The Interstellar Medium of the Galaxy
…the multiphase cloudy gas between the stars.
Clouds are not as bright as stars! We must use a variety of observing techniques to
see them, including: reddening, optical & UV absorption lines, 21 cm emission &
absorption, molecular emission, far-IR observations.
The Orion Deep Field Credit & Copyright: Robert Gendler
Absorption/Emission/Reddening
Interstellar Dust and Reddening
Dust grains: micron-sized aggregates, like soot or cigarette smoke. Often ice
covered in the ISM. Grain size ≈ wavelength of light (or near-IR).
• Dust is: more transparent to red light,
preferentially scatters blue light.
• Rayleigh scattering: mostly why the sky is blue, and sunsets red.
• Dust clouds can absorb visible light heavily. (20 mags. in V to the Milky Way
center.
• Far-IR and radio observations see through dust.
• Can map clouds via dust absorption.
Mapping absorption line clouds
Absorption lines from different clouds have different Doppler shifts because of
differential rotation and random velocity components.
Relative Intensity
-10
0
V (km/s)
10
• With this technique, typically get several clouds along the line of sight, and
some measure of the amount of material in each cloud.
• Except: clouds with nearly the same velocity relative to Earth will be
confused.
East of Antares Credit & Copyright: Johannes Schedler (Panther Observatory)
(Some) Components of the Interstellar Medium
Component
Density (cm-3)
T (K)
Pressure (nT)
Obs. Technique
Hot corona
10-4
106
102
UV (O+5) lines,
X-ray continuum
Warm (HI, HII) 1
≤ 104
104
Ion fine-structure lines,
Optical abs. Lines,
21 cm emission/abs.
Cold (Envelope) 100
80
104
Reddening,
Neutral metal lines
Some CO, H2
Cold (Core)
105
10
106
Molecules
Why are there such different thermal phases?
-Heating and cooling processes across a very broad spectrum.
What determines the phases?
There are several constraints, notably quasi-equilibrium conditions.
- Mechanical equilibrium, and isobaric assumption (within a scale height).
- Thermal equilibrium, heating ≈ cooling,
n2Λ ≈ nΓ.
A little cooling physics
Cooling rate:
nkT
n Λ=
.
τ cool
2
Coolants include: atoms, ions, molecules, grains of various sizes…..
€
What’s τcool?
τcool = <neσv>,
assuming that the atom is excited primarily by collisions with electrons.
Further assume:
<neσv> = <ne> <σv>,
and then,
nn e kT
Λ=
σv .
n2
And finally,
€
<σv> = <σ> <v>,
in terms of the mean cross section and the (Maxwellian) mean velocity.
However, this is a lousy estimate if the cross section has ‘resonances’. Generally
need a quantal calculation.
Some cooling curves (on trans.)….
Some Dynamical and Heating Processes in the ISM
Process (local)
Characteristic Length (pc)
Energy Scale (J)
Cloud collisions
<λ> ≈ 100
mclσ2/2 ≈ 1042
Photoheating
R ≈ 56 n-0.3
tOBLOB ≤ 1045
(HII regions)
Supernovae
40
1044
Stellar winds
“
bit less
Grav. Collapse
lJ ≈ 40 (T/100)3/2(100/n)3/2
GM2/R ≈ 1041
Magnetic effects, Alfven waves ????
Global disturbances:
Spiral density waves, Other large-scale flows, AGN activity, Galaxy collisions…