On the distribution of the Cold Neutral Medium in galaxy discs

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


The Cold Neutral Medium (CNM) is an important part of the galactic gas cycle and a precondition for the formation of molecular and star forming gas, yet its distribution is still not fully understood. In this work we present extremely high resolution simulations of spiral galaxies with time-dependent chemistry such that we can track the formation of the CNM, its distribution within the galaxy, and its correlation with star formation. We find no strong radial dependence between the CNM fraction and total HI due to the decreasing interstellar radiation field counterbalancing the decreasing gas column density at larger galactic radii. However, the CNM fraction does increase in spiral arms where the CNM distribution is clumpy, rather than continuous, overlapping more closely with H2. The CNM doesn't extend out radially as far as HI, and the vertical scale height is smaller in the outer galaxy compared to HI with no flaring. The CNM column density scales with total midplane pressure and disappears from the gas phase below values of PT/kB =1000 K/cm3. We find that the star formation rate density follows a similar scaling law with CNM column density to the total gas Kennicutt-Schmidt law. In the outer galaxy we produce realistic vertical velocity dispersions in the HI purely from galactic dynamics but our models do not predict CNM at the extremely large radii observed in HI absorption studies of the Milky Way. We suggest that extended spiral arms might produce isolated clumps of CNM at these radii.
Original languageEnglish
Pages (from-to)873-885
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date22 May 2023
Publication statusPublished - Sept 2023


  • Methods: numerical
  • ISM: kinematics and dynamics
  • ISM: structure


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