On the origin of counterrotating stellar disks in TNG50. I

TL;DR

CRDs in Milky Way–mass galaxies are rare but informative tracers of past accretion. The authors leverage the high-resolution TNG50 cosmological simulation to identify counterrotating stellar disks by a kinematic decomposition and to trace the in-situ versus ex-situ origin of CRD stars. They find that most CRDs are compact, in-situ dominated, and characterized by bursty star formation histories often triggered by interactions or misaligned gas accretion; extended CRDs are rarer and tied to late gas misalignment. Collectively, the results reveal multiple robust formation channels for CRDs and support their use as sensitive indicators of a galaxy’s assembly history, while highlighting differences with some observational trends.

Abstract

Understanding galaxy evolution is key to explaining the structures we observe in the present-day Universe. Counterrotating stellar disks (CRDs), i.e. co-spatial stellar disks rotating with opposite angular momentum, have been proposed as signatures of past accretion events. Therefore, they constitute potentially valuable tracers of galactic assembly. We aim to investigate the properties, formation, and significance of CRDs in a sample of Milky Way mass galaxies using the IllustrisTNG cosmological simulations. We select 260 central late-type galaxies (i.e. $M_{\rm tot} \approx 10^{12}$, $D/T>0.5$, $N_{\rm star}>10^5$). For each galaxy, we measure the circularity of its stellar particles and define the CRD by considering all particles with circularity $ε< -0.7$, which are located within the spatial extension of the main disk. We then characterize the mass fraction, spatial extent, and star formation history of the CRDs. Out of the 260 galaxies, we find that 26 host significant CRDs (i.e. contributing at least 1\% of the total stellar mass of the disk). This means that CRDs are rare, consistent with the results from observations. We also find that the most of the CRDs are compact (i.e. 88\%), in-situ dominated (i.e.73\%), and exhibit bursty SFHs whose peaks often coincide with external perturbations. This means that external perturbations are able to catalyze 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. 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 structures are sensitive tracers of the galaxy's past accretion history.

Figures (11)

• Figure 1: Distribution of the counterrotating disk mass fraction for the full sample of 260 late-type disk galaxies. The vertical dashed orange line indicates a 1% mass fraction from the CRD. We see that only 10% of the galaxies (when including the boundary cases) have a CRD mass fraction greater than 1%.
• Figure 2: The face-on and edge-on projections of the stellar mass surface density for a sub-sample of 8 galaxies with the most significant counterrotating stellar disks. The galaxies are sorted according to their total mass, from the most to the least massive. Here, we show all the stellar particles that belong to the host galaxy. The yellow solid and dot-dashed circle represent the radii, $R_{95}^{\rm CRD}$ and $R_{50}^{\rm CRD}$, of the counterrotating disk, respectively, while the white dashed circle represents the optical radius, $R_{\rm opt}$, of the galaxy.
• Figure 3: Same as Fig. \ref{['fig:8subhaloes_all']}, but we show here only the stellar particles belonging to the CRDs.
• Figure 4: The CRD mass fraction, $M_{\rm CRD}/M_{\rm Disk}$, as a function of different measurements of the counterrotating stellar disk extension and mass distribution for the sample of 26 significant CRD galaxies. Top & middle panels: the radius containing 50% and 95% of the counterrotating stellar disk mass normalized by the optical radius of the galaxy, respectively. Bottom panel: the stellar concentration of the counterrotating disk. All data points are color-coded by the total stellar disk mass. We see that, while most CRDs are compact with the majority having $R_{50}^{\rm CRD}/R_{\rm opt} < 0.25$, there are a few extended cases. Overall, we find that our sample of counter rotating galaxies show a variety of structural properties.
• Figure 5: The fraction of the in-situ and ex-situ stellar particles in the counterrotating disk for the sample of 26 significant CRD galaxies. Light blue bars represent the fraction of in-situ stellar particles in the CRD, while the remaining colors represent the contribution to the ex-situ fraction in the CRD from different satellite galaxies. Each color refers to a different satellite. The red dashed line indicates an in-situ contribution equals to 80%.