2019A&A...630A.112M

Shells are fine stellar structures that are identified by their arc-like shapes around a galaxy. They are currently thought to be vestiges of galaxy interactions and/or mergers. The study of their number, geometry, stellar populations, and gas content can help us to derive the interaction or merger history of a galaxy. Numerical simulations have proposed a mechanism of shell formation through phase wrapping during a radial minor merger. Alternatively, there could be merely a space wrapping, when particles have not yet radially oscillated, but are bound by their radial expansion, or produce an edge-brightened feature. These can be distinguished because they are expected to keep a high radial velocity. While shells are first a stellar phenomenon, HI and CO observations have revealed neutral gas associated with shells. Some of the gas, the most diffuse and dissipative, is expected to be quickly driven to the center if it is traveling on nearly radial orbits. Molecular gas, which is distributed in dense clumps, is less dissipative, and may be associated with shells. It can then determine the shell velocity, which is too difficult to obtain from stars. We present here a search for molecular gas in nine shell galaxies with the IRAM-30 m telescope. Six of them are detected in their galaxy center, and in three galaxies, we clearly detect molecular gas in shells. The derived amount of molecular gas varies from 1.5x10⁸ to 3.4x10⁹M_☉ in the shells. For two of them (Arp 10 and NGC 3656), the shells are characteristic of an oblate system. Their velocity is nearly systemic, and we conclude that these shells are phase wrapped. In the third galaxy (NGC 3934), the shells appear to participate in the rotation. Follow-up with higher spatial resolution is required to conclude.