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Cosmology and extragalactic astronomy

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the big bang, nucleosynthesis, inflation. the cosmic microwave background ... It bulges in the middle, sixteen thousand light years thick, ... – PowerPoint PPT presentation

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Title: Cosmology and extragalactic astronomy


1
Cosmology and extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
1. Overview and introduction
2
1. Overview
  • This lecture
  • What is going to be covered in the course
  • Before we get too extragalactic or cosmological!
  • Our place in the Galaxy.
  • Motions and dark matter in the Galaxy.

3
What is to be covered in the course
  • Galaxies
  • what they are, the different types, clustering
  • high redshift quasars, giant starbursts and
    evolution
  • The expanding Universe
  • doppler shifts and Hubbles law
  • cosmological redshift and expansion
  • The cosmic distance ladder
  • Cosmology
  • concepts, principles and competing theories
  • the big bang, nucleosynthesis, inflation
  • the cosmic microwave background
  • dark matter, growth of large scale structure

4
Our place in the Galaxy
  • For early Cosmologists our Galaxy was the
    Universe.
  • Its a good place to start our travels.

up until about 100 years ago!
5
Our Galaxy
  • Our galaxy itself contains a hundred billion
    stars.
  • Its a hundred thousand light years side to side.
  • It bulges in the middle, sixteen thousand light
    years thick,
  • But out by us its just three thousand light
    years wide.
  • Were thirty thousand light years from Galactic
    centre point.
  • We go round every two hundred million years,
  • And our galaxy is only one of millions of
    billions
  • In this amazing and expanding Universe.
  • Monty Python

6
What does our galaxy look like?
  • The word galaxy comes from the greek word for
    milk

7
But where are we in the Galaxy?
  • Very hard to see such a lot of the galaxy in such
    fantastic contrast as the last picture
    (especially today from London..)
  • Herschel and Kapteyn tried to determine where we
    are in the Galaxy from star counts.

8
They decided that we are in the middle of the
galaxy!
William Herschels star map
  • They saw the same number and brightness of stars
    in all directions!

9
They were wrong!
  • Interstellar dust in the Galactic plane absorbs
    light.
  • the uniformity is because we can see about the
    same distance in all directions through the plane.

10
So where are we?
  • Harlow Shapley (1920)
  • Looked out of the plane at globular clusters.
  • Apparent magnitude of standard candle gives
    distance (inverse square law)
  • Globular clusters orbiting the galaxy
  • RR Lyrae variables all 100 LO
  • Periods lt 1 day.
  • Standard candle! gives distance (inverse square
    law)

11
So where are we?
But Shapley was off by a factor of 2 because of
extinction by interstellar dust -(Still pretty
good!)
12
The Milky Way in the infrared
  • IRAS at 100 mm emission from dust
    concentrated in plane.
  • Cobe near IR fairly transparent to dust
    starlight

13
What about spiral structure?
  • Need to see through the dust.
  • Need to locate material along the line of sight.
  • 21cm line from spin-flip in atomic hydrogen is
    ideal to trace cool gas

14
Mapping the arms in 21cm
Use velocity in the spectral line to separate the
arms.
15
The observed spiral structure
16
But what are the spiral arms?
Are they specific material rotating
differentially? Well, we know how fast they are
rotating, so lets see what happens
17
But what are the spiral arms?
Are they specific material rotating
differentially? Well, we know how fast they are
rotating, so lets see what happens
18
But what are the spiral arms?
Are they specific material rotating
differentially? Well, we know how fast they are
rotating, so lets see what happens
19
But what are the spiral arms?
Are they specific material rotating
differentially? Well, we know how fast they are
rotating, so lets see what happens
So the arms would be all wound up after just a
few orbits!
20
Density wave model
So the arms must be moving slower than the actual
material! Best model is that they are density
waves in the disc. Fast moving gas and dust
arrives at a region of higher density. The gas is
slowed and compressed causing star
formation. Bright new stars move out of dense
region and travel on. O and B stars form they
are very luminous but short lived so mark the
positions of the arms.
21
(No Transcript)
22
Rotation of the galaxy
We can learn something else very important from
the motions in spiral galaxies. The stars and gas
are orbiting about the mass interior to their
orbits. If we measure how the velocity changes
with radius we can see the radial distribution of
matter. Most of the visible mass is concentrated
in the bulge.
23
Observation vs expectation
24
Idealised motion in the galaxy disc
  • Assume all the mass M is concentrated in the
    bulge
  • Circular motion under gravity (Newton and
    Kepler)
  • MTOTr v2/G

GMTOT
v
r
25
Idealised motion in the galaxy bulge
  • Assume uniform density r, spherical bulge.
  • MINT4prr3/3rv2/G
  • (of course r isnt constant in reality)

26
So there is a problem!
  • Orbital velocities rise OK but then dont fall
    off.
  • Something wrong
  • Either gravity not a 1/r2 (!!)
  • or there is more mass than we can see.
  • This is dark matter

27
Some key points
  • The Universe extends well beyond our Galaxy.
  • Dust can have a profound effect on cosmological
    observations.
  • We live in the disc of a spiral galaxy not at
    the centre.
  • Spiral arms must be density waves rather than
    circulating material.
  • Our Galaxy contains a lot of dark matter
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