# On the distribution of the CNM in spiral galaxies

**Authors:** Rowan J. Smith, Robin Tress, Juan D. Soler, Ralf S. Klessen, Simon C. O. Glover, Patrick Hennebelle, Sergio Molinari, Mordecai-Mark Mac Low, David Whitworth  
**arXiv:** [2301.08265](https://arxiv.org/abs/2301.08265)  
**Date:** 2023-01-19  
**Summary page:** https://www.summarizepaper.com/en/paper/2301.08265v1/

## Key points

- The study investigates the distribution of the Cold Neutral Medium (CNM) in spiral galaxies and its correlation with star formation.
- The CNM is important for the formation of molecular and star-forming gas.
- High-resolution simulations are used to track the formation and distribution of CNM within galaxies.
- There is no strong radial dependence between CNM fraction and total HI.
- The CNM fraction increases in spiral arms and overlaps closely with H2.
- The CNM does not extend radially further outwards like HI.
- The vertical scale height of CNM in outer galaxy regions is smaller and does not flare.
- Column density of CNM scales with total midplane pressure, transitioning out of existence below certain values.
- Star formation rate density follows a similar scaling law with CNM column density as predicted by total gas Kennicutt-Schmidt law.
- Extended spiral arms might generate isolated clumps of CNM at large radii.

## Summary

In this study, the authors investigate the distribution of the Cold Neutral Medium (CNM) in spiral galaxies and its correlation with star formation. The CNM plays a crucial role in the galactic gas cycle and is a prerequisite for the formation of molecular and star-forming gas. However, its distribution within galaxies is not fully understood. To address this, the authors employ extremely high-resolution simulations of spiral galaxies with time-dependent chemistry. This allows them to track the formation of the CNM, its distribution within the galaxy, and its relationship with star formation. The results reveal that there is no strong radial dependence between the CNM fraction and total HI (neutral atomic hydrogen). This lack of dependence is attributed to decreasing interstellar radiation field counterbalancing the decreasing gas column density at larger galactic radii. However, it is observed that the CNM fraction increases in spiral arms where its distribution appears clumpy rather than continuous. Moreover, it overlaps more closely with H2 (molecular hydrogen). Interestingly, it is found that while HI extends radially further outwards in galaxies, the CNM does not exhibit such an extension. Additionally, compared to HI, the vertical scale height of CNM in outer galaxy regions is smaller and does not show any flaring behavior. The authors also establish that column density of CNM scales with total midplane pressure and disappears from gas phase below certain values; specifically when PT/kB (the ratio of total midplane pressure to Boltzmann's constant) falls below 1000 K/cm3 ,CNMs transition out of existence. Furthermore, it is discovered that star formation rate density follows a similar scaling law with CNM column density as predicted by total gas Kennicutt-Schmidt law. While realistic vertical velocity dispersions in HI are produced solely from galactic dynamics in outer galaxy regions these models do not predict CNM at extremely large radii as observed in HI absorption studies of Milky Way. The authors propose that extended spiral arms might generate isolated clumps of CNM at these radii. Overall this study provides valuable insights into distribution and properties of CNM in spiral galaxies shedding light on its role in star formation processes .The findings contribute to better understanding of galactic gas cycle and have implications for our knowledge about interstellar medium .