Green galaxies in the COSMOS field
  We present a research of morphologies, spectra and environments of $\approx$
2350 "green valley" galaxies at $0.2<z<1.0$ in the COSMOS field. The bimodality
of dust-corrected \nuvr\ color is used to define "green valley" (thereafter,
GV), which removes dusty star-forming galaxies from truly transiting galaxies
between blue cloud and red sequence. Morphological parameters of green galaxies
are intermediate between those of blue and red galaxy populations, both on the
Gini--Asymmetry and the Gini--M$_{\rm 20}$ planes. Approximately 60% to 70%
green disk galaxies have intermediate or big bulges, and only 5% to 10% are
pure disk systems, based on the morphological classification with Zurich
Estimator of Structural Types (ZEST). The obtained average spectra of green
galaxies are intermediate between blue and red ones in terms of \oii\,,
H$\alpha$ and H$\beta$ emission lines. Stellar population synthesis on the
average spectra show that green galaxies are averagely older than blue
galaxies, but younger than red galaxies. Green galaxies have similar projected
galaxy density ($\Sigma_{10}$) distribution with blue galaxies at $z>0.7$. At
$z<0.7$, the fractions of $M_{\ast}<10^{10.0}M_{\sun}$ green galaxies located
in dense environment are found to be significantly larger than those of blue
galaxies. The morphological and spectral properties of green galaxies are
consistent with the transiting population between blue cloud and red sequence.
The possible mechanisms for quenching star formation activities in green
galaxies are discussed. The importance of AGN feedback cannot be well
constrained in our study. Finally, our findings suggest that environment
conditions, most likely starvation and harassment, significantly affect the
transformation of $M_{\ast}<10^{10.0}M_{\sun}$ blue galaxies into red galaxies,
especially at $z<0.5$.
