Dissecting the formation of gas-versus-star counter-rotating galaxies from the NewHorizon simulation

Gas-versus-star counter-rotating galaxies are characterized by the presence of a disk of stars and a disk of gas that are co-spatial but rotating in opposite directions. Using the N<ns0:sc>EW</ns0:sc>H<ns0:sc>ORIZON</ns0:sc> simulation, we identified and studied ten such galaxies in field environments with a stellar mass of M* ∼ [1–5] × 1010 M⊙. For all of them, the retrograde accretion of gas either from gas stripping from a nearby companion or from the circumgalactic medium is the starting point of the formation process. This is followed by the coexistence of two distinct disks of gas (or components) rotating in opposite directions, with the pre-existing disk in the inner parts of the galaxy and the accreted gas in the outer parts. The latter progressively replaces the former, leading to the final gas-star kinetic misalignment configuration. During the process, star formation is first enhanced and then progressively decreases. We roughly estimate that a higher fraction of the pre-existing gas is converted into stars rather than being expelled. We also found that the black hole (BH) activity tends to be enhanced during the removal of the pre-existing gas. Furthermore, our analysis suggests that the formation of a counter-rotating gas component is always accompanied with the formation of counter-rotating stellar disks. These stellar disks can have diverse properties, but in general, they host a younger and more metal rich population of stars with respect to the main disk, depending on the star formation history and BH activity. The central part of counter-rotating disks also tend to be characterized by a younger population, an enhanced star formation rate, and a higher metallicity than their outer parts. The high metallicity comes from the progressive metal enrichment of the accreted gas through mixing with the pre-existing gas and by supernovae activity as the accreted gas sinks toward the center of the galaxy. In case of major mergers, a large amount of accreted stars from the companion would be distributed at large distances from the remnant center due to conservation of the initial orbital angular momentum. This process might favor the observation of two distinct counter-rotating stellar disks, particularly in observed projected velocity fields from integral field spectroscopy surveys, as well as stellar streams characterized by specific age-metallicity properties.