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.