. Right: proposed sample selection which would revolutionise our understanding of mergers (and many other science areas).