Evidence of ram-pressure stripping of WLM, a dwarf galaxy far away from any large host galaxy

Abstract

Dwarf galaxies are affected by all the evolutionary processes normally at work in galaxies of any mass. As fainter and less massive galaxies, however, dwarf galaxies are particularly susceptible to environmental mechanisms that can more easily perturb these systems. Importantly, the presence of nearby large galaxies is expected to have a profound effect on dwarf galaxies. Gravitational (especially tidally induced) effects from the large galaxy can cause mass to be lost from the dwarf, and the passage of the dwarf through the gaseous medium surrounding the large galaxy can additionally cause the dwarf to lose its own gas through a process called ram-pressure stripping. Such effects are considered to be the main sources of difference between “satellite” and “field” dwarf galaxy populations. Here, we report on new observations of the gaseous content of Wolf–Lundmark–Melotte (WLM), an archetype of isolated, gas-rich field dwarf galaxies in the Local Universe, which presents a much more complex situation. Previous studies of its gaseous disk suggest it has perturbed kinematics; here, we identify four trailing, extended gas clouds lying in the direction opposite to WLM’s spatial motion, as well as a spatial offset between the WLM gas and stars. Overall, the morphology and kinematics of this gas show that WLM is undergoing ram-pressure stripping, despite being 930 and 830 kpc from the Milky Way and M 31, respectively. This finding indicates the presence of an intergalactic, gaseous reservoir far from large galaxies whose evolutionary role in galaxies, both large and small, may not be fully appreciated.