GREEN GALAXIES IN THE COSMOS FIELD

Abstract

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 $z<0.5$.