X-ray observations of hot, intergalactic gas in galaxy groups provide a
useful means of characterizing the global properties of groups. However, X-ray
studies of large group samples have typically involved very shallow X-ray
exposures or have been based on rather heterogeneous samples. Here we present
the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for
the first time, a redshift-selected, statistically unbiased sample of galaxy
groups using deep X-ray data. Combining this with radio observations of cold
gas and optical imaging and spectroscopy of the galaxy population, the project
aims to advance the understanding of how the properties and dynamics of group
galaxies relate to global group properties. Here, X-ray and optical data of the
first four galaxy groups observed as part of the project are presented. In two
of the groups we detect diffuse emission with a luminosity of L_X ~ 10^41
erg/s, among the lowest found for any X-ray detected group thus far, with a
comparable upper limit for the other two. Compared to typical X-ray selected
groups of similar velocity dispersion, these four systems are all surprisingly
X-ray faint. We discuss possible explanations for the lack of significant X-ray
emission in the groups, concluding that these systems are most likely
collapsing for the first time. Our results strongly suggest that, unlike our
current optically selected sample, previous X-ray selected group samples
represented a biased picture of the group population. This underlines the
necessity of a study of this kind, if one is to reach an unbiased census of the
properties of galaxy groups and the distribution of baryons in the Universe.Comment: 14 pages, 8 figures, accepted for publication in MNRA
