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We present a maximum-likelihood weak lensing analysis of the massdistribution in optically selected spectroscopic Galaxy Groups (G$^3$Cv5) inthe Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital SkySurvey (SDSS) photometric galaxies. The scaling of halo mass, $M_h$, withvarious group observables is investigated. Our main results are: 1) themeasured relations of halo mass with group luminosity, virial volume andcentral galaxy stellar mass, $M_\star$, agree very well with predictions frommock group catalogues constructed from a GALFORM semi-analytical galaxyformation model implemented in the Millennium $\Lambda$CDM N-body simulation;2) the measured relations of halo mass with velocity dispersion and projectedhalf-abundance radius show weak tension with mock predictions, hinting atproblems in the mock galaxy dynamics and their small scale distribution; 3) themedian $M_h|M_\star$ measured from weak lensing depends more sensitively on thelognormal 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 thosein the catalogue, we find that the G$^3$Cv5 mass can give biased results whenused to select subsets of the group sample. Of the various new halo massestimators that we calibrate using our weak lensing measurements, groupluminosity is the best single-proxy estimator of group mass.

Galaxy and Mass Assembly (GAMA): The halo mass of galaxy groups from maximum-likelihood weak lensing