# On the origin of star-gas counterrotation in low-mass galaxies

## TL;DR

This study uses the Illustris simulations to investigate the rare phenomenon of star-gas counterrotation in low-mass galaxies, revealing it results from gas removal events followed by accretion of misaligned gas, with long-lived misalignments observed.

## Contribution

It identifies the mechanisms behind counterrotation in low-mass galaxies and demonstrates its long-term stability, providing insights into galaxy evolution processes.

## Key findings

- Counterrotation occurs in about 1% of low-mass galaxies.
- Gas removal followed by misaligned gas accretion causes counterrotation.
- Counterrotation can persist for over 2 Gyr.

## Abstract

Stars in galaxies form from the cold rotationally supported gaseous disks that settle at the center of dark matter halos. In the simplest models, such angular momentum is acquired early on at the time of collapse of the halo and preserved thereafter, implying a well-aligned spin for the stellar and gaseous component. Observations however have shown the presence of gaseous disks in counterrotation with the stars. We use the Illustris numerical simulations to study the origin of such counterrotation in low mass galaxies ($M_\star = 2 \times 10^9$ - $5 \times 10^{10}\; \rm M_\odot$), a sample where mergers have not played a significant role. Only ${\sim}1\%$ of our sample shows a counterrotating gaseous disk at $z=0$. These counterrotating disks arise in galaxies that have had a significant episode of gas removal followed by the acquisition of new gas with misaligned angular momentum. In our simulations, we identify two main channels responsible for the gas loss: a strong feedback burst and gas stripping during a fly-by passage through a more massive group environment. Once settled, counterrotation can be long-lived with several galaxies in our sample displaying misaligned components consistently for more than $2$ Gyr. As a result, no major correlation with the present day environment or structural properties might remain, except for a slight preference for early type morphologies and a lower than average gas content at a given stellar mass.