An extremely optically dim tidal feature in the gas-rich interacting galaxy group NGC 871/NGC 876/NGC 877

We present Giant Metrewave Radio Telescope H i observations and deep Canada–France–Hawaii Telescope (CFHT) MegaCam optical images of the gas-rich interacting galaxy group NGC 871/NGC 876/NGC 877 (hereafter NGC 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, AGC 749170, that has a gas mass of ∼109.3 M⊙, a dynamical-to-gas mass ratio of ∼1 (assuming that 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 ′ and r ′ bands; if it is physically associated with AGC 749170, the latter has $M/L_{\rm g} >1000 \,\mathrm{M}_{{\odot }}/\mathrm{L}_{{\odot }}$ 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 AGC 749170. Overall, the H i properties of AGC 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.