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The Evolution of Galaxies at Constant Number Density: A Less Biased View of Star Formation, Quenching, and Structural Formation

Due to significant galaxy contamination and impurity in stellar mass selected samples (up to 95% from z=0-3), we examine the star formation history, quenching time-scales, and structural evolution of galaxies using a constant number density selection with data from the UKIDSS Ultra-Deep Survey field. Using this methodology we investigate the evolution of galaxies at a variety of number densities from $z=0-3$. We find that samples chosen at number densities ranging from $3\times10^{-4}$ to 10$^{-5}$ galaxies Mpc$^{-3}$ (corresponding to $z\sim0.5$ stellar masses of M$_{*}= 10^{10.95-11.6}$ M$_{0}$) have a star forming blue fraction of $\sim50$\% at $z\sim2.5$, which evolves to a nearly $100$\% quenched red and dead population by $z\sim 1$. We also see evidence for number density downsizing, such that the galaxies selected at the lowest densities (highest masses) become a homogeneous red population before those at higher number densities. Examining the evolution of the colours for these systems furthermore shows that the formation redshift of galaxies selected at these number densities is $z_{\rm form}>3$. The structural evolution through size and Sersic index fits reveal that while there remains evolution in terms of galaxies becoming larger and more concentrated in stellar mass at lower redshifts, the magnitude of the change is significantly smaller than for a mass selected sample. We also find that changes in size and structure continues at $z < 1$, and is coupled strongly to passivity evolution. We conclude that galaxy structure is driving the quenching of galaxies, such that galaxies become concentrated before they become passive.