  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.
