How does the galaxy rotate and keep the spiral arms together? And what really lies at the center of the galaxy?

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1 Ch 14: Mysteries of the Milky Way How does the galaxy rotate and keep the spiral arms together? And what really lies at the center of the galaxy?

2 The Structure of the Galaxy We know that our galaxy has a dense core (bulge) that is so bright we can t see in the core probably has black holes powering it, a flat disk with 4 major spiral arms, and a halo of globular star clusters.

3 How does a galaxy rotate? So how does this whole thing rotate? The structure of the arms and red shift data can help us figure this out. In general, the farther out from the nucleus, the longer the orbital period. Unfortunately, the orbital data doesn t match our expectations given the amount of mass we currently see.

4 Differential Rotation The spiral arms are bent backward because as the arm rotates, the farther out stars rotate slower and the arm bends. Somehow, the spiral arm doesn t seem to disintegrate over long periods of time.

5 Rotation Across the Arm Here is a cross-section of the speed of rotation across the arms of the galaxy. Notice how the tight clump of matter near the nucleus produces the spike below 10,000 LY. Also, as you move outward the speed dincreases According to Kepler s 3 rd Law, it should drop off around 40,000 LY as the stars thin out. This increasing velocity is due to invisible mass dark matter

6 Finding Dark Matter What makes up dark matter? We have found very cool, brown dwarfs (proto-stars that never got started) by their bending of light from more distant stars. However, brown dwarf stars only account for about 10% of the missing matter.

7 What Do We Know Even without the dark matter, we know that: The Sun takes 230 million years to revolve. The Sun revolves around the galaxy at about 500,000 mph! There is very little matter in between the spiral arms (apparently it is swept up as the arms revolve). The galactic nucleus is very massive and revolving very fast.

8 The Galactic Nucleus Based on this picture of our nucleus in infrared, we know that (if we could see in there) it would be incredibly bright in the visible wavelengths. Based on close-ups of this image, the region appears to be made up of evenly, but densely, distributed type 0 stars. Why would so many supermassive stars form? What does this tell us about how these stars end?

9 Sagittarius A* This close-up of the last picture (in infrared) shows the stars in the region surrounding the center of the galaxy Near the brightest star here (Sag. A*) is our best guess of the location of the galactic nucleus. This area is very dense, very bi bright, and very hard to see through.

10 What Do We See at Other Wavelengths? At radio wavelengths, we see a very different picture of the nucleus It is shaped like a blobby, irregularly shaped ball. It is filled with hot hydrogen gas (ionized H II) forming big arms of matter in the nucleus. These are probably huge regions of star formation. The massive gravitational attraction of the nucleus and the swirling mass of dead stars make this area very complex.

11 The Area Around Sagittarius A* Here is the view of the area around Sagittarius A* in radio. Notice the blobby hot nucleus (red) with the two trailing cooler arms (yellow and blue). The larger arm at the top is actually outside the nucleus.

12 Inside the Nucleus This radio close-up of the inner nucleus from the last image shows the base of the two arms we saw before. Astronomers are perplexed as to what could have generated these huge clouds of swirling hydrogen gas. They are estimated to be about 10 million years old. What happened 10 million years ago to cause this?

13 At the Very Core is the Culprit At the center of the last image is the possible source of the swirling a supermassive black hole. The (+) sign marks where a huge suspected black hole resides. This black hole probably powers the spiral arms AND gives out the energy that we see radiating from the nucleus! The black hole s mass should be about 2.5 million solar masses!

14 Answering Other Questions Some astronomers think that there may be more than one black hole in the nucleus (up to 9 in some theories!) Are these black holes very long lived? Have they always been found in the nucleus? Do other galaxies also have these black holes?

15 Dark Matter in the Milky Way The dark matter that keeps the Milky Way rotating is invisible to our observations. Why? Recent discoveries of dark matter in the spiral arms and halo are helping to decrease the 90% of mass we couldn t find, but What produces this dark matter? Could this dark matter be found everywhere in the galaxy? Is there dark matter in between the galaxies?