Chris Martin et al. (2007) have studied this process by measuring how fast galaxies move from one end to the other. By using SDSS spectra, one can infer the star formation histories of individual galaxies. To this end, spectroscopical indices - more specifically, Dn(4000) and H_delta,A - were measured for a large number of green valley galaxies. While evolving, galaxies trace a path through the Dn(4000) x H_delta,A plane, and this path is strongly dependent on the speed through which star formation is quenched. By measuring masses and number densities of the green valley galaxies, one can infer how much mass is going through the valley at a given time.

Martin and collaborators have found that the mass going through the green valley is consistent with the build-up of the red sequence, with quenching timescales of a few hundred million years for most galaxies. In addition, they have found the fraction of AGN in the green valley is the highest across the color-magnitude plot, which could indicate active nuclei play a prominent role in quenching star formation.

The galaxies studied by Martin et al. are at z~0.2. However, we know the process of galaxy formation and evolution is strikingly different across cosmic time, from z~10 (or more) until today, and that most stars in the present universe were formed at z~2-3. Therefore, one would expect the quenching to be different at different ages of the cosmos.

For my thesis, I have obtained hundred of spectra of green valley galaxies at redshifts 0.55 < z < 0.9, to be compared with the previous low-z results. However, to study absorption lines at those distances, very deep data is needed - and this is achieved by taking 8-hr spectra with the 10-m Keck telescope, with the DEIMOS instrument. This will allow for a measurement of star formation history of galaxies when the universe was half its present age.