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Galaxy Formation
Formation of galaxies in cold dark matter universe
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Hubble Galaxy Sequence
Ellipticals (round through oval)Spirals (centrally condensed with arms)Barred Spirals (distinct bar shape with bulge)
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Shapes of Galaxies
Spirals disk shaped with spiral arms often have bright bulges in center contain interstellar gas, nebulae, star forming
regions, open clusters and globular clusters
Barred Spirals spiral arms emerge from end of bar gas from outer part of galaxy funneled to center
through the bar, forming new stars in bulge
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Shapes of Galaxies
Ellipticals range from spherical to foot-ball shapes lots of old stars and globular clusters star formation is over or just restarting maybe the result of collision and merger of smaller
galaxies
Irregulars lots of gas and new stars forming rather small compared to spirals and ellipticals
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Edge-on Spiral Galaxy
NGC 891 - we think our Galaxy looks like this
This is an infrared
composite image from
KPNO
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Spiral sequence
Sandage Hubble Space Telescope
NGC 7537/Sbc
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Spiral Galaxies
Bulges formed over a short period very early in the young universe, perhaps through the collapse of a single cloud of hydrogen or merger of primeval star clusters.
NGC 1232/VLT
NGC 1288
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Spiral sequence
Sandage Hubble Space Telescope
NGC 5838/S0
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Spiral sequence
Sandage Hubble Space Telescope
NGC 5689/Sa
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Barred Spiral M61
Bar across central region is made of stars, gas, and dust
Small bulge is dominated by a disk of material
Spiral arms begin at both ends of the bar
The bar is funneling material into the hub, which triggers star formation and feeds the bulge
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Barred Spiral NGC1365
Optical/Sandage
Optical/HST WFPC2
IR/HST NICMOS
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M87 Optical
Giant elliptical galaxyAt center of Virgo clusterMany globular clusters surround itLittle dust and gas in the galaxy50 million light years away
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M87 Optical Jet
HST IR and UV composite
Globular clusters also seen
Note shock waves and knot patterns in jet
Bright point at central black hole
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Irregular Galaxies
NGC 4753/I0
Sextans A
5000 light years across
5 million light years away
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Galaxy Evolution
A computer simulation of a flat disk-type galaxy colliding with a dwarf galaxy. The dwarf galaxy cannot be seen, but its gravitational influences trigger the production of spiral arms in the gas-rich disk.
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Collisions and Mergers
Note: galaxies do not make noise when they collide!
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Galaxy Collision
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Galaxy Collisions, Part 2
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M31/Andromeda
Our nearest spiral neighbor2 million light years away
Center of M31 has two optical nuclei
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M31/Chandra X-ray
Image of central part of our neighbor M31 Bright X-ray sources are binaries with black holes or neutron starsCentral BH is very cool, 30 million Mo
Center of M31
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Cartwheel Galaxy
Wheel shape was formed from collision of two galaxiesBright stars are forming at the edges of the wheel (105 light years in diameter)Intruder galaxy is no longer visible
This is from HST
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Active Galaxies
Quasars Quasi-stellar radio source. (A radio source that is
so far away that it looks like a star.) Bright nucleus Highly variable Most produce strong X-rays
Seyfert galaxies Spiral galaxies with bright nucleus Seyfert 2 galaxies are viewed through gas disk
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Active Galaxies
Radio galaxies quasars with radio emission (original quasars) some have two lobes, connected by jets some are wildly variable and have X-rays not especially bright in visible light often giant elliptical galaxies
Blazars wildly variable, have lots of X-rays and gamma-
rays
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Unified Model of AGN
All AGN are the same: central BH plus disk and jetsDifferences in lines, jets, & spectra are due to different viewing angles Also possible that BHs are being fed at different rates
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Centaurus A/Optical
Giant elliptical galaxy10 million light years awayDark dust lane obscures the central region
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Centaurus A/Radio
Double Lobe Radio GalaxyImage shows radio lobes superposed on optical imageCentral black hole must be producing radio jets and lobes
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Centaurus A/X-ray
X-ray image from ChandraBright nucleus can be seen in center at location of black holeSmall jet to lower right ends in shocked region
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Centaurus A/IR
IR map shows a second galaxy (barred spiral) hiding inside Cen A’s dust lanesElliptical’s gravity helps barred spiral maintain its shapeMaterial funneled along the spiral's bar fuels the central black hole which powers the elliptical's radio lobes Symbiotic relationship