In order to comprehend the impact that the various forms of the feedback process have in groups of galaxies, an unbiased sample is essential. With the majority of studies to date being focused only on the obvious places where the feedback mechanism would take place, on jet/cavity systems in massive clusters of galaxies, one important environment where feedback would have the greatest impact on galaxy evolution and formation is left out: groups of galaxies.
Galaxy groups are the environment where most of the galaxies reside (e.g., Eke et al., 2004). The fact that galaxies in groups are into close proximity, ignites more easily pro- cesses such as galaxy interactions. Thus the understanding of feedback requires the close investigation of the processes involved in a variety of groups, as the governing process of galaxy dynamics in groups is still unknown (e.g., Gaspari et al., 2011; McCarthy et al., 2010; Randall et al., 2011). The gap in our knowledge for galaxy groups is also enhanced by the fact that to date there is no complete radio/X-ray sample of nearby groups making automatically the study of AGN feedback in groups a necessary, but also a very difficult, task. The samples that are considered to be the best available for galaxy groups to date, are selected in such a manner (e.g., Osmond & Ponman, 2004; Rasmussen & Ponman, 2007; Sun, 2009) that are ‘vulnerable’ to any potential biases that would most probably lead to a misinterpretation of any results drawn from them (O’ Sullivan et al., 2014).
For example, X-ray selected samples until recently were based on the shallow obser- vations of ROSAT All-Sky Survey (RASS), which is known for its bias at low luminosities to centrally-concentrated cool core systems (Eckert et al. 2011, discussion about cool-core bias in cluster samples). A similar cool-core bias manifests itself in cluster samples selected by virtue of the Sunyaev-Zel’dovich effect, since the latter preferentially picks up centrally condensed systems (Lin et al., 2015): group samples based on the S-Z effect are not feasible with current technology anyway. While the detection of diffuse X-rays ensures the presence of a substantial dark halo binding the group or cluster together, optically selected group cat- alogues (e.g. Berlind et al. 2006; Eke et al. 2004) suffer from the limitation that a significant fraction of the groups selected by percolation or friends-of-friends like algorithms turn out to be chance superpositions, or systems in the early stages of collapse.
The Complete Local-Volume Groups Sample (CLoGS) project targets to fill this gap for galaxy groups and become the first statistically complete survey of galaxy groups observed in the X-ray, optical and radio wavelengths (http://www.sr.bham.ac.uk/ ejos/CLoGS.html). The main science goals of this project in which I am performing the radio data analysis include:
• Investigate the X-ray temperature of groups and their hot halo filled with gas (Sander- son, Ponman, & O’Sullivan, 2006).
• Characterise the population of AGN in groups, and investigate their impact on their environment.
• Study the process of star formation, the radio activity presented in the core of central galaxies and the statistics concerning these two processes of the overall population of galaxies in groups.
1.6.1
Sample
X-ray or optical catalogues utilised by most studies of galaxy groups so far were inserting biases in their own way. Systems that were not yet verified as formed groups were most certainly included by optically selected group catalogues while the low number of galaxies that most optically selected samples had was making this method not trustworthy for safe results (O’ Sullivan et al., 2014). On the other hand, the X-ray selection of samples of galaxies relies in the nearby Universe on the ROSAT All-Sky Survey (RASS), which has a preference in ‘picking up’ more cool core systems (Eckert et al., 2011). This bias is also obvious from the results of many statistical and representative samples of cluster galaxies. For example, in Sanderson et al. (2006) it is found that half of the galaxies in the sample are characterised as cool-core systems whereas the equivalent finest sample that exists for groups of galaxies (but non statistical), reveals that only ∼20% of galaxies are non cool-core (Dong et al., 2010). Lastly, most of the observations that were performed for most of the X-ray catalogues to date, were not deep enough in order to avoid the non detection of fainter X-ray systems.
The only method to certify that a group is formed, is the detection of the hot intra-group medium from the X-rays. Combining systems selected using this method with optical and radio detections, will give the opportunity for the identification and investigation of more AGN and star forming galaxies in the group environment (O’ Sullivan et al., 2014).
The CLoGS sample consists of 53 candidate groups that reside in the local Universe (≤80 Mpc) and aims to be the first carefully selected optical sample of groups with high- quality X-ray and radio data. X-ray data are from Chandra and XMM-Newton satellite telescopes that allow us to have accurate group temperatures and detailed information of the hot group gaseous haloes. On the other hand, the detection of older, radio AGNs, will be
Figure 1.3 Locations of the CLoGS groups among the filamentary structures of the lo- cal volume, projected on the sky. Blue circles mark member galaxies, red points are dominant ellipticals and all other galaxies in the local volume are marked in black [http://www.sr.bham.ac.uk/ ejos/CLoGS_Sample.html].
possible with low-frequency radio data from the Giant Metrewave Radio Telescope (GMRT) that is used to observe the groups using for the observations two frequencies, at 235 and 610 MHz, with average time spent on source of 3-4 hours. The sample has also been selected to overlap with the NVSS 1.4 GHz and TGSS 150 MHz surveys so more multifrequency data will need to be analysed in the future.
In order for our sample selection to be secure we also include a threshold determined by the richness parameter R. As R we set the number of galaxies with log LB ≥ 10.2 within
1 Mpc and 3σ (σ =mean group velocity) of the BGE. Systems then that have richness pa- rameter greater than 10 are excluded from our sample as they end up being clusters already known, and on the other side, six groups that have R = 1 are also excluded, as they are not rich enough to give a trustworthy determination of their physical parameters. Finally, in a total of 53 groups, we divided the CLoGS groups into two statistically complete sub-
samples: i) the 26 high richness groups sub-sample with R = 4−8 and ii) the 27 low richness groups sub-sample with R = 2−3.
Lastly, GMRT observations of the full 53-group sample have been succesfully finished, while in the X-ray only the high-richness subsample of 26 groups is observed, with a limit- ing sensitivity of L0.5−0.7KeV> 1.2×1042 erg s−1.