On the distribution of the cold neutral medium in galaxy discs

Author:

Smith Rowan J1ORCID,Tress Robin2,Soler Juan D3,Klessen Ralf S45ORCID,Glover Simon C O4ORCID,Hennebelle Patrick6,Molinari Sergio3,Mac Low Mordecai-Mark7ORCID,Whitworth David4ORCID

Affiliation:

1. Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester , Oxford Road, Manchester M13 9PL , UK

2. École Polytechnique Fédérale de Lausanne, Observatoire de Sauverny , Chemin Pegasi 51, CH-1290 Versoix , Switzerland

3. Istituto di Astrofisica e Planetologia Spaziali (IAPS), INAF, Via Fosso del Cavaliere 100 , I-00133 Roma , Italy

4. Zentrum für Astronomie, Institut für Theoretische Astrophysik, Universität Heidelberg , Albert-Ueberle-Straße 2, D-69120 Heidelberg , Germany

5. Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg , Im Neuenheimer Feld 205, D-69120 Heidelberg , Germany

6. Laboratoire AIM, Paris-Saclay, CEA/IRFU/SAp, CNRS, Université Paris Diderot , F-91191 Gif-sur-Yvette Cedex , France

7. Department of Astrophysics, American Museum of Natural History , 200 Central Park West, New York, NY 10024 , USA

Abstract

ABSTRACT 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 neutral atomic hydrogen (H i) 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 does not extend out radially as far as H i, and the vertical scale height is smaller in the outer galaxy compared to H i with no flaring. The CNM column density scales with total mid-plane pressure and disappears from the gas phase below values of PT/kB = 1000 K cm−3. 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 H i purely from galactic dynamics, but our models do not predict CNM at the extremely large radii observed in H i absorption studies of the Milky Way. We suggest that extended spiral arms might produce isolated clumps of CNM at these radii.

Funder

STFC

DFG

European Research Council

NSF

Durham University

MWK

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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