  The bimodality in galaxy properties has been observed at low and high
redshift, with a clear distinction between star-forming galaxies in the blue
cloud and passively evolving objects in the red sequence; the absence of
galaxies with intermediate properties indicates that the quenching of star
formation and subsequent transition between populations must happen rapidly. In
this paper, we present a study of over 100 transiting galaxies in the so-called
"green valley" at intermediate redshifts (z ~ 0.8). By using very deep
spectroscopy with the DEIMOS instrument at the Keck telescope we are able to
infer the star formation histories of these objects and measure the stellar
mass flux density transiting from the blue cloud to the red sequence when the
universe was half its current age. Our results indicate that the process
happened more rapidly and for more massive galaxies in the past, suggesting a
top-down scenario in which the massive end of the red sequence is forming
first. This represent another aspect of downsizing, with the mass flux density
moving towards smaller galaxies in recent times.
