The morphological evolution, AGN fractions, dust content, environments,
  and downsizing of massive green valley galaxies at 0.5<z<2.5 in
  3D-HST/CANDELS
  To explore the evolutionary connection among red, green, and blue galaxy
populations, based on a sample of massive ($M_* > 10^{10} M_{\odot} $) galaxies
at 0.5<z<2.5 in five 3D-HST/CANDELS fields, we investigate the dust content,
morphologies, structures, AGN fractions, and environments of these three galaxy
populations. Green valley galaxies are found to have intermediate dust
attenuation, and reside in the middle of the regions occupied by quiescent and
star-forming galaxies in the UVJ diagram. Compared with blue and red galaxy
populations at z<2, green galaxies have intermediate compactness and
morphological parameters such as Sersic index, concentration, Gini coefficient,
and the second order moment of the 20% brightest pixels of a galaxy. Above
findings seem to favor the scenario that green galaxies are at transitional
phase when star-forming galaxies are being quenched into quiescent status. The
green galaxies at z<2 show the highest AGN fraction, suggesting that AGN
feedback may have played an important role in star formation quenching. For the
massive galaxies at 2<z<2.5, both red and green galaxies are found to have a
similarly higher AGN fraction than the blue ones, which implies that AGN
feedback may help to keep quiescence of red galaxies at z>2. A significant
environmental difference is found between green and red galaxies at z<1.5.
Green and blue galaxies at z>0.5 seem to have similar local density
distributions, suggesting that environment quenching is not the major mechanism
to cease star formation at z>0.5. The fractions of three populations as
functions of mass support a "downsizing" quenching picture that the bulk of
star formation in more massive galaxies is completed earlier than that of lower
mass galaxies.
