The Masses of Galaxies

Remember Hubble’s ‘Tuning Fork’

Evolution Between Types? The ‘tuning fork’ diagram suggests evolution from one kind of galaxy to another, but this may be misleading. How can we test this?

Remember the Stars?

A First Speculation On seeing the main sequence, we wondered if hot stars might become cool faint 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.

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?

Relevant Evidence We would like to know (among other things): Masses of galaxies Masses of galaxies Whether galaxies interact (which will depend on they are distributed in space) Whether galaxies interact (which will depend on they are distributed in space)

Masses of Galaxies To determine this: note how much gravitational influence they have on the stars and gas that orbit them. Reconsider the solar system first…

Remember Kepler’s 3 rd Law The planets farther out take longer to orbit the sun. Why? take longer to orbit the sun. Why? 1 They follow longer orbital paths; and 2. They move more slowly

To Be Precise..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)

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

Now, Back to Galaxies! 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!).

StellarMotions This galaxy, seen edge-on, is spinning like a Frisbee. So one side is approaching, the other is receding.

The Gas Too… We measure its motion as well

Surprise! For many spirals, the stars and gas in the very outer parts are moving just as quickly as the material somewhat closer to the center.

…The Outer Parts of the Milky Way Included

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. Analogously, why don’t the stars and gas in the outer parts of the galaxies fly off into deep intergalactic space?

Extra Mass There must be a lot of mass in the galaxy. Its gravity holds onto the outermost stars 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. It has to be widely distributed. But what is it? Just stars, gas and dust?

NO! Dark Matter We can see the stars, and detect the gas and dust at other wavelengths (radio and infrared). 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.)

An Unsettling Implication Galaxies must 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...

The Present Understanding: a HUGE Dark Halo Note the relatively small “visible Milky Way” at the very centre!

What Is It? We Just Don’t Know! [SNOLAB experiments may eventually tell us] Black holes? Tiny planet-like objects? Exotic new sub-atomic particles? WIMPS?MACHOS? Myriads of neutron stars?

The Dynamical Evidence in Summary

Let’s Reconsider Galaxy Masses The Milky Way is ~ solar masses. Irregular galaxies (like the Magellanic Clouds) are low- mass (only ~10 9 solar masses), even counting 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.

Hierarchical Build-up? It is, however, possible that small galaxies merge to create yet larger systems. (This is something we didn’t have to consider when we were studying stars!) So we need to understand interactions between galaxies.