  We present new MUSE observations of quasar field Q2131-1207 with a log
N(HI)=19.50+/-0.15 sub-DLA at z_abs=0.42980. We detect four galaxies at a
redshift consistent with that of the absorber where only one was known before
this study. Two of these are star forming galaxies, while the ones further away
from the quasar (>140 kpc) are passive galaxies. We report the metallicities of
the HII regions of the closest objects (12+log(O/H)=8.98+/-0.02 and
8.32+/-0.16) to be higher or equivalent within the errors to the metallicity
measured in absorption in the neutral phase of the gas (8.15+/-0.20). For the
closest object, a detailed morpho-kinematic analysis indicates that it is an
inclined large rotating disk with V_max=200+/-3 km/s. We measure the masses to
be M_dyn=7.4+/-0.4 x 10^10 M_sun and M_halo=2.9+/-0.2 x 10^12 M_sun. Some of
the gas seen in absorption is likely to be co-rotating with the halo of that
object, possibly due to a warped disk. The azimuthal angle between the quasar
line of sight and the projected major axis of the galaxy on the sky is 12+/-1
degrees which indicates that some other fraction of the absorbing gas might be
associated with accreting gas. This is further supported by the galaxy to gas
metallicity difference. Based on the same arguments, we exclude outflows as a
possibility to explain the gas in absorption. The four galaxies form a large
structure (at least 200 kpc wide) consistent with a filament or a galaxy group
so that a fraction of the absorption could be related to intra-group gas.
