Active Galactic Nuclei (AGN) and Cosmology

1. Active Galactic Nuclei (AGN) and Cosmology Chapters 17-18

2. AGN Again? • Galaxies that emit powerful radiation in many parts of the EM spectrum are called active galaxies. However, the power source is in the nucleus—hence, AGN. • One type of active galaxy is called the seyfert galaxy, in honor of Carl Seyfert. • The spectra of normal galaxies contain the combined light of billions of stars (mostly absorption lines), but seyfert galaxies also contain broad emission lines.

3. Seyfert Characteristics • Seyfert galaxies contain large amounts of hot, low-density gas. They are broadened because this gas is moving extremely fast (30 times greater than normal). • Their luminosities fluctuate quickly*, suggesting a compact power source. • A large majority of these galaxies show signs of interaction with another galaxy. • The evidence points to the presence of a supermassive black hole in their centers.

4. Supermassive Evidence? • *1. An astronomical body cannot change its brightness in a time shorter than the time it takes light to cross its diameter! A black hole is the only power source that is small enough to explain this. • 2. These galaxies have matter around their centers that are swirling so fast, that nothing short of a black hole is massive enough hold it in orbit. Ex. M87 Dust orbiting only 60 ly from its center is traveling at 750 km/s! • Mass = 2.4 billion suns!!

5. The Core of M87 Note the Doppler shift in the above picture and the jet in the right one.

6. Disky Evidence: NGC 7052 The gas in this disk (only 57 pc from the center) spins at 155 km/s, which suggests a black hole of 300 million solar masses.

7. Disks and Jets: NGC 4261

8. Modeling AGNs • There are 2 types of Seyfert galaxies that each behave differently—their properties may depend on the angle at which we see the accretion disk (or its jet) in the center (NOT necessarily related to the galaxy as a whole). • Seyfert Type 1: bet. 0o & 90o from the disk • Seyfert Type 2: 0o (looking at the side of the disk) • What would we see if we looked straight down the jet (at 90o)? A blazar!

9. 90o 0o

10. Blazars and Quasars • Blazar: what we see when we look straight down the jet: 10,000 x more luminous than the MW; fluctuates in only hours! • A quasar (or quasi-stellar object, or QSO) looked like stars, but had strange spectra, were thousands of times brighter than average galaxies, and gave off powerful radio emissions. It turns out that they have highly redshifted lines. The first quasar had a redshift of 0.158 (15.8%). Largest = 6!!

11. A picture (left) and spectrum (below) of the first quasar, 3C273. Note how it looks very “starlike.”

12. Invisible Lenses • Gravity can bend light in such a way that a galaxy can act like a “gravitational lens” and create distorted or multiple images.

13. The galaxies that appear stretched and bent result from gravitational lensing.

14. Calculating z for Galaxies • Classical (slow) redshift (z): • For “normal” galaxies • Relativistic redshift: • For quasars • Ex. If z = 6, then we have to use the relativistic redshift equation. We get: • or 96% the speed of light! • Such a large redshift also implies a huge look-back time (several billion years!). There were more quasars long ago, when galaxies were closer and collisions more common. v =zc

15. Cosmological Assumptions • The cosmological principle assumes homogeneity and isotropy in the universe. • Homogeneity is the assumption that matter is spread evenly throughout the universe on the large scale. • Isotropy is the assumption that the universe looks the same in all directions on the large scale. • Universality is the idea that the laws we observe on Earth operate the same way in any part of the universe.

16. Cosmological Observations • The fact that the night sky is dark seems obvious, but it has a profound answer: the universe is finite in size and age! • The universe is expanding! The redshifts that we measure do NOT result from galaxies flying through space (so NOT Doppler shifts!), but space-time itself expanding and carrying the galaxies with it—as if on an expanding balloon. There is no edge or center of the universe!

18. Which Universe Do We Live In? • Space-time could have a positive curvature [curvature is due to gravity according to General Relativity]: like the surface of a sphere, finite and closed. • Space-time could have zero curvature: like a flat surface, infinite or it would have an edge. An open universe. • Space-time could have negative curvature: like a saddle, infinite or it would have an edge. An open universe.

19. Curvature Possibilities

20. Most evidence supports a flat universe (middle) and inflation. More later.

21. The Ultimate Assumption! • If our universe is expanding, then it must have started from a small, high-density, high-temperature state (run the “video” backwards). There must be a beginning! • This beginning was derisively called “the big bang” by Fred Hoyle, a strong critic. Many astronomers were repulsed at the idea of a finite universe—favored alternatives that made the universe infinite. • Myths: not an explosion in space and time, but of space and time; did not occur at one point (everywhere).

22. The Ultimate Evidence? • Radiation from the big bang should still be detectable from hot gas formed just after the bang. About 3000 K (near IR). • However, this light has a redshift of 1000! Therefore, its observed wavelength and temperature would be about 1000 times larger. Expected about 3 K (microwave). • Arno Penzias and Robert Wilson detected this in the mid-sixties: about 2.7 K! Now called the cosmic microwave background radiation. Seen in all directions! 1978 Nobel Prize

23. Other Theories • The steady state model posited that the universe is eternal and unchanging. New matter continuously appeared in order to maintain a constant density. Abandoned. • The oscillating universe model stated that the universe undergoes stages of alternating expansion and collapse so that the universe can be “infinite.” Abandoned. • Current ideas: infinite universes (“branes”); time can flow both forward and backward; time does not really exist.

24. Refinements • The work of Penzias and Wilson was rough, but later measurements have refined their work. • The COBE (Cosmic Background Explorer) satellite mapped the entire sky, refined the black body temperature (2.735  .06), and discovered tiny variations in the background temperature (from density differences, “seeds” of larger structure). • BOOMERANG, WMAP, and other satellite data support a big bang/inflationary/flat universe model.

25. CMBR Blackbody Curve

26. COBE and WMAP Data The COBE team, including George Smoot, won the 2006 Nobel Prize.

27. A (Very) Brief History of Time**Based on the book by Stephen W. Hawking • Early: What occurred within the first fraction of a second is uncertain. The laws of Physics can’t operate as we understand them today. Inflation (sudden expansion) and separation of 4 forces. • Matter Era: Cool enough for basic particles (protons, neutrons, electrons) and photons to appear. Matter wins; atoms form; finally ~75% H; 25% He. • Structure Era: Small structures become bigger structures; quasars and early stars form, then conventional galaxies, later stars, and planets.

29. Future Fate of the Universe • The fate of the universe depends on its density. The critical density ( ), which determines the universe’s geometry and its fate equals 4 x 10-30 g/cm3. • If the universe is less than or equal to , then the universe will expand forever. If the universe is greater than , then it will eventually collapse (the “big crunch”). • The % of dark matter and dark energy (?) determines the density. Probably flat.