Galactic Structure Lecture #2 Paul Woodward 4/16/03 Perseus Arm Orion Arm (spur?) Rotation Sun Optical features Concentrated radio features ? General neutral hydrogen (radio &gt; 1 atom/cm3) + Center Sagitarius Arm Fig. 20.11 Centaurus Arm Parsecs Milky Way Galaxy (from above) currently known or estimated features Radio observations of atomic H and molecular clouds reveal asymmetry of our galaxy. Gas density not same as you go around in circle at one radius. <a href="/keyword/spiral-arms/" >spiral arms</a> give slight wiggles in rotation curve. Atomic H map shows them most clearly right near us. 3 arm segments near us are named by constellations in their directions: Orion arm, Saggitarius arm (toward Galactic center), and Perseus arm (away from Galactic center). These marked also by visual light from massive young stars. Cleanest maps of <a href="/keyword/spiral-arms/" >spiral arms</a> come from CO observations. CO emission is from large, cold molecular clouds, mostly in central half of the Galactic disk, where large numbers of stars form. Ring of Molecules around the core of the Milky Way 1 The galaxy NGC 2997, shown in your textbook, has unusually regular spiral structure. <a href="/keyword/spiral-arms/" >spiral arms</a> dominated by light from massive, young, blue stars. These stars emit very, very much more light for their mass than the redder, low mass, older stars of the central bulge region. So, don't be fooled. The light distribution is not the mass distribution. Don't be fooled into thinking that material in <a href="/keyword/spiral-arms/" >spiral arms</a> stays there. Galaxies rotate differentially (remember the rotation curve?). In age of typical galaxy, arms would wind up like watch spring, but we never see this. So either <a href="/keyword/spiral-arms/" >spiral arms</a> are crests of waves (like white caps on lake) or they are transient, caused by some recent phenomenon (like the striking of a bell leaves it ringing for a short while). Molecular Clouds Lower plot on slide: horizontal axis = galactic longitude, vertical axis = component of velocity along our line of sight. Longitude 0 or 360 , at center, marks Galactic center direction. Longitude 180 , at edges, marks anticenter direction. Velocities are from Doppler shifts. Can trace lanes of enhanced CO emission in plot which show relationship of velocity to longitude. These are the <a href="/keyword/spiral-arms/" >spiral arms</a> . Converting this plot to a view of the disk from above is very difficult, and people still argue about it. NGC 2997 As line of sight approaches Galactic center direction, maximum velocity decreases in magnitude. This velocity arises form CO clouds at the &quot;tangent point.&quot; This decrease proves that the mass in the central region of the Galaxy is distributed, and not all concentrated at very center. Right near the center direction, the maximum CO velocity shoots up. This indicates a mass concentration right at center. This central mass not significant if get just a bit away from it. To see what our Galaxy might look like from above, it is best to look at other galaxies. They show various irregularities in their spiral patterns, with spiral arm spurs, etc. They also show that very bright, young stars trace out the pattern. Fig. 12.24 from Shu. 2 Spiral waves are common. Just stir your coffee. C. C. Lin, at MIT, with Frank Shu, first showed that <a href="/keyword/spiral-arms/" >spiral arms</a> could be waves. Even if they are transient, it is now clear that they are still waves. If you strike a bell, it rings for a while. The ringing is still a wave phenomenon. It simply cannot sustain itself and must be set off by an external event. Lin and Shu showed that under certain conditions that can and do occur in rotating disk galaxies: 1. Can find a spiral gravitational distortion that reinforces itself. 2. This distortion, as a fixed pattern, must itself rotate. 3. Because the galaxy rotates differentially, the pattern and the stars can rotate at the same speed only at one radius. 4. At other radii, stars must move into and out of <a href="/keyword/spiral-arms/" >spiral arms</a> periodically. 5. Compression of density of stars is modest in <a href="/keyword/spiral-arms/" >spiral arms</a> . 6. Compression of gas, since clouds collide inelastically, is huge. 7. Compression of gas can lead to star formation. 