GALAXY FORMATION AND EVOLUTION 1

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GALAXY FORMATION ANDEVOLUTION - 1

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The Expansion of Galaxies
In the 1920s, Edwin Hubble demonstrated that all
the nebulae observed in the sky, which were
previously thought to reside in our own Galaxy,
were actually Island Universes, I.e., other
galaxies. They also have a recession velocity
relative to us (they run away from us)
How did he prove it?
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Emission spectra
V0
Light becomes redder
V1000
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Redshift
Redshift is defined as the fraction by which the
lights wavelength changes.
?1
z ????????? / ??????(????? / ?
?2
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How to calculate a redshift
H?, one of the hydrogen recombination lines, has
a well known wavelength of emission??? 6563 A
In the galaxy NGC7714, H??is measured at
wavelength ?? 6624 A
z (????? / ?
NII H??NII
z (6624-6563) / 6563 0.0093
SII
Very small for this galaxy
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How to convert a redshift into a recession
velocity
If the redshift of galaxies is a Doppler shift
(I.e., the shift in light is caused by the
galaxies moving away from us, and the faster
their velocity from us, the stronger the shift)
think of an ambulance first approaching (high
pitch) and then receding (low pitch) from you
z (????? / ?
V c (????? / ?
Speed of light
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How to calculate a recession velocity
H?, one of the hydrogen recombination lines, has
a well known wavelength of emission??? 6563 A
In the galaxy NGC7714, H??is measured at
wavelength ?? 6624 A
v c (????? / ?
NII H??NII
v 300,000 (6624-6563) / 6563 2788 km/s
SII
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Hubble noticed that dimmer (farther away)
galaxies had larger recession velocities
Ho is the Hubble constant
v Ho d (Ho 71 km/s/Mpc)
The Hubble Law
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How to measure distances
Very difficult, and until recently astronomers
were in disagreement on the value of the Hubble
constant (some said 100 km/s/Mpc, some said 50
km/s/Mpc). Some methods used to measure
distances involve supernovae, and Cepheids
(pulsating stars) The HST solved the issue of
the value of the Hubble constant.
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Use the Hubble Law to know distances!
Once the Hubble Constant is known, you can use
the Hubble Law to derive distances to
galaxies d V/Ho 49,000 km/s / (71
km/s/Mpc) 690 Mpc
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All observers see the same expansion
30
40
10 Mpc
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2100, an alien
2800
1400
You V0 km/s
700
700 km/s
V2100-21000
V2100-02100
V2100-1400700
V2800-2100700
The alien will observe the same Hubble Law
Cosmological Principle
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The Hubble expansion is expansion of space
between galaxies
Think of buttons strung across a rubber
band Stretch the rubber band and the buttons
will separate from each other.
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Because light has finite speed, a further away
galaxy is also younger
Thus redshift can also be used to tell time,
and by how much the size of the Universe has
changed Z (change in ?) /?? (change in size
of Universe) / (size of Universe)
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What is in the Cosmos?
4 baryons (ordinary matter) 27 dark matter 69
dark energy
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