Abstract
We explore the role of environment in the evolution of galaxies over 0.1<z<0.7 using the final zCOSMOS-bright data set. Using the red fraction of galaxies as a proxy for the quenched population, we find that the fraction of red galaxies increases with the environmental overdensity and with the stellar mass, consistent with previous works. As at lower redshift, the red fraction appears to be separable in mass and environment, suggesting the action of two processes: mass and environmental quenching. The parameters describing these appear to be essentially the same at z~0.7 as locally. We explore the relation between red fraction, mass and environment also for the central and satellite galaxies separately, paying close attention to the effects of impurities in the central-satellite classification and using carefully constructed samples matched in stellar mass. There is little evidence for a dependence of the red fraction of centrals on overdensity. Satellites are consistently redder at all overdensities, and the satellite quenching efficiency increases with overdensity at 0.1<z<0.4. This is less marked at higher redshift, but both are nevertheless consistent with the equivalent local measurements. At a given stellar mass, the fraction of galaxies that are satellites also increases with the overdensity. At a given overdensity and mass, the obtained relation between the environmental quenching and the satellite fraction agrees well with the satellite quenching efficiency, demonstrating that the environmental quenching in the overall population is consistent with being entirely produced through the satellite quenching process at least up to z=0.7. However, despite the unprecedented size of our high redshift samples, the associated statistical uncertainties are still significant and our statements should be understood as approximations to physical reality, rather than physically exact formulae.
Highlights
Correlations between various galaxy properties and their environment have been known for many years (e.g. Hubble 1939; Oemler 1974; Davis & Geller 1976; Dressler 1980) and have been measured up to redshifts of about z = 1 − 1.5 (e.g. Balogh et al 2004; Kauffmann et al 2004; Blanton et al 2005; Hogg et al 2004; Cucciati et al 2006, 2010; Cooper et al 2007, 2010; Kovac et al 2010b; Chuter et al 2011; Quadri et al 2012)
We explore the relation between red fraction, mass and environment for the central and satellite galaxies separately, paying close attention to the effects of impurities in the central-satellite classification and using carefully constructed samples well-matched in stellar mass
The stellar masses which we use are obtained by integrating the star formation rate (SFR) and they include the mass of gas processed by stars and returned to the interstellar medium
Summary
Mergers of galaxies are expected to be more frequent in dense (but not the most dense) environments, and according to the semi-analytical models (SAMs), the merger rate is 4-20 times higher in regions with a high number overdensity of galaxies than in regions with a ∼ 100 lower number overdensity (Jian et al 2012) It is clear from the list above that, from a theoretical point of view, the majority if not all of the environmental differences in the overall galaxy population may be caused by the transformation of galaxies after they infall into a larger halo, i.e. after they become a satellite. Peng et al (2010) suggested from an initial analysis of the first half of the zCOSMOS-bright survey that separability was maintained to z ∼ 1 and that the differential effects of stellar mass and environment, i.e. the mass and environmental quenching efficiencies were more or less constant. A sampling rate of ∼ 50% is achieved (e.g. Figure 1 in Knobel et al 2012a) with the redshift uncertainty about 100 km s−1
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