Green Valley Galaxies
  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.
