2020MNRAS.495.1433Y

We present interferometric H I observations of six double-disc stellar counterrotator ('2σ') galaxies from the ATLAS^3D sample. Three are detected in H I emission; two of these are new detections. NGC 7710 shows a modestly asymmetric H I disc, and the atomic gas in PGC 056772 is centrally peaked but too poorly resolved to identify the direction of rotation. IC 0719, the most instructive system in this study, shows an extended, strongly warped disc of ∼43 kpc diameter, with a faint tail extending towards its neighbour IC 0718. The gas has likely been accreted from this external source during an encounter whose geometry directed the gas into misaligned retrograde orbits (with respect to the primary stellar body of IC 0719). In the interior, where dynamical time-scales are shorter, the H I has settled into the equatorial plane forming the retrograde secondary stellar disc. This is the first direct evidence that a double-disc stellar counterrotator could be formed through the accretion of retrograde gas. However, the dominant formation pathway for the formation of 2σ galaxies is still unclear. The ATLAS^3D sample shows some cases of the retrograde accretion scenario and also some cases in which a scenario based on an unusually well-aligned merger is more likely.