We present GMRT H i observations and deep CFHT MegaCam optical images of the gas-rich interacting galaxy group NGCÂ 871/NGCÂ 876/NGCÂ 877 ( hereafter NGCÂ 871/6/7 ) .
Our high-resolution data sets provide a census of the H i and stellar properties of the detected gas-rich group members .
In addition to a handful of spiral , irregular and dwarf galaxies , this group harbours an intriguing H i feature , AGCÂ 749170 , that has a gas mass of \sim 10 ^ { 9.3 } M _ { \odot } , a dynamical-to-gas mass ratio of \sim 1 ( assuming the cloud is rotating and in dynamical equilibrium ) and no optical counterpart in previous imaging .
Our observations have revealed a faint feature in the CFHT g ^ { \prime } - and r ^ { \prime } -bands ; if it is physically associated with AGCÂ 749170 , the latter has M / L _ { g } > 1000 M _ { \odot } / L _ { \odot } as well as a higher metallicity ( estimated using photometric colours ) and a significantly younger stellar population than the other low-mass gas-rich group members .
These properties , as well as its spectral and spatial location with respect to its suspected parent galaxies , strongly indicate a tidal origin for AGCÂ 749170 .
Overall , the H i properties of AGCÂ 749170 resemble those of other optically dark/dim clouds that have been found in groups .
These clouds could represent a class of relatively long-lived H i -rich tidal remnants that survive in intermediate-density environments .