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)

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

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

Edge-on Spiral Galaxy

NGC 891 - we think our Galaxy looks like this

This is an infrared

composite image from

KPNO

Spiral sequence

Sandage Hubble Space Telescope

NGC 7537/Sbc

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

Spiral sequence

Sandage Hubble Space Telescope

NGC 5838/S0

Spiral sequence

Sandage Hubble Space Telescope

NGC 5689/Sa

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

Barred Spiral NGC1365

Optical/Sandage

Optical/HST WFPC2

IR/HST NICMOS

M87 Optical

Giant elliptical galaxyAt center of Virgo clusterMany globular clusters surround itLittle dust and gas in the galaxy50 million light years away

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

Irregular Galaxies

NGC 4753/I0

Sextans A

5000 light years across

5 million light years away

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

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

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

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

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

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

Centaurus A/Optical

Giant elliptical galaxy10 million light years awayDark dust lane obscures the central region

Centaurus A/Radio

Double Lobe Radio GalaxyImage shows radio lobes superposed on optical imageCentral black hole must be producing radio jets and lobes

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

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