Because galaxies are formed in the highest peak of matter density, they are naturally clustered, and form in local over density naturally. Indeed observations indicate that galaxies live preferentially in groups environment, and therefore groups must play a very important role in galaxy evolution (Cooper et.al 2006, Cucciati et. al 2005). To explain the way galaxies are shaped into their current appearances, and explain the observed relation between morphology and local density (Poggianti et al. 2008), we need to effectively separate two (combined) kind processes involved in galaxy evolution, we characterise as
”nature” and ”nurture”.
Natural, also called ”secular”, processes are related to the evolution of galaxies themselves, without any external trigger, such as Active Galactic Nuclei, star-formation bursts or bars in spiral galaxies. Such processes may quench star-formation and act on the morphological evolution passively, for instance reducing the size of the disc when star formation has stopped (e. g. Kovac et al. 2010).
Nurtural, also called ”peer”, processes are resulting from an external phenomenon, due to the interaction of the galaxy with other galaxies or its local environment. Major mergers (between galaxies of similar masses) for instance will directly affect the stellar kinematics and galactic structure, triggering a burst of star-formation and eventually nuclear activity (e. g. Kovac et al. 2010). Major mergers are more common in groups than in clusters, as the velocity dispersion of galaxies in clusters is to high to favourite direct interactions (Cavaliere et al. 1992). On the other hand, ram pressure is a pressure exerted on a body which is moving through a fluid medium (see figure 1), which is favorised by cluster environments (Gunn & Gott 1972). It causes a strong drag force to be exerted on the body, this pressure will strip cold gas when a galaxy infall a hot gas environment.
Harassment is phenomenon in cluster that pulls and drags between galaxy and galaxy, although they do not merge, but it lead to the gas of star formation that will be destroy
(Dressler et al. 1980).
The red fraction is defined as the ration between the number of red galaxies and total number of galaxies in a given structure (field, group or clusters). It is know since long time (e. g. Butcher & Oemler 1978) that red passive galaxies are more frequent in denser
environments, hence that the red fraction is higher in more dense environment. However it is also known that more massive galaxies are typical redder in the local Universe (Bell E.
F. et al., 2004) and that the most massive galaxies are located in denser environment (Davis & Geller et al. 1976) . There is therefore a degeneracy between nurtural and
natural effects. Then what is really need is to investigate the red fraction for similar stellar masses (as measured from long-wavelength Infrared photometry) and determine how this varies with redshifts. By comparing the same group mass at different redshift, a higher red fraction is observed in the local universe (Cooper et al. (2007)). However red galaxies present a strong clustering at z > 1.5 (Daddi et al. 2003; Quadri et al. 2007; Hartley et al.
2008; Hartley et al. 2010) which suggests that a colour-density relation may also exist at these higher redshifts. All these observations are in agreement with a ”Cosmic Downsizing”
scenario, in which more massive galaxies are evolving more early than less massive galaxies and therefore present a higher red fraction.
Figure 2, extracted from Kovac et al. (2010), illustrates well this mixed evolution between secular evolution and environment effect. Within the same panel, galaxies in group have higher red fraction, i.e. are more subject to transformation linked to quenching, than field and isolated galaxies. When comparing different panels with different luminosity bins, hence mass bins, massive group galaxies obviously present an earlier evolution than the galaxies of low mass.
As demonstrated by figure 3, our current understanding is that environment is acting on galaxy evolution as a catalyser, accelerating the evolution processes which will happen
anyway (Balogh et al. 2009).
Fig. 1.— From Steinhauser et al. (2012): This simulation presents the evolution with time of the Surface density of the Inter-stellar Medium (green) and of the newly formed stars (isolines) for a ram-pressure stripping scenario of model galaxy in an Intra-Cluster Medium with a density of 1027g.cm−3. Note the colorbars for both distributions. Four different time steps are shown. New stars in the wake are formed in the dense gas knots. After 200 Myr many stars are present also in front of the disc. The stars formed in the wake are gravitationally attracted by the disc and due to the collisionless dynamics, these stars from the wake are falling through the disc. Furthermore, with the initial onset of ram pressure, the gas disc is pushed back from the stellar disc, as can be seen in the timestep after 50 Myr of evolution in the ICM.
Fig. 2.— From Kovac et al. (2010): The redshift/look-back time evolution of the fraction of early type galaxies in different environments. The red and yellow symbols represent the group galaxies, black circles represent the field galaxies and the green triangles represent the isolated galaxies in the indicated MB-bins of galaxies.
Fig. 3.— From Balogh et al. (2009): Galaxies show a halo-mass dependence: Red fractions of groups intermediate between cluster and field environments.
