Radio Astronomy and the Galaxy

The problem with living in the middle of the Galactic disk is that the interstellar medium blocks our view of most of the Galaxy. We are very fortunate that there is a way around this, provided by radio astronomy. This comes by way of the "21-cm" line, which arises from neutral hydrogen. It is a special sort of energy transition, involving a "spin flip" of the electron relative to the proton. The atom is in a lower energy state when the spins are not aligned - the energy difference is equal to a photon with wavelength of 21-cm. This transition is very weak ( a typical interstellar atom can stay in the higher energy state for millions of years), but since most of the Universe is hydrogen, this radiation arises everywhere. Thus, the Galaxy is both transparent and visible in 21-cm radiation.
The transparency makes it difficult to tell where along a line-of-sight the detected emission is coming from. Because it is a spectral line, one can make use of the Doppler shift to help with this problem. In general, the inner part of the Galaxy rotates at a different speed than the outer parts, so there will be some projected velocity difference between gas at different distances along the line-of-sight. Given a model for the Galactic rotation, one can use it to assign each velocity bin in an observed broadened spectral line to a different location. In this way, we can build up an overall map of the neutral hydrogen. This map can be supplemented for nearby locations by observing star forming regions containing massive stars; we know how to assign distances to stars based on their main sequence location. Another useful molecule is carbon monoxide (CO), whose rotational changes can also give rise to radio photons. This is a good tracer of molecular clouds. We end up with a somewhat fuzzy impression of our whole Galaxy.

The picture which arises is that we live in a flattened disk of stars and gas, which is organized into spiral arms. We see many examples of similar structure for other galaxies. Indeed, the nearest major galaxy, M31 in Andromeda, we think looks very much like the Milky Way. The spiral structure may at first seem sensible, since the Galaxy rotates at different rates at different radii, it would take radial structures and wind them into spirals. The problem is that the spirals are never too tightly wound, even though the galaxies have had time for up to 100 rotations. The solution is that these arms are not composed of particular stars and clouds; they are "density waves". As on a freeway where there is an accident, the density of cars behind and up to the wreck is much higher than in front of it, but individual cars move through the density wave and then speed up away from it. So stars, including our Sun, find themselves moving a little slower while in the spiral arms and faster between them. The arms maintain themselves by their own extra gravity, and serve the function of moving angular momentum to the outer regions of the Galaxy, allowing it to slowly collapse. Due to the higher density in the arms, this is where molecular clouds and star formation tend to be concentrated. In other galaxies where we can look down from above, the spiral arms are clearly traced out by the young bright stars. Thus, we can use the locations of O&B stars, and HII regions, to try to trace the spiral arms in our own Galaxy. These work nearby; 21-cm observations are need for the far side of the Galaxy.

A very nice version of most of this lecture can be found from Oregon...

The Galactic Nucleus

When you look at the center of the Galaxy with an optical telescope you can't see anything because of all the dust in the way. In order to see through the dust you have to look at infrared and radio wavelengths because these waves are long enough that they don't get scattered by the dust.  Radio images of the center of the Galaxy show a rotating disk of molecular gas about 5 parsecs across.  This is an accretion disk, where gas and dust being pulled towards the center piles up.  Just inside this disk is a spiral of hot ionized gas which is falling in towards the middle and converging at the dynamical center of the Galaxy. Near where the spiral converges there is an extraordinarily bright spot of radio emission called Sgr A*. Astronomers think this is a supermassive black hole.  By measuring the velocities of stars and gas orbiting Sgr A* we can measure its mass and we find that it has to be about 2.6 million solar masses. The central mass has to be contained in a region about 10 AU and the only way to put a million solar masses in a region 10 AU across is if it is a black hole.  As we study the centers of other galaxies, it looks like supermassive black holes are pretty common in the centers of galaxies on the whole.  Black holes in the centers of galaxies contain a million to a billion solar masses.
Here you can see stars orbiting our central Black Hole (457kb animated gif).

Stellar populations

 We divide stars up into 2 groups, based largely on their time of birth.  The characteristics of each group are as follows: Pop I: young stars (1 billion years), have circular orbits in the
disk of the galaxy, color tends to be blue, they are metal-rich. Pop II: old stars (10 billion years), found in the bulge and halo of our galaxy, orbits may be highly elliptical and inclined to plane of galaxy, generally red, and metal-poor. Of course, many stars formed in between 1 and 10 billion years ago and hence there is a continuum of stellar properties, not a sharp division between Pops I and II.  The Sun is classified as a Population I star despite its age, because it is metal-rich and has a circular orbit in the disk.  Astronomers have recently begun to search for Population III stars, which are supposed to be the leftovers of the very first stars formed in the galaxy (so they have characteristics even more extreme than thos of Pop II stars).  What appear to be Pop III white dwarfs (very cool) were discovered in 2000. These also appear to be the objects that cause microlensing in the MACHO survey (see "dark matter" later on). It begins to seem that the Pop III stars were mostly of relatively high mass, since we don't see the population of low mass stars that should go with the white dwarfs. Thus Pop III may only have lasted a short time, and produced the "metals" which contaminated the Pop II stars.

Properties and Structure of the Milky Way
(1kpc = 3200 ly)

 Mass (in stars): ~10^11 Msun
 Luminosity: ~10^11 Lsun
 # of stars: ~10^11
 diameter of disk: 25 kpc
 diameter of bulge: 3 kpc
 diameter of halo: at least 50 kpc
 thickness of disk: ~500 pc
 distance from Sun to center of galaxy: 8.5 kpc
 age: 10-12 billion years (11-13 billion for globular clusters)

 Disk: Pop I stars, blue, spiral arms, open clusters, lots of gas and dust
 Bulge: Pop II stars, red, little gas or dust
 Halo: Pop II stars, globular clusters, dark matter (Pop III white dwarfs + ??)

Formation of the Milky Way

 We are only now learning about how galaxies form and their early evolution, because the galaxies we can see that are in this stage of their existence are very far away and hence difficult to examine in detail.  Nevertheless, we believe that we know roughly how galaxies form.  They collapse out of large gas clouds, similar to the way we learned that small gas clouds collapse to form stars.  The globular clusters around a galaxy form first, perhaps a billion years earlier than the galaxy itself.  The Milky Way probably formed between 10 and 12 billion years ago - this estimate is based on the ages of the oldest stars in globular
clusters.  When we look at distant galaxies, we see that they are smaller and more numerous than galaxies are today, implying that galaxy mergers are an important part of the formation process.

(Thanks to Ast. 122 in Oregon for this image)