Friends-of-friends galaxy group finder with membership refinement.
  Application to the local Universe
  Context. Groups form the most abundant class of galaxy systems. They act as
the principal drivers of galaxy evolution and can be used as tracers of the
large-scale structure and the underlying cosmology. However, the detection of
galaxy groups from galaxy redshift survey data is hampered by several
observational limitations.
  Aims. We improve the widely used friends-of-friends (FoF) group finding
algorithm with membership refinement procedures and apply the method to a
combined dataset of galaxies in the local Universe. A major aim of the
refinement is to detect subgroups within the FoF groups, enabling a more
reliable suppression of the fingers-of-God effect.
  Methods. The FoF algorithm is often suspected of leaving subsystems of groups
and clusters undetected. We used a galaxy sample built of the 2MRS, CF2, and
2M++ survey data comprising nearly 80000 galaxies within the local volume of
430 Mpc radius to detect FoF groups. We conducted a multimodality check on the
detected groups in search for subgroups. We furthermore refined group
membership using the group virial radius and escape velocity to expose unbound
galaxies. We used the virial theorem to estimate group masses.
  Results. The analysis results in a catalogue of 6282 galaxy groups in the
2MRS sample with two or more members, together with their mass estimates. About
half of the initial FoF groups with ten or more members were split into smaller
systems with the multimodality check. An interesting comparison to our detected
groups is provided by another group catalogue that is based on similar data but
a completely different methodology. Two thirds of the groups are identical or
very similar. Differences mostly concern the smallest and largest of these
other groups, the former sometimes missing and the latter being divided into
subsystems in our catalogue.
