6.1 Phases of the Interstellar Medium

Ionized Interstellar Medium

The interstellar medium (ISM) is the gas and dust filling the space between stars. It exists in several distinct phases, classified by temperature, density, and ionization state. These phases coexist in a dynamic, roughly pressure-balanced system that governs how galaxies recycle matter and form new stars.

Hot and Warm Ionized Media

The Hot Ionized Medium (HIM) fills roughly 50% of the ISM by volume, yet its density is extremely low (on the order of $n \sim 10^{-3} \, \text{cm}^{-3}$). Temperatures exceed $10^6 \, \text{K}$, sustained by the cumulative energy input from supernova explosions. This highly ionized plasma occupies large-scale structures like galactic bubbles and superbubbles, and it emits primarily at X-ray wavelengths due to thermal bremsstrahlung and line emission from highly ionized metals.

The Warm Ionized Medium (WIM) accounts for about 20–30% of the ISM volume, with temperatures near $8000 \, \text{K}$ and densities of $n \sim 0.1 \, \text{cm}^{-3}$. It consists of partially ionized hydrogen, kept ionized mainly by Lyman continuum photons from O and B stars. The WIM is detected through optical emission lines, particularly $\text{H}\alpha$ at 656.3 nm, and through pulsar dispersion measures, which probe the free electron column density along the line of sight.

Both media contribute to:

• Galactic magnetic field dynamics (ionized gas couples to magnetic fields via the Lorentz force)
• Energy and mass transfer between different ISM phases
• Regulation of star formation by pressurizing and heating surrounding neutral gas

HII Regions and Supernova Remnants

HII regions are localized zones of ionized hydrogen surrounding hot, young O and B stars. The central star's UV flux (photon energies $\geq 13.6 \, \text{eV}$) ionizes a surrounding sphere of gas out to the Strömgren radius, where the ionization rate balances the recombination rate. Typical properties include:

• Temperatures of 7,000–10,000 K
• Electron densities of $n_e \sim 10^1 \text{–} 10^4 \, \text{cm}^{-3}$
• Bright optical emission (the Orion Nebula is a classic example)
• Strong forbidden lines such as $[\text{O III}]$ and $[\text{N II}]$, used as diagnostics of temperature and density

HII regions are direct tracers of recent star formation, so mapping them across a galaxy reveals its spiral structure and star-forming activity.

Supernova remnants (SNRs) form when a massive star explodes and its ejecta slam into the surrounding ISM. They evolve through well-defined stages:

1. Free expansion — Ejecta expand nearly unimpeded at speeds up to $\sim 10{,}000 \, \text{km/s}$, with shock-heated gas reaching $> 10^7 \, \text{K}$.
2. Sedov-Taylor (adiabatic) — The swept-up ISM mass exceeds the ejecta mass; the remnant expands as a blast wave with energy approximately conserved. The radius grows as $R \propto t^{2/5}$.
3. Snowplow (radiative) — The shell cools efficiently and radiates away energy, forming a dense, thin shell that decelerates.
4. Dissipation — The remnant merges with the ambient ISM.

SNRs enrich the ISM with heavy elements synthesized during stellar evolution and the explosion itself. Their shocks can also compress nearby clouds, potentially triggering new star formation.

Neutral Interstellar Medium

Warm and Cold Neutral Media

Neutral atomic hydrogen exists in two thermally stable phases, a result of the balance between heating (primarily photoelectric emission from dust grains) and cooling (mainly fine-structure line emission from species like $\text{C II}$ at 158 μm). This two-phase equilibrium was described by the Field, Goldsmith, and Habing model.

The Warm Neutral Medium (WNM) makes up roughly 30% of the ISM volume:

• Temperatures of 6,000–10,000 K
• Low densities, $n \sim 0.3 \, \text{cm}^{-3}$
• Composed of diffuse, neutral atomic hydrogen (HI)
• Detected through the 21 cm hyperfine emission line, which arises from the spin-flip transition of the hydrogen atom ($\Delta E = 5.9 \times 10^{-6} \, \text{eV}$)

The Cold Neutral Medium (CNM) occupies only 1–5% of the ISM volume but is much denser:

• Temperatures of 50–100 K
• Densities of $n \sim 30\text{–}50 \, \text{cm}^{-3}$
• Found in compact clouds and filamentary structures
• Observed through 21 cm absorption lines against background radio continuum sources

Both phases serve as reservoirs of gas that can eventually be incorporated into molecular clouds. The CNM, being denser and cooler, is closer to the conditions needed for molecule formation and gravitational collapse.

Molecular Gas and Interstellar Dust

Molecular gas occupies less than 1% of the ISM volume but contains roughly 20% of its mass, making it disproportionately important. It resides in giant molecular clouds (GMCs) with masses of $10^4\text{–}10^6 \, M_\odot$ and temperatures of only 10–20 K.

The dominant molecule is $\text{H}_2$, but $\text{H}_2$ lacks a permanent dipole moment, so its rotational transitions are weak and require high temperatures to excite. Instead, molecular gas is traced indirectly using CO rotational emission lines (the $J = 1 \rightarrow 0$ transition at 2.6 mm). The CO-to-$\text{H}_2$ conversion factor (the $X_{\text{CO}}$ factor) is used to estimate total molecular mass from CO luminosity, though this factor varies with metallicity and local conditions.

Molecular clouds are the direct sites of star formation. Within them, dense cores with $n > 10^4 \, \text{cm}^{-3}$ can become gravitationally unstable and collapse when they exceed the Jeans mass.

Interstellar dust makes up only about 1% of the ISM mass but has an outsized influence on ISM physics and observations:

• Grain sizes range from $\sim 0.001$ to $\sim 1 \, \mu\text{m}$, with compositions including silicates, carbonaceous materials (graphite, PAHs), and icy mantles
• Dust absorbs and scatters UV and optical starlight, causing interstellar extinction that reddens observed stellar spectra (quantified by $A_V$ and the reddening law $R_V = A_V / E(B-V)$)
• Absorbed energy is re-radiated thermally in the far-infrared, making dust emission a key tracer of star-forming regions
• Dust grain surfaces catalyze $\text{H}_2$ formation, a reaction that is extremely inefficient in the gas phase; without dust, molecular clouds could not form
• Dust also shields cloud interiors from UV radiation, allowing fragile molecules to survive

Together, molecular gas and dust create the cold, shielded environments where gravitational collapse leads to star formation, connecting the ISM phases directly to the topics covered in the rest of this unit.