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Astronomy II, Fall 2005
Lectures on galaxies and cosmology
Dr Martin Hendry
University of Glasgow, UK
(Basler Chair, 2005)
[email protected]
Tel: 94252
Brown Hall, Room 373
Office Hrs: Mon 10.30 – 11.30
Tue 14.30 – 15.30
http://www.astro.gla.ac.uk/users/martin/basler/astro2/
Measuring the intrinsic diameter of a galaxy
1 radian = 206265 arcsec

D

2 2d
1 degree = 60 arcmin
= 60 x 60 = 3600 arcsec
Angular
diameter

(in RADIANS)
D  d
Diameter
(See also Astronomy Today pg 28)
Distance
Hubble’s Expanding Universe
Through the 1920s Hubble
also measured the shift in
colour, or wavelength, of the
light from distant galaxies.
Galaxy
Laboratory
He found that most galaxies displayed a redshift which, by
applying the Doppler formula indicated that these galaxies
were moving away from us.
Spectroscopic Binaries
Orbits, from above
B
A
A
B
A
B
B
A
To Earth
Spectral lines
0
B
0
A
A+B
0
A
0
B
A+B
Doppler Shift Formula: (OK if v << c)
Radial velocity
(can be +ve or –ve)
Change in wavelength
(can be +ve or –ve)

redshift
v
z

0 c
Wavelength of light as
measured in the laboratory
Speed of light
Hubble’s Law: 1929
Distant galaxies are receding from us with a
speed proportional to their distance
Hubble’s Interpretation
‘Recession of the Nebulae’
caused not by the motion
of galaxies through space,
but the expansion of space
itself between the galaxies
Einstein’s General Relativity
Einstein’s Relativity
“Matter tells space how
to curve, and space tells
Matter causes space
matter how to move”
to curve or warp
Hubble’s Law
Radial
velocity
Hubble’s constant
v = H0 d
distance
H0 has units of (time)-1 – usually measured
in kilometres per second per Megaparsec
It tells us how fast the Universe is expanding
H0-1 = Hubble time = timescale for the
expansion age of the Universe
So provided Hubble’s law holds for more distant galaxies,
the measured recession velocity of a galaxy gives us an
accurate estimate of its distance, assuming we know the
value of the Hubble constant.
Let’s look at Hubble’s original data again!
H0 ~ 500 kms-1 Mpc-1
So Hubble’s original data led to a gross over-estimate of the
Hubble constant, and hence a gross under –estimate of the age of
the Universe. (The Universe can’t be younger than the Earth!)
Main reason for Hubble’s error was that he got his distance
estimates badly wrong (see later).
But his linear relation between distance and recession velocity
was correct, and holds well at much greater distances.
Even if we don’t know the Hubble constant precisely, we can use
the measured redshifts (radial velocities) of galaxies to
estimate their relative distance. This lets us make 3-D maps of
the Universe, which we call redshift surveys.
On this map, even without a scale, we still know:
 the pattern of how buildings are laid out on campus
 e.g. the CPA is twice as far from Brown Hall as the Culp Center.
Mapping the Universe with redshift surveys
Largest feature presently seen is the so-called Great Wall in the Sloan
Digital Sky Survey. This feature spans about 250 Mpc.
On larger scales than this the Universe begins to look uniform (at least
once we account for undersampling of dimmer galaxies very far away).
A hierarchy of clustering
Galaxy clusters are collections of galaxies which are held together by
their mutual gravity. The galaxies in a cluster are not receding from
each other due to the expansion of the Universe; locally (i.e. within
the cluster, the gravity is strong enough to overcome the expansion).
On even larger scales we see patterns of galaxy clusters which we
refer to as superclusters. These are not gravitationally bound.
A hierarchy of clustering
The nearest rich cluster of galaxies to the Local Group is the Virgo
Cluster, which contains about 2500 galaxies.
It lies at a “distance” of about 1400 km/s (or ~20 Mpc).
The Local Group and Virgo Cluster belong to the “Local Supercluster”:
a concentration of many rich galaxy clusters within 5000 km/s
Measuring the Hubble Constant - 1
We need to find (exactly as Edwin Hubble himself did!) some galaxies
for which we can measure:
o their redshift ( recession velocity)
o their distance (independently of their redshift)
If we plot recession velocity versus distance on a graph, and our data
lies on (or close to) a straight line, then the slope of the line is the
Hubble constant.
(Astro II students will get the chance to do this for themselves in
the Laboratory!)
Measuring the Hubble Constant – 2
So why did Hubble get it so wrong?...
He was using the wrong calibration for his galaxy distances. (Like
thinking measurements are in feet, when in fact they’re in inches!)
The Virgo Cluster distorts Hubble’s Law in our Local Universe.
Our Local Group has a “peculiar motion” towards the Virgo Cluster,
due to the gravitational pull of its galaxies + dark matter.
Hubble’s galaxies weren’t distant enough to overcome this problem.
To measure the true value of the Hubble constant, we need to
measure galaxy distances well beyond the Virgo Cluster
Distortion of Hubble’s Law due to Virgo cluster
Problem:
Need to determine H0
from remote galaxies,
where peculiar motions
are less important….
….but….
We cannot use
Cepheids to measure
their distance
Need Distance Ladder!!
The Tully Fisher Relation for Spirals
The more luminous a spiral galaxy is, the faster it rotates.
B
Neutral hydrogen emission
v rot ~ 150kms -1
A
v rot ~ 150kms -1
Direction towards the Earth
A
1000
radial velocity
B
1500
(kms -1 )
Problem:
Need to determine H0
from remote galaxies,
where peculiar motions
are less important….
….but….
We cannot use
Cepheids to measure
their distance
Need Distance Ladder!!
Type I Supernova
White dwarf star with a massive binary companion. Accretion
pushes white dwarf over the Chandrasekhar limit, causing
thermonuclear disruption
Good standard candle because:Narrow range of luminosities at maximum light
Observable to very large distances
www.space-art.com
Some examples of Type I supernova light curves
Narrow range of absolute magnitude
at maximum light indicates a good
Standard Candle
No. of days since maximum light
Luminosity and flux
Apparent brightness, or flux, falls off with the square of the
distance, because the surface area of a sphere increases with
the square of its radius
Distance, (metres)
L  4 D F
2
Luminosity, (watts)
Flux, (watts / square metre)
Distance, (metres)
L  4 D F
2
Luminosity, (watts)
Flux, (watts / square metre)
A Type I supernova has a luminosity 10 times that of the Sun.
9
17
As seen from the Earth, the supernova appears 10
How far away is the supernova?
fainter than the Sun.
Measuring the Hubble Constant – 3
Although the errors which caused Hubble to find H0 ~ 500 kms-1 Mpc-1
were gradually eliminated, even by the late 1980s, the value of the
Hubble constant was still controversial, because of disagreements
over the different steps of the Distance Ladder.
Some astronomers argued that H0 ~ 100 kms-1 Mpc-1
Others that
H0 ~ 50 kms-1 Mpc-1
The Hubble Space Telescope launched in 1990 was to come to the
rescue!
H0?
HST has ‘bypassed’
one stage of the
Distance Ladder, by
observing Cepheids
beyond the Local
Group of galaxies
HST Key Project, led by Wendy Freedman
Measure Cepheid distances to ~30 nearby galaxies,
Link Cepheids to Secondary distance indicators
Virgo Cluster galaxy
M100, 60 million light years distant…..
HST has ‘bypassed’
one stage of the
Distance Ladder, by
observing Cepheids
beyond the Local
Group of galaxies
This has dramatically
improved
measurements of H0