Exploring a new definition of the green valley and its implications

The distribution of galaxies on a colour-magnitude diagram reveals a bimodality, featuring a passively evolving red sequence and a star-forming blue cloud. The region between these two, the green valley (GV), represents a fundamental transition where quenching processes operate. We exploit an alternative definition of the GV using the 4000 angstrom break strength, an indicator that is more resilient than colour to dust attenuation. We compare and contrast our GV definition with the traditional one, based on dust-corrected colour, making use of data from the Sloan Digital Sky Survey. Our GV selection - that does not need a dust correction and thus does not carry the inherent systematics - reveals very similar trends regarding nebular activity (star formation, AGN, quiescence) to the standard dust-corrected (0.1)(g - r). By use of high-SNR stacked spectra of the quiescent GV subsample, we derive the simple stellar population (SSP) age difference across the GV, a rough proxy of the quenching time-scale (Delta t). We obtain an increasing trend with velocity dispersion (sigma), from Delta t similar to 1.5 Gyr at sigma = 100 km s(-1), up to 3.5 Gyr at sigma = 200 km s(-1), followed by a rapid decrease in the most massive GV galaxies (Delta t similar to 1 Gyr at sigma = 250 km s(-1)), suggesting two different modes of quenching, or the presence of an additional channel (rejuvenation).