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. What’s an Active Galaxy?
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 1015 LSun
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!
Infrared
Ultraviolet
X-rays

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 – 1010 MSun
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 waves mostly 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 Methods for Measuring Distance
Radar Distances
Parallax
Spectroscopic Parallax
Main Sequence Fitting
Cepheid Variable Stars
White Dwarf Supernovae
Hubble’s Law
Geometric Methods
Standard Candles

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. nearest stars several ly away
Parallax
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 p
(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 is consistently the same luminosity
Like 100 W light bulbs, or G2 main sequence 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