Peter Watson, Dept. of Physics Galaxies Origin of the Milky Way, Tintoretto Peter Watson Galaxies Peter Watson First: some things that aren’t galaxies • A lot of the Messier objects are globular clusters of stars: relatively bright and close, mostly old stars • e.g. M2 in Aquarius: about 100000 stars Credit & Copyright: D. Williams, N. A. Sharp, AURA, NOAO, NSF Peter Watson • e.g. M3 (note lots of red giants) Peter Watson • ~200 round our galaxy: all galaxies seem to have them. • M87 (more about it later) has about 1000 globulars. Anglo - Australian Telescope photograph by David Malin Copyright: Anglo - Australian Telescope Board Peter Watson • Dynamics are easy to understand
Transcript
Peter Watson, Dept. of Physics
Galaxies
Origin of the Milky Way, Tintoretto
Peter Watson
Galaxies
Peter Watson
First: some things that aren’t galaxies
• A lot of the Messier objects are globular clusters of stars: relatively bright and close, mostly old stars
• e.g. M2 in Aquarius: about 100000 stars
Credit & Copyright: D. Williams, N. A. Sharp, AURA, NOAO, NSF
Peter Watson
•e.g. M3 (note lots of red giants)
Peter Watson
•~200 round our galaxy: all galaxies seem to have them.
•M87 (more about it later) has about 1000 globulars.
Anglo-Australian Telescope photograph by David Malin Copyright: Anglo-Australian Telescope Board
Peter Watson
•Dynamics are easy to understand
Peter Watson
Spiral Galaxies• Some are spread out,
like NGC6946
• About 10 billion stars
• About 100,000 light years across
• Can’t see individual stars: red patches are “star nurseries”
• “Hot spot” in centre
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• Some are tightly wound up,like M31 (the Andromeda galaxy)
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•Some are seen side on
•NGC4565
•Note the dust clouds
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• Some have “bars” across the centre
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M101 aka Pinwheel
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As usual, see a lot more if we look at them in different ways
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• Purple =x-rays
• Blue = UV
• Yellow = optical
• Red = IR
• Note arms seem to be hot, means young stars
• spiral arms are young new stars
• old stars don’t show arms
• Note rotation of galaxy ~ 500 million years
• lifetime of large stars ~ 10 million
• Density wave theory
Why spirals?
Group of young stars
go supernova and compress gas
small stars remain, new group of young stars are born
Why spirals?
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The Milky Way is hard to see, since we are inside it!
• But it looks roughly like this
• With the sun about here
If you are in Scotland• Go to the Crawick multiverse
• Site of old open pit mineM31 (Andromeda)
Milky Way
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The centre of the Milky Way• This is the Milky way, showing the whole sky
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• we can pick out the same general structure in radio waves, but note very intense source at centre
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•And gamma-rays
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•Galactic Centre Not visible directly (too much dust)
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•But we can zoom in with radio waves
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•And X-rays
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The stars there are swirling round something~1000000 Mo
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•In addition to the stars, there is a huge clump of gas feeding it
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•Something like this!
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•Whole picture is consistent with massive black hole (4 million xMo) at centre
•Can see this in other galaxies
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•Some galaxies have grabbed hold of other galaxies
•This is M51
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•Then there are galaxies that seem to having problems:
• NGC 1512
• Starburst galaxy: stars are forming in huge numbers round the outside
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Elliptical galaxies are much less fun!
Credit: W. A. Baum (U. Washington), WFPC2, HST, NASA
NGC 4881 in Coma
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M87 looks dull But it’s huge: one trillion stars
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And it has a huge jet emerging from the
black hole at its centre
Which seems more complicated the closer you look!
Centaurus A is a strange galaxy
With jets coming from the centre
Sometimes the jets are far larger than the galaxy!
• Hercules A
• aka 3C348~ 1 Mpc ~ 3 million light-years
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Jets• We seem to see jets
on all scales, from small new stars to giant BH’s
• This is how they might work: spinning BH produces wrapped up mag field that focusses particles
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First “real” BH photo• from the Event Horizon Telescope
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April 10, 2019 BH in M 876.5 billion M0
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Galaxies often come in groups
• 3 galaxies in Draco
Copyright: Giovanni Benintende
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•Which means they can collide
•These are the Antennae galaxies
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•When galaxies collide the stars almost never do, but the clouds of gas do
•X-ray picture of the antennae
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•We can see how this might have happened
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•M81 and M82 get very close every 100 million years:
Credit & Copyright: Leonardo Orazi
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•M 82 is getting ripped apart
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•And the “cartwheel” galaxy is a remnant of a much older collision
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•Some are tightly packed
•Stefan’s quintet
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•As we look out we see more and more galaxies
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•The Coma cluster is made up of 10000 galaxies
•Apart from one bright star, almost all the objects are galaxies
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•But there are more
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•And the further out we go, the more we see Quasars• Bright objects were observed in early radio maps
which had no obvious optical counterpart
• Several hundred seen in the 3rd Cambridge catalogue
• In 1960 a faint blue ‘star’ seen at location of 3C48
• Detailed studies made when another blue star found at 3C273
•Quasi-stellar objects…
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Position of 3C273 found v. accurately by lunar occultation, so could be identified with 13 mag. blue "star" with jet projecting from it
Except stars don’t have jets!
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This shows the problem: it shows a galaxy (maybe 2) a quasar and a star. Which is which?
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Quasars�• The quasars are much more abundant in the early universe
• They appear to be an early stage of galaxy formation
• As quasars feed on local matter they get heavier
• Most luminous quasars consume 1000 solar masses per year
• Life of the quasar stage is only ~ 106 years
• The resulting massive black holes are clearly related to galaxy formation
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Radio image of Quasar
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• Only object we know that would work is massive hungry black hole
• Expect up to 20% of the rest energy of infalling matter gets converted to some form of radiation
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Quasars�• The quasars are much more abundant in the early
universe
• They appear to be an early stage of galaxy formation
• As quasars feed on local matter they get heavier
• Most luminous quasars consume 1000 solar masses per year
• Life of the quasar stage is only ~ 106 years
• The resulting massive black holes are clearly related to galaxy formation
HST
Seyfert galaxy: very bright star-like centre
NGC 6814
• Quasars very common in early universe.
• Seyfert galaxies seem to be intermediate between quasars and normal galaxies
• Quasars evolve to Seyferts then to normal spirals as black hole consumes most of central core
Gravitational lensing and quasars• About 10000 quasars known
• Each is very characteristic: red-shifts and spectrum are very distinct.
• However several pairs which lie very close in sky: e.g. 0957 +561A & 0957 +561B are 6" apart in sky and have identical red-shifts. Note "fuzz" sticking out of lower one
HST
Galaxies and “double quasar”
Can be understood via radio image: massive galaxies will
