Sometimes many galaxies collide and merge Large masses mean collisions will be high speed

1. Giant Elliptical Galaxies • Sometimes many galaxies collide and merge • Large masses mean collisions will be high speed • Gas will get heated very hot • Giant galaxy becomes an elliptical Giant Elliptical Q. 96: Seeing The Past

2. Looking Out = Looking Back Galaxies in the Past • Smaller than modern galaxies • Irregulars are more common Why were they different? • Galaxies collided a lot in the past • Galaxies got bigger from mergers • Light travels at one light-year per year • If you look at very distant galaxies, you are seeing them as they were, not as they are • 1 kly = 1000 years • 1 Mly = 1 million years • 1 Gly = 1 billion years • You can see back almost to the beginning of the Universe

3. Galaxies Long Ago

4. Galaxies Long, Long Ago

5. Active Galaxies What’s an Active Galaxy? • Most of the power of ordinary galaxies come from the stars • Some galaxies have very bright sources right at the center • Can be as bright as a galaxy, or even brighter • Called Active Galactic Nuclei (AGN’s) • They can be incredibly bright, up to 1015LSun • Their power comes out with different spectra • Visible • Radio • Some of them vary their power in a day or less • This proves they are very, very small! • X-rays • Infrared • Ultraviolet

6. Fast  Small • A large source will not – can not – change all at once • Roar from a stadium crowd • Light (and other EM radiation) travels at c • If it changes in a time t it must be no larger than d = ct • If it changes in a day, its size is no bigger than

7. Types of AGN’s • Seyfert Galaxies • Relatively dim as AGN’s go • Spiral galaxies with lots of gas in plane of galaxy • Radio Galaxies • Produce enormous amounts of radio energy • Often, the radio emission is mostly not from the nucleus • Quasars • Similar to Seyferts, but much brighter • Radio Noisy Quasars • Blazars / BL Lacartae Objects • Visible and radio signal can vary in as little as an hour

8. Seyfert Galaxies

9. Core of a Seyfert Galaxy

10. Quasars Other Galaxies Quasar Foreground Star

11. Quasars

12. Centaurus A, a Radio Galaxy Visible Composite visible / radio

13. Radio Galaxy in Radio

14. Black Hole in M87 Galaxy in Radio • Image released April 10, 2019 • First image of black hole • 6.5 billion solar masses

15. Radio Galaxy in Visible and Radio

16. What Causes an AGN? • Black hole in center • 104 – 1010MSun • Size of Earth up to size of Solar system • Source of gas - a gas torus (doughnut) • Gas getting dumped into the center • An accretion disk of gas falling in • Rotating very fast • Friction makes it hot – X-rays • Very efficient – 50% mass  energy • Thin gas surrounding the center • Heated by X-rays from the accretion disk • Sometimes, jets shooting out

17. Jets and Lobes • Magnetic fields trapped in gases can explode outwards • Gases swept along • Flung away from AGN very fast • Beams light forward • Beams radio wavesmostly forward

18. Core of an Active Galaxy

19. Unified Picture of Active Galaxies • Some are brighter, some are dimmer • What you see depends on what angle you see it from Blazar Seyfert or Quasar or Radio Quasar Radio Galaxy

20. Active Galaxies Used to be More Common • Active galaxies, especially bright ones, are rare now • But common in the past Why? • Active galaxies require fuel to be fed into the black hole • Colliding galaxies allow gas to flow to center • Galaxy collisions were much more common in the past

21. The Cosmic Distance Ladder Geometric Methods Standard Candles Methods for Measuring Distance • Radar Distances • Parallax • Spectroscopic Parallax • Main Sequence Fitting • Cepheid Variable Stars • White Dwarf Supernovae • Hubble’s Law

22. Radar Distance 0 - few AU Earth Venus d 2d = ct, solve for d Radar distances • We know what an AU is • Effectively no error

23. Parallax p p • The farther apart you put your “two eyes”, the better you can judge distance • The smaller p is, the farther away the star is. 1AU – 1000 ly d • p in arc-seconds (The distance 3.26 ly is also known as a parallax second) parsec nearest stars several ly away Q. 97 Which Distance Method For Galaxies?

24. Standard Candles • Radar Distances • Parallax • Spectroscopic Parallax • Main Sequence Fitting • Cepheid Variable Stars • White Dwarf Supernovae • Hubble’s Law • A standard candle is any object that isconsistently the same luminosity • Like 100 W light bulbs, or G2 mainsequence stars How the technique works: • Figure out how luminous your standard candles are • If you know distance d and brightness B, you can figure this out from: L = 4d2B • To find the distance to another of the same class: • It should have the same luminosity L • Measure its brightness B • Deduce distance from: L = 4d2B

25. Spectroscopic Parallax • Has nothing to do with parallax • Works only on main sequence stars How it works: • Observe the star – determine it’s brightness B • Measure its spectral type from spectrum • Deduce its luminosity from the Hertzsprung-Russell Diagram • Find its distance from: L = 4d2B

26. Spectroscopic Parallax 10 ly – 200 kly Limitations: • The main sequence is a band, not a line • Because stars are different ages • Causes significant error • Main sequence stars are not the most luminous stars • You can’t measure it if you can’t see it • Limits maximum distance