Interstellar

The Interstellar Medium

By Keith Cowing
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astro-ph.GA
April 3, 2025
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The Interstellar Medium
Atomic and molecular gas across the local galaxy population. In each panel, the galaxies of the xCOLDGASS survey (Saintonge et al., 2017) are shown in comparison to the full galaxy population at low redshift (SDSS; gray countours), and color-coded by a specific ISM-related parameter (black dots are non-detections in the Hi and/or CO emission lines). The quantities shown are the atomic gas mass fraction (fHI≡MHI/M∗; left), the molecular gas mass fraction (fH2≡MH2 /M∗; middle), and the molecular gas depletion timescale (tdep(H2)≡MH2 /SFR; right). The solid and dashed lines shows the position and width of the main sequence. Figure reproduced from Saintonge and Catinella (2022). — astro-ph.GA

The interstellar medium (ISM) is the material that fills the space between the stars in all galaxies; it is a multi-phase medium in pressure equilibrium, with densities and temperatures covering over 6 orders of magnitude.

Although accounting for only a small fraction of the mass of any given galaxy, it is a vital component, since it holds the material responsible for galaxy growth through star formation. Studying the ISM requires careful observations at all wavelengths of the electromagnetic spectrum.

This article describes the multi-phase nature of the ISM, and then puts it in the context of galaxy evolution models, emphasising the importance of the cycling of baryons in and out of galaxies.

Within this framework, the ISM plays a central role: it connects the physical processes operating on very large physical- and time-scales which control the accretion of gas onto galaxies, and the small scale processes that regulate star formation.

Key points

  • The interstellar medium (ISM) is the material that fills the space between stars within galaxies. It is typically made of 99% gas, and < 1% dust particles.
  • The ISM is a multi-phase medium, comprised of specific components with characteristic temperatures ranging from 10 to 106 K.
  • Studying this multi-phase medium requires observations across the electromagnetic spectrum. Gaining a full picture of the ISM in galaxies is therefore a
    technical challenge.
  • The cold neutral medium has a characteristic temperature of 10-100 K and the highest density within the ISM, and is the birth place of new stars.
  • The warm medium has typical temperatures of 5000-10000 K and has both an atomic and an ionised component.
  • The hot ionised medium is the component with typically fills a large fraction of the volume, but only accounts for a small fraction of the mass of the ISM,
    due to the low densities and high temperatures reached through heating by supernova explosions.
  • The relative contribution of the different phases to the overall ISM mass budget varies from galaxy to- galaxy, with spiral and elliptical galaxies forming
    two markedly different populations in terms of their ISM.
  • Dwarf galaxies typically have more mass in their ISM than the amount of mass they have in stars.
  • Observing the ISM in high-redshift galaxies (i.e. galaxies we are seeing as they were only hundreds of millions of years after the Big Bang) is a challenge,
    but an area of intense study especially with the James Webb Space Telescope (JWST).
  • The baryon cycle refers to the lifecycle of gas which flows into galaxies along the cosmic web, cools and settles into the ISM where it can contribute to star
    formation, and finally partly returned to the environment through energetic winds powered by young stars and supermassive black holes.
  • The ISM provides astronomers with vital information regarding all components of the baryon cycle. Put together, this evidence supports a model for galaxy
    evolution where the baryon cycle plays a crucial role.

Amelie Saintonge

Comments: This is a pre-print of a chapter for the Encyclopedia of Astrophysics (edited by I. Mandel, section editor S. McGee) to be published by Elsevier as a Reference Module
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2504.01410 [astro-ph.GA] (or arXiv:2504.01410v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2504.01410
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Submission history
From: Amelie Saintonge
[v1] Wed, 2 Apr 2025 06:52:57 UTC (9,974 KB)
https://arxiv.org/abs/2504.01410
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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