Updates: 11.05.2012 (Aliş)
3.3 Galaxy Clusters
As the most massive bounded objects in the universe, galaxy clusters have a special interest in observatinal cosmology studies. The main research projects can be gathered in three different groups:
- Constraining the cosmological parameters and dark energy
- Structural properties, scaling laws and structure formation
- Internal physics and connection to stellar activity
The state of the art cosmological simulations, like The Millennium Simulation (produced mainly by MPA and ICC (Springel et al. 2005)), show that clusters result from the hierarchical structure formation. Besides enabling the undertanding of internal physics of clusters, numerical simulations play an important role in the determination of selection function of any cluster sample. It became a standard approach today producing numerical simulations before the observations. Comparisons between the simulations and observations give clues about the fine-tuning in the physics as well as technical capabilities and systematics.
Beyond their cosmological interest, galaxy clusters often considered as laboratories because they are closed boxes where all their gaseous matter retained due to high potential well. Main components of clusters are galaxies and the intracluster medium (ICM). In the standard CDM scenario these baryonic components consist the 20% of the total mass in clusters. The remaining part is attributed to the so-called Dark Matter. First claims about such matter was belong to Zwicky (1933) where he noticed the difference between the light and the implied mass in clusters. The very famous Bullet Cluster (1E 0657-558) is a perfect example to investigate different components in clusters, especially Dark Matter. Properties of galaxies within clusters help to understand better the galaxy formation and evolution. With increasing redshift blue galaxies appear more in clusters (Butcher-Oemler effect). Detailed studies of individual clusters are then important to understand evolution of galaxies in clusters. There are many mechanisms (e.g. ram pressure stripping, tidal mergers, harassment) proposed for galaxy interaction in clusters lead to an evolution. This is known as the morphology-density relation of galaxies implies denser environments show more interaction between the member galaxies in clusters (Dressler, 1980).
Interaction related to galaxy clusters not only seen between the member galaxies it is also seen between clusters as merging events. Cluster merging provides information on many physical processes seen in clusters. These events can be studied in the optical, radio and X-rays (Ferrari et al., 2006; Ferrari et al., 2003; Maurogordato et al., 2008; Bourdin et al., 2011). Cluster merging and internal interactions within the clusters are important for star formation studies (Poggianti et al., 2004; Ferrari et al., 2005; Ferrari et al., 2005).
The central galaxies in clusters (aka Brightest Cluster Galaxies - BCG) are also very important to understand and constrain galaxy formation. Hiearchical structure formation fails or not well explains BCGs. Formation of these galaxies mainly happen in the form of galaxy infalls and tidal mergers where they gain gas in some cases. Therefore investigating star formation histories in clusters can be possible with studying BCGs (Quillen et al. 2008; O'Dea et al. 2008; Hicks et al. 2010; Edge et al. 2010). Thanks to the large surveys BCGs spread over a wide range of redshift are known and can be studied.
The first approaches to study galaxy clusters were started with inspecting the Schmidt plates obtained at Palomar Observatory. These efforts was based on visual inspection where today with wide field CCD imagers multi-band observations and cluster detections are done automatically. ESO Imaging Survey (EIS), Sloan Digital Sky Survey (SDSS), 2dF (Two Degree Field Survey), Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) are the most important digital surveys designed for galaxy cluster studies or enabled those studies in the modern era.
These various aspects described briefly above show the need for large, multi-band, homogeneous samples to study clusters as large scale structures of the universe. Starting from SDSS it became possible and will become easier with the recent and future planned surveys. Studying evolution of galaxy clusters needs to go deep where it is needed to extent recent optical multi-band surveys into the IR. The current deepest survey for galaxy clusters, CFHTLS, has just started new programs to add IR data to the survey fields. For this purpse WIRCAM observations started and planned at CFHT under two different programs: CFHQSIR and WIRCAM Deep Survey. The DAG Project becomes even more important in this aspect. VLT VIRMOS is also available at the moment to study extensively clusters depending on time allocation. There are ~69.000 galaxy clusters detected from the SDSS DR6 (Szabo et al. 2011) in 9600 square degrees which makes 7 cluster per sq.degree. On the contrary in CFHTLS ~3500 clusters detected in 174 sq.deg. (Olsen et al., 2007; Olsen et al., 2007; Benoist 2012) which makes 20 cluster per sq. degree. The difference comes from the depth of the CFHTLS which reaches z=1.2-1.3. Certainly, the next generation surveys (e.g. LSST) will change this view significantly.
Benoist et al., in prep
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What can be done with DAG
- Brightest cluster galaxies can be observed in IR to determine star formation rates.
- Metallicity of the ICM can give clues about the evolution of clusters.
- MOS spectroscopy of the member galaxies lead to the mass determination and obtain the dynamical status of clusters.
- With the assumption of a FOV of 30 arcmin it is possible to capture clusters in the redshift range of z>0.05. Which means that with the telescope implied here we cannot do imaging (single shot) of nearby clusters such as Virgo, Fornax, Centaurus, Hydra.