PHYS 182 Lecture Notes - Lecture 19: Photon Gas, Structure Formation, Outer Space
PHYS182: Our Evolving Universe
2017-11-16 LEC 19 The Formation & Evolution of Galaxies
Guest lecturer: Tracy Webb
What is a galaxy?
- Composed of the galactic bulge and a disk
- We live ~2/3 of the way out from the center (8.5kpc)
- Galactic halo around the galaxy, which is primarily made up of dark matter
Components of a galaxy
- Stars (~100 billion suns)
- Gas/Dust (mostly hydrogen)
o In the Milky way, the gas/dust occupies the same mass as the stars in our galaxy
- Supermassive black hole (millions of times the mass of the sun)
o What we see is the accretion disk around the black hole and the jets that flow out of the poles
of the magnetic fields
- Dark matter: 10-100x the mass of the stars
o Most massive component of the galaxy, spread out in the galactic halo that surrounds the
bulge & disk of the galaxy
Hubble Deep Field Telescope
- Took a photo of the galaxy, the size of a grain of sand held at arm’s length
o This small area filled with thousands of galaxies
- Integrating this over the entire area of the sky, you get 100 billion galaxies in the observable universe
- Galaxies are separated by millions or billions of lightyears
o Galaxies are “islands of stars” separated by vast amounts of space
Galaxy Categories
- Elliptical: Tends to be large & yellow, not a lot of gas or dust, lots of dark matter
o Stars orbit the center of the galaxy without order
o Made up of older stars (cool & red)
- Spiral: Lower mass (in everything except for gas; i.e. fewer stars, less dark matter), have arms that
emanate outward like a pinwheel
o Continually making new generations of stars & therefore appear blue to us
o Outer regions are younger than the central regions
- Irregular: range of size, have a lot of gas & dust, a lot of star formation
Empirically observing galaxy formation
- Because light travels at a finite speed, looking out far into space means looking back in time
o E.g. the sun is 8 light-minutes away, so we see the sun 8 minutes back in time
o E.g. Andromeda galaxy is 2.5 million light years away
o E.g. Hubble deep field: 10s of billions of light years
Galaxy formation and evolution
- We cannot watch a single galaxy evolve (timescale is too long)
- But we can study galaxies at different evolutionary phases
- If we assume that galaxies evolve together (I.e. were roughly born at the same time), then we can
piece together galaxy evolution
o As we go back in time, there is a higher fraction of irregular galaxies
- Redshift: in addition to mapping of galaxies onto a timescale, you can also map galaxies on a redshift
scale
o The most distant galaxies that we can study are around a redshift of 9
Document Summary
2017-11-16 lec 19 the formation & evolution of galaxies. Composed of the galactic bulge and a disk. We live ~2/3 of the way out from the center (8. 5kpc) Galactic halo around the galaxy, which is primarily made up of dark matter. In the milky way, the gas/dust occupies the same mass as the stars in our galaxy. Supermassive black hole (millions of times the mass of the sun: what we see is the accretion disk around the black hole and the jets that flow out of the poles of the magnetic fields. Dark matter: 10-100x the mass of the stars: most massive component of the galaxy, spread out in the galactic halo that surrounds the bulge & disk of the galaxy. Took a photo of the galaxy, the size of a grain of sand held at arm"s length: this small area filled with thousands of galaxies.
