2026A&A...707A..51C

Context. Understanding galaxy evolution is key to explaining the structures we observe in the present-day Universe. Counterrotating stellar disks, i.e., co-spatial stellar disks rotating with opposite angular momentum, have been proposed as signatures of past accretion events. Therefore, they constitute potential tracers of galactic assembly.
Aims. In this work, we aim to investigate the properties, formation channels, and significance of coplanar counterrotating disk components (CRDs) in a sample of Milky Way mass galaxies using the IllustrisTNG cosmological simulations.
Methods. We selected an initial sample of 260 central late-type galaxies (i.e. M_tot ≈ 10¹², D/T > 0.5, N_star > 10⁵). For each galaxy, we measured the circularity of its stellar particles, and we defined a CRD by considering all particles with circularity ϵ < -0.7, which are located within the spatial extension of the main disk. We then characterized the mass fraction, spatial extent, and star formation history of the CRDs.
Results. Out of the 260 late-type galaxies, we find 26 host significant CRDs (i.e., contributing at least 1% of the mass of the stellar disk). This means that CRDs are rare, and this outcome is consistent with the results from recent observations. We also find that most of the CRDs are compact (i.e., 88%) and in situ dominated (i.e., 73%), and they exhibit bursty star formation histories whose peaks often coincide with external perturbations. This means that external perturbations are able to catalyze retrograde star formation, even when a majority of the CRD's star population is in situ. Finally, we find that a variety of formation pathways can lead to CRDs, including interaction-induced in situ bursts and smooth accretion of misaligned gas.
Conclusions. Overall, our results suggest that CRDs are rare but diverse in origin. In most cases, their formation is linked to the accretion of retrograde gas, either through mergers or environmental inflow, suggesting that these are sensitive tracers of the galaxy's past accretion history.