1. Nebular Astrophysics

2. Visible Appearance of the ISM
1) Emission Nebulae
Emission line radiation from radiatively ionized/excited gas
The Trifid Nebula
The Fox Fur Nebula
NGC 2246

3. Visible Appearance of ISM
2) Reflection Nebulae
Star light reflected by dust
Appear blue because blue light is scattered by larger angles than red light.

4. Visible Appearance of ISM
3) Dark Nebulae
Dense clouds of gas and dust absorb the light from the stars behind;
Barnard 86
Horsehead Nebula

5. Observing Neutral Hydrogen: The 21-cm (radio) line
Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation.
Opposite magnetic fields attract => Lower energy
Equal magnetic fields repel => Higher energy
Magnetic field due to proton spin
21 cm line
Magnetic field due to electron spin

6. Observations of the 21-cm Line
G a l a c t i c p l a n e
All-sky map of emission in the 21-cm line

7. H2 = Molecular Hydrogen  Ultraviolet absorption and emission:
Molecules in Space
CO = Carbon Monoxide
OH = Hydroxyl
 Radio emission from rotational (and vibrational) transitions
H2 = Molecular Hydrogen  Ultraviolet absorption and emission:
Difficult to observe!
But: Where there’s H2, there’s also CO.
Use CO as a tracer for H2 in the ISM!

8. The Central Region of our Milky Way in H2 emission

9. Molecular Clouds
Molecules are easily destroyed (“dissociated”) by ultraviolet photons from hot stars.
 They can only survive within dense, dusty clouds, where UV radiation is completely absorbed.
“Molecular Clouds”:
Largest molecular clouds are called “Giant Molecular Clouds”:
UV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there.
Molecules survive
Cold, dense molecular cloud core
Diameter ≈ 15 – 60 pc
Temperature ≈ 10 K
HI Cloud
Total mass ≈ 100 – 1 million solar masses

10. Dust in Infrared
Spitzer Space Telescope Infrared Image of the Orion Nebula

11. Dust in Infrared

12. Keplerian Orbits: v ~ r-1/2
Dark Matter
Keplerian Orbits: v ~ r-1/2

13. Galactic Magnetic Fields
Radio Emission and Polarization (B-fields) in M51

14. Faraday Rotation

15. Cosmic Rays E < 3*1015 eV: Galactic (Supernova remnants)
Mostly protons
3*1015 eV < E < 3*1018 eV:
Extragalactic
Heavier elements (up to Fe)
Active Galaxies?
Gamma-Ray Bursts?
Heavy supersymmetric particle decay? …

16. Cosmic Rays E > 3*1018 eV: “Ultra-high-energy Cosmic Rays” (UHECRs)
New, extragalactic component
Composition???
Sources???
E ~ 1020 eV:
Greizen-Zatsepin-Kuzmin (GZK) cutoff:
UHECRs interacting with Cosmic Microwave Background through
pg -> p p0
pg -> n p+
pion production.

17. Hillas Plot Magnetic-field – Size requirements
to accelerate cosmic rays to
100 EeV (= 1020 eV)
and
1 ZeV (= 1021 eV).

18. Pierre Auger Observatory
Array of 1,600 water tanks with fluorescence detectors, spread over ~ 3,000 km2
In Pampa Amarilla (Western Argentina).

19. UHECR All-Sky Map
* = Nearby (< 100 Mpc) Active Galactic Nuclei
O = Arrival Direction of UHECRs
Association with nearby Active Galactic Nuclei was claimed, but no longer significant.

20. Cosmic Rays colliding with gas of the ISM
The GeV Gamma-Ray Sky
Cosmic Rays colliding with gas of the ISM
All-Sky Map by the Fermi Gamma-Ray Space Telescope

21. Collisional Excitation and De-excitation
Critical Densities ID
l [A]
log ncr [cm-3]
[CII
1909
9.0
[O II]
3726.1
3.5
3728.8
2.8
[Fe VII]
3760.3
7.6
[Ne III]
3868.8
7.0
3967.5
[S II]
4068.6
6.4
[O III]
4363.2
7.5
5006.9
5.8

22. Collisional Excitation and De-excitation
O III (O2+) forbidden lines

23. Density-Dependent Line Ratios

24. Thermal Equilibrium in Nebulae
Tequi