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Giovanni Covone » 4.Matter components of the ISM

Five components of the gaseous ISM

In this lecture we will discuss the thermal phases of the gas in the ISM.

Classification according to temperature and ionization state. We can identify four main component: molecular clouds, cold neutral medium, warm neutral medium, warm ionized medium, and the hot ionized medium.

Simple model of the ISM: description of the ISM with a limited set of such states rather than as a smooth continuum of conditions.

Physical properties of the four main components of the ISM.

Thermal phases in the ISM (1)

The ISM is described in terms of thermodynamic properties: density, temperature, pressure, etc.

Most of the mass in the ISM is found in the form of clouds of cold, neutral gas with $T\sim 80$ K and density $n \sim 10 - 100 {\rm cm}^{-3}$ and in giant molecular clouds with $T\sim 10$ K and density $n \sim 10^2 - 10^5 {\rm cm}^{-3}$ .

Molecular clouds (molecular hydrogen).

30% of the mass, but only 0.05 % of the volume.
Gravitationally bound, unstable, sites of star-formation.

Main observational tracer: mm-wavelength emission lines from CO molecules.

Cold neutral medium

Distributed in sheets, filaments, occupy 1-5 % of the ISM volume.
Main observational tracer: UV and optical absorption lines towards bright background sources.

Thermal phases in the ISM (2)

Most of the volume is occupied by hotter and less dens gas.

(1) warm neutral component

About 30% of the ISM volume. Located mostly at boundaries of HII regions and molecular clouds (PDRs).
Typical values: $T \sim 5000$ K, $n \sim 0.1 {\rm cm}^{-3}$ .
Observational tracer: 21cm emission line from HI.

(2) warm ionized component

About 25% of the volume. 90% of ionized hydrogen isin this phase.
Typical values: $T \sim 8000$ K, $n \sim 0.1 {\rm cm}^{-3}$ .
Main observational tracer: H-alpha emission.

(3) hot diffuse component

Heated by SN. Main structure: hot Galactic “corona”.
Typical values: $T \sim 10^6$ K, $n \sim 10^{-3} {\rm cm}^{-3}$ .
Observational tracer: X-ray emission; absorption lines in the far-UV.

Thermal phases in the ISM (3)

Pressure equilibrium (all phases, but in molecular clouds):

$P = n \, k_B \, T \sim 10^{-13} {\rm dyn \, cm}^{-2} \, = {\rm const}.$

Dust

Solide phase of the ISM.
Grain size: few microns to scale of macro-molecules.

Interstellar dust is mixed with all the gas phases (but the hottest ones).
Dust grains are rensponsbile for interstellar extinction, sites of chemistry in space.

In order to understand its important role: mass versus luminosity.

Summary table: components of the ISM

Schematic view of the interactions among the ISM components.

Figure: different regions of the ISM

A schematic view of the different regions in the ISM.

The ISM as a complex system

Other components in the ISM

Chemical elements

Heavy elements

He (about 10 % by number)
C, N, O (at “cosmic” abundances)
Si, Ca, Fe (depleted onto dust grains)

Photons

CMB
Star light—average interstellar radiation field
X-rays—from hot gas & the extragalactic background

Magnetic fields & cosmic rays

Neutral atomic gas (1)

The neutral atomic gas is mainly neutral hydrogen (symbol: HI).

Most of the gaseous hydrogen in the ISM (about 97%) is neutral; a small percentage (about 3%) is ionized.

Observations of HI clouds at 21 cm (via hyperﬁne ground state transitions of hydrogen) in emission and absorption provide information about temperatures and column densities.

The brightness of the emission provides a measurement of the HI column density

$N_{\rm H} = \int n_{\rm H} {\rm d} s$ ,

where $n_{\rm H}$ is the atomic hydrogen density.

Decrease in the brightness of a background source at 21 cm is proportional to $\int \frac{n_{\rm H}}{T_H} {\rm d} s$ where $T_H$ is the temperature of the cloud.