Nebular Astrophysics

Visible Appearance of the ISM1) Emission Nebulae

Emission line radiation from

radiatively ionized/excited gas

The Fox Fur Nebula NGC 2246The Trifid

Nebula

Visible Appearance of ISM

Star light reflected by dust

Appear blue because blue

light is scattered by larger angles than red light.

2) Reflection Nebulae

Barnard 86

Visible Appearance of ISM

Dense clouds of gas and dust absorb the light from the stars behind;

Horsehead Nebula

3) Dark Nebulae

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.

Magnetic field due to proton spin

Magnetic field due to electron

spin

Opposite magnetic fields attract => Lower energy

Equal magnetic fields repel => Higher energy

21 cm line

Observations of the 21-cm Line

All-sky map of emission in the 21-cm line

G a l a c t i c p l a n e

Molecules in Space

• CO = Carbon Monoxide• OH = Hydroxyl

Radio emission from rotational (and vibrational) transitions

Difficult to observe!

Use CO as a tracer for H2 in the ISM!

• H2 = Molecular Hydrogen Ultraviolet absorption and emission:

But: Where there’s H2, there’s also CO.

The Central Region of our Milky Way in H2 emission

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”:

Diameter ≈ 15 – 60 pcTemperature ≈ 10 K

Total mass ≈ 100 – 1 million solar masses

Cold, dense molecular cloud core

HI Cloud

UV emission from nearby stars destroys molecules in the outer parts of the cloud; is

absorbed there.

Molecules survive

Dust in Infrared

Spitzer Space Telescope Infrared Image of the Orion Nebula

Dust in Infrared

Dark Matter

Keplerian Orbits: v ~ r -1/2

Galactic Magnetic Fields

Radio Emission and Polarization (B-fields) in M51

Faraday Rotation

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?• …

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 throughp -> p 0

p -> n +

pion production.

Hillas Plot

Magnetic-field – Size requirements to accelerate cosmic rays to

100 EeV (= 1020 eV) and 1 ZeV (= 1021 eV).

Pierre Auger ObservatoryArray of 1,600 water tanks with fluorescence

detectors, spread over ~ 3,000 km2

In Pampa Amarilla (Western Argentina).

UHECR All-Sky Map

Association with nearby Active Galactic Nuclei was claimed, but no longer significant.

* = Nearby (< 100 Mpc) Active Galactic Nuclei

O = Arrival Direction of UHECRs

The GeV Gamma-Ray Sky

All-Sky Map by the Fermi Gamma-Ray Space Telescope

Cosmic Rays colliding with gas of the ISM

Collisional Excitation and De-excitation

ID [A] log ncr [cm-3]

[CII 1909 9.0

[O II] 3726.1 3.5

[O II] 3728.8 2.8

[Fe VII] 3760.3 7.6

[Ne III] 3868.8 7.0

[Ne III] 3967.5 7.0

[S II] 4068.6 6.4

[O III] 4363.2 7.5

[O III] 5006.9 5.8

Critical Densities

Collisional Excitation and De-excitation

O III (O2+) forbidden lines

Density-Dependent Line Ratios

Thermal Equilibrium in Nebulae

Tequi