1. The Masses of Galaxies: Dark Matter

2. Back to Hubble’s ‘Tuning Fork’
The ‘tuning fork’ diagram suggests evolution from one kind of galaxy to another, but this may be misleading.

3. Remember Stars: A First Speculation
On seeing the main sequence, we wondered if hot stars might become cool cinders as they use up their fuel; or (conversely) if cool stars might move up the main sequence as the fires grow hotter with age. Neither is correct, as we now know.

4. Similar Speculations Here!
Perhaps galaxies start out as gas-rich irregulars, develop into spirals as time passes, and eventually wind up as ellipticals when the gas is all used up? Or could there be evolution in some other sense? To assess this, we would like to know (among other things):
Masses of galaxies
Whether galaxies interact (which will depend on how they are distributed in space and their motions)

5. Masses of Galaxies
As always, we work out the masses of galaxies by determining how much gravitational influence they have on the stars and gas that orbit them.
We do this by measuring states of motion. These are not inferred from changing positions, but by measuring Doppler shifts -- the actual velocities with which the gas and stars are moving.

6. As Here: We can measure the motions of the stars in the
brighter central parts.
The gas in galaxies
extends well beyond
the visible stars, and can
also be measured (at
radio wavelengths!).

7. Stellar Motions First This galaxy, seen edge-on, is spinning.
(Imagine a
Frisbee!) One
side is approaching, the other is receding, at the velocities
shown. We get the spectrum from the integrated
(summed) light of the stars, and use the Doppler shift.

8. We Can Study The Gas Too…
We measure
its motion as
well, using
radio telescopes.
(We study the
21-cm emission
from neutral
Hydrogen gas.)

9. A Big Surprise! For almost all spiral galaxies, the stars and gas
in the very outer
parts are moving
just as quickly
as the material
somewhat closer
to the center.

10. Compare This to the Solar System
..the speed falls
off like the square
root of the distance
from the Sun
(So one object 4x
as far out as
another travels at
½ the speed)

11. Why?
In the solar system, objects at large distances feel the sun’s gravity only weakly. If they moved much faster, they would simply escape.
Objects that are closer in feel the sun’s gravity strongly. If they moved more slowly, they would fall in towards the sun.

12. Bye-bye Pluto!
Pluto is in the outer parts of the Solar System. If it moved at the speed of the Earth, it would escape the Solar System.
Why don’t the stars and gas in the outer parts of the galaxies fly off into deep intergalactic space?

13. In the Milky Way…

14. The Answer: Lots of Extra Mass
There must be a lot of mass in the galaxy. Its gravity holds onto the outermost stars and gas so they don’t escape. Moreover, it can’t all be concentrated at the center or else we would see a Kepler-like falloff of speed. The mass has to be widely distributed (and we can work out just how). But what is it? Just stars, gas and dust?

15. Invisible Dark Matter
We can see the light of the stars, and detect the gas and dust at other wavelengths (radio and infrared), because all of these things glow.
But if you add all of them up, there’s not nearly enough to explain the total gravitational influence. There must be something else.
(We know it is there by its gravity. But it emits no detectable radiation – it is dark.)

16. This is Very Unsettling!
Galaxies contain a lot of widely-distributed invisible matter which contributes to the gravitational influence.
Indeed, as much as 90% of the mass of the galaxies may be in this invisible form...
…and we don’t yet know what it is!

17. Our Present Understanding: The Milky Way Has a HUGE Dark Halo
Note the relatively small visible Milky Way at the very centre of this huge gravitating mass.

18. What Is It? We Don’t Know! [SNOLAB experiments may eventually tell us]
WIMPS? (“weakly interacting massive particles”)
MACHOS? (“massive compact halo objects”)
Black holes?
Exotic new sub-atomic
particles? (gravitinos,
axions, neutralinos….)

19. Let’s Reconsider Galaxy Masses
The Milky Way is ~ 1012 solar masses.
Irregular galaxies (like the Magellanic Clouds) are low-mass (only ~109 solar masses), even counting the dark matter.
Individually, they are simply too puny to turn into the big spirals and ellipticals (~ solar masses). No single galaxy changes form in this way.

20. Hierarchical Build-up?
It is, however, possible that small galaxies merge to create yet larger systems.
Note that we didn’t have to consider this sort of thing when we were studying stars! Small stars don’t gradually merge to form yet more massive ones.
So we will need to understand interactions between galaxies.