Download cosmo_01_overview - Mullard Space Science Laboratory

Cosmology and
extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
1. Overview and introduction
Slide 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.
Slide 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 Hubble’s 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
Slide 4
Our place in the Galaxy
• For `early*’ Cosmologists our Galaxy was
the Universe.
• It’s a good place to start our travels.
*up until about 100 years ago!
Slide 5
Our Galaxy
Our galaxy itself contains a hundred billion stars.
It’s a hundred thousand light years side to side.
It bulges in the middle, sixteen thousand light years thick,
But out by us it’s just three thousand light years wide.
We’re 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
Slide 6
What does our galaxy look like?
The word galaxy comes from the greek word for milk
Slide 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.
Slide 8
They decided that we are in the middle of
the galaxy!
William Herschel’s star map
• They saw the same number and
brightness of stars in all directions!
Slide 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.
Slide 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 < 1 day.
• Standard candle! gives
distance (inverse square
law)
Slide 11
So where are we?
But Shapley was off by a factor of 2 because of
extinction by interstellar dust -(Still pretty good!)
Slide 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
Slide 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
Slide 14
Mapping the arms in 21cm
Use velocity
in the
spectral line
to separate
the arms.
Slide 15
The observed spiral structure
Slide 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…
Slide 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…
Slide 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…
Slide 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
So the arms would be all wound up
happens…
after just a few orbits!
Slide 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.
Slide 21
Slide 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.
Slide 23
Observation vs expectation:
Slide 24
Idealised motion in the galaxy disc
• Assume all the
mass M is
concentrated
in the bulge
• Circular
motion under
gravity
(Newton and
Kepler):
• MTOT=r v2/G
v=
GMTOT
r
Slide 25
Idealised motion in the galaxy bulge
• Assume uniform
density r, spherical
bulge.
• MINT=4prr3/3=rv2/G
• (of course r isn’t
constant in reality)
v = r 4prG
3
Slide 26
So there is a problem!
• Orbital velocities rise OK but then don’t
fall off.
• Something wrong
– Either gravity not a 1/r2 (!!)
– or there is more mass than we can see.
– This is “dark matter”
Slide 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