GEMS Sample Selection

It is the intention of this work to assemble the largest sample of galaxy groups for which a detailed X-ray analysis could feasibly be performed. We therefore compiled a list of 4320 groups from 14 optical catalogues (shakhbazian 1973, Hickson 1982, Huchra & Geller 1982, Geller & Huchra 1983, Maia 1989, Fouque et al. 1992, Garcia 1993, Nolthenius 1993, Prandoni et al. 1994, Barton et al. 1996, Ramella et al. 1997, Trasarti 1998, Giudice 1999, White et al. 1999) and compared it to the ROSAT PSPC observation log. Groups were then selected from this list so as to meet the following criteria:

A group position within 20' of the pointing co-ordinates.

An exposure time t > 10000 s.

A recessional velocity 1000 < V < 3000 km/s.

This was to ensure that we received enough photon counts to carry out a detailed analysis and that the X-ray emission was neither too close to fill the PSPC field of view nor too distant to be detected. Duplicate entries resulting from the overlap between catalogues were removed, along with 7 groups in or around the Hydra and Virgo clusters, and 1 group with no optical membership. To our sample of 35 selected groups we added a further 25 which had been previously studied with the PSPC (Helsdon & Ponman 2000, Ponman et al. 1996) for comparison purposes.

It is important when constructing a sample such as this to identify, and take into account, any sources of biasing. Selection effects in our sample may originate from either the ROSAT pointing agenda or the sample of Helsdon & Ponman 2000, from where our groups have been taken. It is not clear what biasing may be inherent in the ROSAT pointing agenda, but the sample of Helsdon was chosen so as to include only the brightest X-ray groups. There will therefore be a tendency towards brighter X-ray emission in the groups we have chosen. Details of the final 60 groups in our sample can be found in the GEMS catalogue table where the names given are those of the brightest, and most central galaxy.