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

Abstract

We present Giant Metrewave Radio Telescope HI 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 HI 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 HI feature, AGC 749170, that has a gas mass of ~109.3M⊙, 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/Lg > 1000M⊙/L⊙ 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 HI 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 HI-rich tidal remnants that survive in intermediate-density environments. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.