Chapter 21 Galaxy Evolution . How do we observe the life histories of galaxies?. Deep observations show us very distant galaxies as they were much earlier in time (Old light from young galaxies). How did galaxies form?. Our best models for galaxy formation assume: Matter originally
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Deep observations show us very distant galaxies as they were much earlier in time (Old light from young galaxies)
Our best models for galaxy formation assume: • Matter originally • filled all of space • almost uniformly • Gravity of denser • regions pulled in • surrounding • matter
Denser regions contracted, forming protogalactic clouds H and He gases in these clouds formed the first stars
Supernova explosions from first stars kept much of the gas from forming stars Leftover gas settled into spinning disk Conservation of angular momentum
NGC 4414 M87 But why do some galaxies end up looking so different?
Conditions in Protogalactic Cloud? Spin: Initial angular momentum of protogalactic cloud could determine size of resulting disk
Conditions in Protogalactic Cloud? Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk
Distant Red Ellipticals • Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe
Collisions were much more likely early in time, because galaxies were closer together
Many of the galaxies we see at great distances (and early times) indeed look violently disturbed
Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical
Shells of stars observed around some elliptical galaxies are probably the remains of past collisions
Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together
Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies
Starburst galaxies are forming stars so quickly they would use up all their gas in less than a billion years. Likely the result of galactic collisions. Few have been observed.
Intensity of supernova explosions in starburst galaxies can drive galactic winds
X-ray image Intensity of supernova explosions in starburst galaxies can drive galactic winds
A galactic wind in a small galaxy can drive away most of its gas. This may explain the lack of young stars and cool gas in elliptical galaxies.
Radio astronomy 1960s Radio astronomy found bright objects, 107 X brighter than normal galaxies at radio wavelengths, many looked like either normal galaxies or stars. Turned out to be a number of different types with what is now believed to be similar power source. Seyfert Galaxies Radio Galaxies: core-halo, radio lobe QSOs or quasars
Very luminous Different in the distribution of the energy—clearly nonstellar in origin [ different intensity, distribution in wavelength and space]. More energy in radio wavelengths than anything seen before. Location in Space : more found at great distances Quasars are all very remote.
Energy Profile Active galaxies Are intense radio sources. Over all more energy. Not blackbody.
Seyfert Galaxies TYPE 1: very luminous at X-ray and uv wavelengths and have broad emission lines of highly ionized atoms. Emission lines = low density gas Ionized = excited gas Broad lines = fast rotation TYPE 2: lack the strong X-ray emissions, emission broadening not as pronounced.
Seyferts look like spirals Seyfert galaxies (1943 Carl Seyfert) : Most have strong redshifts. 100s of Mpc away. All have active nuclei.
‘Nearby’ Seyfert Galaxy Circinus galaxy at 4 Mpc is one of the closest Seyfert Galaxies. Cores alone in radio and IR emit up to 10X energy of our whole galaxy. Energy from small source (<1 lyr.). Fluctuations. Spectral broadening suggests rotating matter near core. Velocities at cores are roughly 10,000 km/s. 30X normal. Spectra not star like. About 2% of spirals appear to be Seyferts.
Seyfert Galaxies are intermediate between normal spirals and the most violent . Optical images look like spirals. But the overall energy emission shows the largest part of the energy is from the galactic nucleus and is in the form of invisible radio and infrared radiation, & nonstellar in distribution. Fluctuations in the energy output shows the energy is produced in a compact source.[ luminosities may vary by 50% in less than a month] Seyferts are 3x more likely to be interacting and 25% have shapes suggesting tidal forces. Seyferts may have been kicked into activity by collisions with other galaxies.
Quasar History 1960’s Objects whose images looked like distant stars were found with strange radio emissions. Difficult to identify at first. The problem solution began with the recognition of the spectra as being redshifted farther than anything previously seen. Large redshift = far away. Correct energy output for the implied distance leads to a huge energy output. Factors of 107 larger than the entire Milky Way in the radio region.
If the center of a galaxy is unusually bright we call it an active galactic nucleus Quasars are the most luminous examples Active Nucleus in M87
The highly redshifted spectra of quasars indicate large distances From brightness and distance we find that luminosities of some quasars are >1012LSun Variability shows that all this energy comes from region smaller than solar system
Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant B. They were more common early in time C. Galaxy collisions might turn them on D. Nearby galaxies might hold dead quasars All of the above!
Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures
Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed
The lobes of radio galaxies can extend over hundreds of millions of light years
If they were visible the radio lobes of Centarus A would be 10X the size of the full Moon. p.280
An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light Speed of ejection suggests that a black hole is present
Radio galaxies don’t appear as quasars because dusty gas clouds block our view of accretion disk
Characteristics of Active Galaxies • Luminosity can be enormous (>1012LSun) • Luminosity can rapidly vary (comes from a space smaller than solar system) • Emit energy over a wide range of wavelengths (contain matter with wide temperature range) • Some drive jets of plasma at near light speed
What is the power source for quasars and other active galactic nuclei?
Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars
Energy from a Black Hole • Gravitational potential energy of matter falling into black hole turns into kinetic energy • Friction in accretion disk turns kinetic energy into thermal energy (heat) • Heat produces thermal radiation (photons) • This process can convert 10-40% of E = mc2 into radiation (compared to 1% in fusion)