Why is the Milky Way Flat?

Transcription

1 Why is the Milky Way Flat?

2 Why the Flattening? Analogous question: why is the solar system flat? Could the same reason, whatever it is, be responsible for the flattened Milky Way?

3 Ordered Motion The Solar System is in a state of ordered motion, with all planets orbiting in the same direction. This is a consequence of how it formed.

4 This is Easily Monitored! In the Solar System, we can easily watch the planets move. Even the orbit of remote Pluto takes less than three centuries.

5 A Challenge! Consider the Equivalent Numbers for the Milky Way

6 Unlike the Solar System The Galaxy rotates once every few hundred million years! How can we hope to monitor that? Will it be possible to map out all the different motions? No. In fact, we can t readily study the motions of stars tens of thousands of light years away, but must work more locally. Given that the stars near the Sun all move more-or-less together, roughly in parallel, we need to think carefully about what significant evidence we can hope to find.

7 How Might the Milky Way Rotate? Three Possibilities 1. Solid-body rotation: the galaxy turns as a unit, like a frisbee or a merry-go-round does 2. Keplerian rotation: the Milky Way rotates under the gravitational control of a hugely dominant central lump, so the stars orbits in ways that mimic the behaviour of the planets orbiting the sun 3. Something in between

8 1. Solid Body Behaviour Example: A merry-go-round Notice that the distances between the riders do not change. They can even hold hands! Do the stars likewise move in parallel paths, with essentially unchanging separations?

9 One Consequence In solid body rotation, the stars farther out would have to be moving faster than those near the center of the Milky Way. In skating, we can play crack the whip.

10 2. Keplerian Motion Here, small low-mass objects orbit a single dominant lump (like the planets around the Sun). In this case, there is a dramatic fall-off of velocity with distance from the centre. Objects farther out move considerably more slowly.

11 3. The Actual Situation: Something in Between The matter in the galaxy has a strong degree of central concentration, but quite a lot of the mass is widely distributed. Consequently, the rotation of the galaxy is not controlled by a single dominant central lump (not even the SMBH it contains). The gas and the individual stars move in response to the combined gravitational effects of the whole distributed ensemble. As a result, the stars nearer the centre move somewhat faster than the Sun, but the effect is not as strong as Kepler s law implies.

12 Still, There is Differential Rotation In the inner parts of the Milky Way, the stars that are closer to the centre than we are gain on the Sun (which is marked with a red arrow). Those inner stars move ahead as shown.

13 Like Cars on a Motorway (faster cars are in the leftmost passing lane, slower cars are to the right)

14 Observable Implications Consider stars nearer the centre than we are. They will be moving somewhat faster, and: n stars ahead of the Sun and closer to the center should be gradually pulling away from us; while n stars behind the Sun but closer to the center should be gradually catching up to us

15 Use Doppler Shifts to Test This Find the average speeds of stars near us in the Milky Way. Is there is a systematic pattern? (The analogy again: On Highway 401, consider the cars in the lane to your left. Generally, the ones ahead of you should be pulling away from you, but the ones behind you should be catching up. This tells you that the fast lane is indeed to your left!) We can do this by studying stars that are reasonably close to the Sun no need to look at all the stars, or especially remote ones. The side that the fast lane is on tells us the direction to the centre of the Milky Way.

16 Why Consider Average Speeds? Remember that individual stars have small random motions superimposed on their general orbits, just as individual drivers speed up and slow down on the highway from time to time. But on average we should see the expected behaviour. Indeed, this should work more reliably than on the highway, where you may find impatient people passing you at high speed on the inside lane. (The stars are more lawabiding!)

17 Oort s Important Discovery (in the 1920s) The Dutch astronomer Jan Oort made the necessary stellar observations, and deduced that the Milky Way is indeed rotating. Moreover, he deduced that the centre lay off in the direction of the constellation Sagittarius. This is, of course, exactly what Shapley s study of the globular clusters had implied, but comes from a completely independent argument! So this is exciting confirmation

18 Conclusion: the Galaxy Spins (and is flattened as a consequence) [but it s not like the paper pinwheel, which spins as a unit ]