# SPH Simulations of Counterrotating Disk Formation in Spiral Galaxies

**Authors:** A. R. Thakar, B. S. Ryden  
**arXiv:** [astro-ph/9805082](https://arxiv.org/abs/astro-ph/9805082)  
**Date:** 1998-05-06  
**Summary page:** https://www.summarizepaper.com/en/paper/astro-ph--9805082v1/

## Key points

- Thakar and Ryden's study focuses on Smoothed Particle Hydrodynamics (SPH) simulations of massive counterrotating disks in spiral galaxies
- They explore adiabatic gas infall and retrograde gas-rich dwarf mergers as processes for generating counterrotating disks
- Experiments involve cold primary models resembling our Galaxy and hot, compact primary models like NGC 4138
- Significant differences are found in the structure of SPH-generated counterrotating disks compared to those formed with sticky particles, with SPH disks being thinner
- SPH simulations show shorter time-scales for forming counterrotating disks due to rapid loss of kinetic energy and angular momentum by the gas
- Spiral structures are evident in most disks, but an exponential radial profile is not naturally observed through these processes
- Infalling gas shells result in the formation of counterrotating bulges and rings rather than traditional disks
- Presence of preexisting prograde gas leads to swift inflow towards the center, causing a void in the counterrotating disk when star formation is absent
- SPH simulations suggest that accretion of massive counterrotating disks plays a pivotal role in driving host galaxies towards earlier Hubble types (S0/Sa)

## Summary

In their study, Thakar and Ryden present the results of Smoothed Particle Hydrodynamics (SPH) simulations on the formation of massive counterrotating disks in spiral galaxies. They explore adiabatic gas infall and retrograde gas-rich dwarf mergers as potential processes for generating such disks, building upon previous work using sticky particle gas dynamics. The researchers conduct experiments with both cold primary models resembling our own Galaxy and hot, compact primary models akin to NGC 4138. They also investigate varying amounts of prograde gas in the primary disk and alternative infall models, such as a spherical shell with retrograde angular momentum. The findings reveal significant differences in the structure of counterrotating disks produced with SPH compared to those formed with sticky particles. The SPH-generated disks are notably thinner than both primary disks and those created using sticky particles. Additionally, the time-scales for forming counterrotating disks are shorter with SPH due to more rapid loss of kinetic energy and angular momentum by the gas. While spiral structures are evident in most disks, an exponential radial profile is not naturally observed through these processes. Experiments involving infalling gas shells result in the formation of counterrotating bulges and rings rather than traditional disks. Furthermore, the presence of substantial preexisting prograde gas in the primary leads to a swift inflow of gas towards the center, causing a subsequent void in the counterrotating disk when star formation is absent. Overall, the SPH simulations provide compelling evidence suggesting that accretion of massive counterrotating disks plays a pivotal role in driving host galaxies towards earlier Hubble types (S0/Sa). This research sheds light on how these dynamic processes contribute to galaxy evolution and morphology.