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Australian Human Resources Institute Diversity Awards 2013 Finalist

Ned Taylor (USyd)

"Galaxy And Mass Assembly (GAMA): Deconstructing Bimodality -- Red ones and blue ones, live ones and dead ones, round ones and flat ones"

Abstract. It has long been understood that there is an apparently fundamental dichotomy between 'developing' galaxies -- the blue, star forming spirals, which tend to be less massive, and to be live in isolation or in small groups -- and the 'developed' galaxies -- the massive, red and dead ellipticals, which dominate the population in cluster environments. But there is still no good theoretical understanding of when, how, or why galaxies make the evolutionary transition from developing to developed. I will present new observational constraints on how this process procedes. Using the technique of mixture modelling, which provides a means for objective classification, I have characterised the properties of the red/blue, quiescent/active, and spheroid-/disk-like galaxy populations. These results are based on a z < 0.12 sample of log M* > 8.7 Msol galaxies from the GAMA survey. Based on these results, I look at the overlaps between these different populations. The basic observational result is that all quiescent galaxies are also red, and all red galaxies are also bulge-like. There is no overlap between the red and the disk-like populations, nor between the blue and the quiescent populations. This implies an order of events. Galaxies go from disk-like to bulge-like *first*; that is, the structural transition happens on the blue sequence. Only after this process has completed do galaxies migrate from the blue to the red populations, while remaining active. And only then do galaxies become quiescent: that is, quenching only happens to red sequence galaxies. This empirical picture presents a major challenge to idea of AGN feedback being the mechanism behind star-formation quenching.

Held in the AAO Large Meeting Room (Room 1, Ground Floor, Building 3) at 11:00 AM on Tuesday, 06 November 2012