# Exploring a new definition of the green valley and its implications

## TL;DR

This study introduces a dust-insensitive method to define the Green Valley in galaxy evolution, revealing insights into quenching timescales and suggesting multiple quenching mechanisms based on galaxy velocity dispersion.

## Contribution

It proposes an alternative Green Valley definition using 4,000 Angstrom break strength, reducing dust-related systematics and providing new insights into galaxy quenching processes.

## Key findings

- Green Valley definition using 4,000 Angstrom break strength aligns with traditional dust-corrected colour methods.
- Quenching timescales increase with velocity dispersion up to a point, then decrease in the most massive galaxies.
- Evidence suggests multiple quenching modes or channels, including rejuvenation in massive galaxies.

## Abstract

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).