Galaxy and Mass Assembly (GAMA): The halo mass of galaxy groups from
  maximum-likelihood weak lensing
  We present a maximum-likelihood weak lensing analysis of the mass
distribution in optically selected spectroscopic Galaxy Groups (G$^3$Cv5) in
the Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital Sky
Survey (SDSS) photometric galaxies. The scaling of halo mass, $M_h$, with
various group observables is investigated. Our main results are: 1) the
measured relations of halo mass with group luminosity, virial volume and
central galaxy stellar mass, $M_\star$, agree very well with predictions from
mock group catalogues constructed from a GALFORM semi-analytical galaxy
formation model implemented in the Millennium $\Lambda$CDM N-body simulation;
2) the measured relations of halo mass with velocity dispersion and projected
half-abundance radius show weak tension with mock predictions, hinting at
problems in the mock galaxy dynamics and their small scale distribution; 3) the
median $M_h|M_\star$ measured from weak lensing depends more sensitively on the
lognormal dispersion in $M_\star$ at fixed $M_h$ than it does on the median
$M_\star|M_h$. Our measurements suggest an intrinsic dispersion of
$\sigma_{\log(M_\star)}\sim 0.15$; 4) Comparing our mass estimates with those
in the catalogue, we find that the G$^3$Cv5 mass can give biased results when
used to select subsets of the group sample. Of the various new halo mass
estimators that we calibrate using our weak lensing measurements, group
luminosity is the best single-proxy estimator of group mass.
