Lecture 35 - Formation of Our Galaxy (4/12/96)

Seeds: Chapter 12

1. Disk Kinematics
   • The galaxy consists of a rotating disk and a halo of stars with randomly oriented orbits, approximately spherical.
   • The disk is relatively thin, with the stars in a thickness of less than 300 pc.
   • The H I gas, as measured by the 21-cm line, is confined to within 125 pc of the galactic plane.
   • The molecular clouds, were stars are being formed, are within 100 pc or less from the plane.
   • The disk stars are in nearly circular orbits.
   • The rotation curve v(R) is nearly constant at the Sun's orbit (R=8.5 kpc).
   • This means the angular velocity is proportional to v(R)/R, or 1/R for constant v.
   • This causes differential rotation, since the stars inside the Sun's orbit take a shorter time to make one orbit, and stars outside the Sun's orbit take a longer time to orbit the center of the galaxy.
   • Stars inside Sun's orbit gain on Sun, outside they lag the Sun.
   • Thus, if you like up a structure radially from the center of the galaxy, it will be wound up into a spiral by the differential rotation.
2. Spiral Arms
   • If you look at some nearby galaxies like M31 the Andromeda galaxy, you find that they have spiral arms which wind outward from the center of the galaxy.
   • Does our Milky Way galaxy have spiral arms? Its hard to tell since we are in the disk ourselves and cannot see it from above.
   • If you plot out the distances to O and B stars, H II regions, and molecular clouds, you find them concentrated in bands toward and away from the galactic center.
   • These are believed to coincide with spiral arms in the galaxy.
   • There is an overdensity of gas in the spiral arms, causing enhanced star formation in these regions.
   • The leading theory for the formation of spiral arms, at least the "grand design" two arm spirals, is the density wave theory.
   • Density waves are regions of enhanced density, like low-speed high density "shocks" in traffic patterns found on crowded freeways.
   • Clouds move in an out of the arms, but while there they bump into each other causing star formation.
   • These density waves are examples of gravitational instabilities in the galactic disk.
   • Because of the differential rotation, the waves get wound up into spirals. Eventually they get wound up so much that they disappear.
   • What triggers the density wave instability? Most likely a close encounter with a neighbor galaxy.
   • Another sort of strong gravitational instability is the bar in the center of a galaxy, like our own, and certain other spiral galaxies.
   • Another theory for the formation of spiral patterns is the propagating star formation model, where the supernovae from star formation compresses clouds and triggers nearby star formation.
   • In this model, weak spiral structures are formed by the differential rotation. This model works best to explain spirals without strong two-arm grand design pattern, called flocculent spirals.
   • In average galaxies like our Milky Way, both density waves and propagation star formation are likely to have roles.
3. Halo Kinematics
   • Halo star orbits are highly elliptical and randomly oriented through the three-dimensional roughly spherical halo.
   • There is low net angular momentum in the halo system, unlike the high angular momentum disk.
   • Unlike the disk, you cannot use the velocity of a halo star to infer the mass inside its orbit, since for any one star you do not know where it is in its elliptical orbit.
   • Instead, you use the velocity dispersion, or the root mean square velocity, of a group of stars at the same distance.
   • This can be related to the mass through the energy equation: E/m = v^2 / 2 - G M / R = - G M / 2 R and thus the mean square velocity < v^2 > is proportional to the mass M.
   • There is little gas, and thus almost no star formation in the halo. The halo seems to have formed all its stars a long time ago, and the leftover gas collapsed to form the disk.
4. History of Our Galaxy
   • A possible scenario for the formation of our galaxy:
   • The first fast collapse of the low angular momentum gas into the halo and the beginnings of the bulge in the center.
   • The globular clusters are formed at this time, before the infalling gas crosses the center and gets randomized.
   • The high density bulge star formation is enriched by supernovae.
   • The higher angular momentum gas left over from the halo formation collapses to the disk through dissipation from cloud-cloud collisions.
   • There is on-going star formation in the disk, at the average rate of 1 solar mass per year or more.
   • Note that this rate of star formation would use up all the gas (10^10 Msun) in the disk in only 10^10 years or less. Since it seems unlikely that we would just now be on the verge of using all the disk gas up, it is likely that there is continuous replenishment by infalling pristine gas from the far halo.
   • An infall rate of 1 Msun per year would be enough to support the star formation rate of the disk.
   • This gas may reside in the galactic corona which surrounds the halo and has a temperature of 10^5 to 10^6 K. Calculations show that this gas could cool at the rate of 1 Msun/yr and rain down onto the disk.
5. The Center of Our Galaxy
   • The galactic center region is heavily obscured by dust in the intervening spiral arms, and it took Shapley's study of the globular clusters to show where the center of the galaxy was located.
   • However, dust is transparent to other parts of the electromagnetic spectrum than visible light. Radio waves, and infrared light, for example, can pass through dust clouds.
   • When radio astronomy began in the years after World War II, one of the first sources of celestial radio waves found was the center of our galaxy. This radio source was designated Sagittarius A (Sgr A) after the constellation in which it was located.
   • The galactic center region is a busy place. There appears to be rings of hydrogen gas and molecular clouds about 150 to 250 pc from the center, likely stirred by the central bar of our galaxy.
   • Inside these rings are some relatively empty regions (cleared by the bar), until the inner few parsecs.
   • There is a high density of stars in the central region, seen in the IR. The average spacing in the center is about 1000 AU, compared with 1.5 pc (300000 AU) near the Sun!
   • The stars at a distance of R = 0.3 pc (62000 AU) from the center seem to be orbiting with a velocity of 200 km/s (P=9000 yrs). Kepler's law tells us that there must be 3 million solar masses within this 0.3 pc!!!!
   • What could this be? One possibility is a super dense star cluster. However, even the largest globular clusters with 10^6 Msun are almost 30 pc in diameter. This is 50 times smaller, or 125000 times denser!
   • The most popular explanation is that there is a supermassive black hole at the center of our galaxy.
   • This would explain the large amounts of radio and X-ray energy being emitted from Sgr A. These sorts of things are also seen in many other galaxies.
   • The presence of some strange object in the nucleus of a galaxy emitting radio, IR, optical, or Xray energy is called an active galactic nucleus. We will discuss these in a more general context later on.
   • The presence of a central million solar mass black hole in the center of our galaxy has no particularly dire consequences. It makes the stars nearby move fast, occasionally swallowing one that gets too close, and turns it into energy that it emits in radio waves or Xrays. No need to worry - it won't swallow up the rest of our galaxy!