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

TL;DR

This paper reports the discovery of a unique, optically dim tidal HI feature in a gas-rich galaxy group, suggesting it is a long-lived tidal remnant with distinct properties from typical galaxies.

Contribution

It provides high-resolution HI and optical observations revealing a new optically dim tidal feature with detailed properties, indicating its likely tidal origin and long-term survival in the environment.

Findings

Discovery of a faint, optically dim HI feature AGC 749170.

AGC 749170 has a high gas mass (~10^9.3 M_sun) and no previous optical counterpart.

Properties suggest a tidal origin and possible long-lived nature.

Abstract

We present GMRT HI observations and deep 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 ~10^9.3 M_sol, a dynamical-to-gas mass ratio of ~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'- and r'-bands; if it is physically associated with AGC 749170, the latter has M/L_g > 1000 M_sol/L_sol 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.