Green Valley Galaxies

TL;DR

The paper reviews the properties and evolution of green valley galaxies, emphasizing their role in understanding star formation quenching and resurgence, and discusses biases and new evolutionary scenarios based on UV and optical data.

Contribution

It provides a comprehensive review of green valley galaxy characteristics, biases in their definition, and introduces a quasi-static evolutionary model based on recent evidence.

Findings

Green valley galaxies are mostly in a slow decline of star formation.

Optical colors can bias the identification of green valley galaxies.

A new quasi-static evolutionary scenario is proposed.

Abstract

The "green valley" is a wide region separating the blue and the red peaks in the ultraviolet-optical color magnitude diagram, first revealed using GALEX UV photometry. The term was coined by Christopher Martin in 2005. Green valley highlights the discriminating power of UV to very low relative levels of ongoing star formation, to which the optical colors, including u-r, are insensitive. It corresponds to massive galaxies below the star-forming "main" sequence, and therefore represents a critical tool for the study of the quenching of star formation and its possible resurgence in otherwise quiescent galaxies. This article reviews the results pertaining to morphology, structure, environment, dust content and gas properties of green valley galaxies in the local universe. Their relationship to AGN is also discussed. Attention is given to biases emerging from defining the "green valley" using optical colors. We review various evolutionary scenarios and we present evidence for a new, quasi-static view of the green valley, in which the majority of galaxies currently in the green valley were only partially quenched in the distant past and now participate in a slow cosmic decline of star formation, which also drives down the activity on the main sequence, presumably as a result of the dwindling accretion/cooling onto galaxy disks.