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Galaxy Lecture 6
Fichas del conjunto (12)
- 1The Interstellar MediumMgalaxy - 10^12Msun
M*-2x10^11Msun
Mgas-4x10^9Msun
Mdust-4x10^7Msun
Mgas/M*=2%
Mdust/Mgas=1% - 2DustSmall, solid grains
Size range 0.1 to 1 micrometer
R>>lambda= geometric optics
R< - 3More dustConfined to layer 300pc thick, centred on the galactic plane. Scale height:100pc
Dust makes up 1% of ISM but causes almost all the visible obscuration
General obscuration: 1 mag per kpc in V band in MW plane down to zero at high galactic latitude (patchy)
Dark clouds associated with star formation regions can give obscuration >30mag in V band - 4Effects of dust on starlightDust absorbs and scatters more strongly at short wavelengths
Reddened starlight: less affected by dust
Reflection nebulae in denser regions - reflection of starlight by dusty clouds located behind bright stars
Polarised starlight: scattering, absorption by aligned grains may make a star seamless bright. Blue is scattered more than red hence scattering also leads to reddening
Young stars which are bright are difficult to see because they produce large amounts of dust which radiates in the infrared - 5Silicate and CarbonIR emission bands at 9.7micrometers and 18 micrometers due to silicate - absorption at specific resonances.
Broad absorption feature in UV due to carbon graphite (220nm)
Silicates - needle shaped
Carbon - flaky
Hence polarisation if grains aligned my galactic magnetic field - 6Dust grainsAlmost all H2 formed on dust grains - site for molecule formation - building blocks for planets
- 7The ISMMostly hydrogen - 75% by mass, 25% He, 3% metals
- 8Phases of the ISM1. Molecular clouds
2. Cold Neutral Helium
3. Warm Neutral Medium
4. Warm Ionised Medium
5. Hot Ionised Medium - 9Molecular CloudsPrimarily H2
Very cold - 25K
30% of ISM by mass but only 0.05% by volume
H2 not readily observable - molecular clouds are traced by extinction and dust emission
Several thousand catalogued in milky way with masses 10^3sun 10^5sun. They're confined to plane: scale height <100pc
Mostly in spiral arms, less molecular gas in outer galaxy - 10Why is dust seen in emission at sub millimetre wavelengths?x
- 11Observing neutral hydrogenH atoms excited by collisions to slightly higher energy level - once per 10^6 years per H atom. Average lifetime in excited state is 10^7 years followed by spontaneous decay back to ground state emitting lambda = 21cm but 10^66 H atoms in MW so observable line emission
All sky surveys provide:
column densities (no of atoms per m^2 along line of sight)
doppler shift of moving H clouds
Get distribution of HI in MW and the rotation curve - determine circular speed as a function of distance from galactic centre - 12—
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- Descripción
- Fichas sobre Galaxy Lecture 6, creado por Lucy Mathieson el 07/11/2016.
- Creado por
- Lucy Mathieson · nov 2016
- Actualizado
- 10 nov 2016
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