# Formation of Massive Counterrotating Disks in Spiral Galaxies

**Authors:** Aniruddha R. Thakar, Barbara S. Ryden  
**arXiv:** [astro-ph/9510053](https://arxiv.org/abs/astro-ph/9510053)  
**Date:** 1995-10-09  
**Summary page:** https://www.summarizepaper.com/en/paper/astro-ph--9510053v1/

## Key points

- Formation of a massive counterrotating gas disk in a spiral galaxy investigated through numerical simulations
- Three mechanisms explored: episodic and continuous gas infall, merger with a gas-rich dwarf galaxy
- Episodic and continuous gas infall can generate significant counterrotating disk without disrupting stability
- Gas must be well-dispersed in phase-space to avoid forming concentrated clumps before absorption by disk galaxy
- Initial angular momentum of gas critical for scale length and formation time of counterrotating disk
- Control of infall rate necessary to prevent excessive heating of preexisting disk
- Easier to produce smaller counterrotating disk compared to extensive one with similar scale length as prograde disk
- Merger with gas-rich dwarf galaxy may not be viable for producing massive counterrotating disk; only very small dwarf galaxies can create such disks without increasing thickness significantly
- Time-scale for this process deemed prohibitively long, making it unlikely that multiple mergers could accumulate a massive counterrotating disk over cosmic timescales

## Summary

In this study, the formation of a massive counterrotating gas disk in a spiral galaxy is investigated through numerical simulations. The researchers utilized a hierarchical tree gravity solver and sticky-particle gas dissipation scheme to explore three mechanisms: episodic and continuous gas infall, as well as a merger with a gas-rich dwarf galaxy. The results indicate that both episodic and continuous gas infall can effectively generate a significant counterrotating disk without disrupting the stability of the existing disk. However, it is crucial for the gas to be well-dispersed in phase-space and not form concentrated clumps before being absorbed by the disk galaxy. The initial angular momentum of the gas plays a critical role in determining the scale length of the formed counterrotating disk and its formation time. Careful control of the infall rate is necessary to prevent excessive heating of the preexisting disk. Generally, it is easier to produce a smaller counterrotating disk compared to an extensive one with a similar scale length as the original prograde disk. Furthermore, this study suggests that a merger with a gas-rich dwarf galaxy may not be viable for producing a massive counterrotating disk. Only very small dwarf galaxies can create such disks without significantly increasing the thickness of the existing disk. Additionally, this process's time-scale is deemed prohibitively long, making it unlikely that multiple mergers could accumulate a massive counterrotating disk over cosmic timescales. Overall, these findings shed light on complex processes involved in forming massive counterrotating disks in spiral galaxies and highlight important factors such as angular momentum, rate of infall, and merger dynamics that influence their formation and stability.