Galaxy groups in the low-redshift Universe
  We apply a halo-based group finder to four large redshift surveys, the 2MRS,
6dFGS, SDSS and 2dFGRS, to construct group catalogs in the low-redshift
Universe. The group finder is based on that of Yang et al. but with an improved
halo mass assignment so that it can be applied uniformly to various redshift
surveys of galaxies. Halo masses are assigned to groups according to proxies
based on the stellar mass/luminosity of member galaxies. The performances of
the group finder in grouping galaxies according to common halos and in halo
mass assignments are tested using realistic mock samples constructed from
hydrodynamical simulations and empirical models of galaxy occupation in dark
matter halos. Our group finder finds $\sim 94\%$ of the correct true member
galaxies for $90-95\%$ of the groups in the mock samples; the halo masses
assigned by the group finder are un-biased with respect to the true halo
masses, and have a typical uncertainty of $\sim0.2\,{\rm dex}$. The properties
of group catalogs constructed from the observational samples are described and
compared with other similar catalogs in the literature.
