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 4,000 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 timescale ($\Delta$t). We obtain an
increasing trend with velocity dispersion ($\sigma$), from
$\Delta$t$\sim$1.5Gyr at $\sigma$=100km/s, up to 3.5Gyr at $\sigma$=200km/s,
followed by a rapid decrease in the most massive GV galaxies
($\Delta$t$\sim$1Gyr at $\sigma$=250km/s), suggesting two different modes of
quenching, or the presence of an additional channel (rejuvenation).