2. Datasets
2.1. The UKIDSS-Ultra Deep Survey and Subaru-XMM Deep Survey The Ultra Deep Survey (UDS)1 is the deepest component of the UKIRT Infrared Deep Sky Survey (UKIDSS - Lawrence et al. 2007) and is the deepest near-infrared survey ever conducted over such a large area (0.8 sq. degrees - see figure 4). The aim of this survey is to understand how and when galaxies are formed and trace their evolution over the last 12 billion years. Using the WFCAM camera (Casali et al. 2007) on the UKIRT 4m telescope, the survey began in 2005 and continued until end of 2012. The Data Release 8 (DR8) used in this work is public worldwide since April 2012, and the latest release DR10 is public to ESO member since January 2013 and will be released worldwide in June 2014. The final release incorporating all data taken during the 7 years of the survey will be release at the end of 2013 to ESO. The DR8 data reach depths of JAB = 24.9, HAB = 24.2 and
KAB = 24.6, and comprises over two hundred thousand detected galaxies.
The UDS is centred on the Subaru/XMM-Newton Deep Survey (SXDS), in order to take advantage of the broad wealth of data available on this field. Indeed the SXDS is a major multi-wavelength survey of a 1.3 sq. degree region of sky observed using the SuprimeCam on the Subaru 8.2m telescope (Masanori et al. 2004). The SXDS optical imagery represents an unprecedented combination of depth and area coverage, and will be combined with suitably deep images at other wavelengths to provide an accurate census of the contents of the Universe without suffering from the biasing effects of large-scale structure. A total area of 1.22 sq. deg. is covered in five contiguous sub-fields, each of which corresponds to a single Suprime-Cam field of view (340× 270), in five broad-band filters B, V , Rc, i0 and z0, to the depths of B = 28.4, V = 27.8, Rc = 27.7, i0 = 27.7 and z0 = 26.6 (AB, 3σ, 2”
aperture). The data are reduced and compiled into five multi-waveband photometric
1http://www.nottingham.ac.uk/astronomy/UDS
catalogs, separately for each Suprime-Cam pointing. The i’-band catalogs contain about 900,000 objects, making the SXDS catalogs one of the largest multi-waveband catalogs in corresponding depth and area coverage. The SXDS catalogs can be used for an extensive range of astronomical applications such as the number density of the Galactic halo stars to the large scale structures at the distant universe (Furusawa et al. 2008).
We also use the u∗-band data from the CFHT-Megacam, with a magnitude limit of u∗ = 27 (AB, 3σ, 2” aperture). MegaCam is the wide-field optical imaging facility at CFHT, covering a full 1 × 1 sq. degree field-of-view with a resolution of 0.187 arcsecond per pixel, which is very efficient in the blue throughput (Boulade et al. 1998). The data were acquired in 2007-2008, during two programs lead by O. Almaini and S. Foucaud. In addition archival data were also added to these two campaigns. The average seeing of the image is < 0.9”.
Finally, the UDS Spitzer Legacy Program (SpUDS, PI:Dunlop) provides deep data in Mid-Infrared waveband with the channels 1 and 2 of IRAC, as well as Multiband Imaging Photometer for Spitzer (MIPS) 24µm data, all of which are used during our analysis.
SpUDS data reach 5σ depths of 24.2 and 24.0 (AB) at 3.6µm and 4.5µm respectively, while the public 24µm catalogue used here is limited to 300µJy (15σ).
The co-incident area of all these different data sets after masking of bad regions and bright stars is 0.62 sq. degrees.
In addition to our deep photometry, the UDS field was also the target of a unique
spectroscopic survey: the UDSz (Almaini et al., in preparation). The UDSz is based on an European Southern Observatory Large Programme targeting a large sample of galaxies (∼ 3500) at zphot > 1 with KAB < 23.0, plus a low-redshift control sample. The survey comprised eight pointings of VIsible MultiObject Spectrograph (VIMOS) in LR-Blue and LR-Red and 20 FORS2 masks with the GRS 300I grating. This survey has produced
∼ 1500 secure redshifts to date which are used along with archival redshift (of ∼ 4000 objects), details of which can be found in Simpson et al. (2012) and references therein.
Fig. 4.— The UKIDSS-UDS field, The UKIDSS-UDS field. This is a composite image of 3 bands (BzK). Zooming into a small section of the UDS field from K-band image. Light from many of the faint red galaxies has travelled over 12 billion light years to reach our telescopes. The right two fields, represent for the upper one the GOODS-South field, covered with VLT/ISAAC near-infrared imaging on areas of 172.5, 159.6 and 173.1 arcmin2 in J, H, and Ks bands, respectively; and for the lower one the FIRES field, which depths reach approximately 26.3, 25.8, 25.5 in J, H, and Ks, for a total coverage of about 23.6 arcmin2.
Fig. 5.— This figure show the spatial distribution of galaxies on UKIDSS-UDS field, this field included in the Subaru/XMM-Newton Deep Survey field, which lies at the centre of one of the DXS fields, the XMM-LSS field. Its total coverage is 0.8 square degrees, and we select KAB < 24.6.