8. Changes in gas velocity at arms explain wiggles in rotation curves of galaxies (like those in our own galaxy's curve). Following diagram (ignore upper left panel): Top left = stellar orbit in axisymmetric galaxy. A circle. Gravity always the same, centrifugal force always same too. Lower left = stellar orbits in galaxy with bar distortion. Orbits are elliptical. Since distortions in gravity at each radius are felt at same angles, all the long axes of the ellipses line up. Lower right = stellar orbits in galaxy with spiral distortion. Orbits are elliptical. Distortions in gravity at each radius are felt at angles that change so as to trace a spiral. Thus long axes of ellipses twist as you go out. Note how the orbits tend to bunch together along spiral paths. Lower right orbits show that a spiral distortion in gravity can set up a star density distribution that will reinforce the distortion. Toomre, at MIT, showed that such a wave disturbance can be set up by a very massive object, a companion galaxy, that orbits a disk galaxy. The orbital speed of the companion sets the rotation speed of the spiral pattern. The spiral pattern is transient, although it can last a while, even on a galaxy's natural time scale. No mechanism has yet been shown to be successful for stimulating and continuously maintaining a spiral pattern in a disk galaxy. Bar distortions of stellar systems have been shown to be stable. These are just elliptical galaxies. Bars can have attached spirals, and these can presumably be driven continuously by the gravitational distortion of the bar. Fig. 12.25 of Shu. The dark line shows the surface of the pond as a water wave travels along it. Each of the little ellipses represents the path that is followed by the bit of the water that forms the black dot on the pond's surface at the moment of this snap shot. The water at the black dot follows the elliptical path indicated as the wave goes by. When we look down onto the surface of the pond, the wave appears to travel. However, what is traveling is merely a pattern on the pond's surface; the water itself does not travel far. The pattern is formed because each bit of water is, at any time like the one in the diagram, located at a different place, or phase, in its elliptical path. From the arrows in the diagram it is clear that as these bits Kinematic waves their elliptical paths of water go through the wave pattern moves smoothly to the right. 3 The theory of the orbits of stars in the disk of our galaxy was worked out by Bertil Lindblad, a Swedish astronomer. He found that orbits which are nearly circular can be described by epicycles, as are indicated on the slides that follow. [Fig. 12.12 of Shu] Our star travels counterclockwise around a small elliptical orbit around its guiding center, as indicated in the diagrams. The result of this motion is the black ellipse shown in each drawing. This elliptical orbital motion itself rotates clockwise about the galactic center, as is clear from the 3 snap shots drawn here. For the orbit of this star to participate in an organized spiral wave, the black, rotating, orbital ellipse must rotate along with the spiral pattern. Then, clearly, our star rotates in the same direction as the spiral pattern, but faster, overtaking the pattern. Our star is only one of a great many, of course. Other representative stars are indicated in each of the 3 drawings by the unfilled circles. Each of these other stars orbits a guiding center which follows the same circular orbit as that of our featured star (the black dot). All of these other stars behave just like our star, and they follow the same rotating black elliptical orbit, but they have different phases in this orbit. Together a great many such stars trace out the black elliptical orbit, and this elliptical arrangement of stars rotates with the spiral pattern. Here is illustrated the behavior of a single star that is participating in a spiral wave in a disk galaxy. In the 3 drawings, which start at the left and show the star at successive intervals as we go to the right, our star is indicated by the dark black dot. Another black dot indicates the center of the galactic disk. A dashed line in each drawing connects the galactic center to a point that is called the guiding center of our star's epicyclic orbit. The guiding center travels in a regular circular motion about the galactic center, going clockwise, as indicated by the black arrows. To make an entire spiral wave, we of course need to put together many such elliptical arrangements of stars. We have one such ellipse for each guiding center radius (or length of the dashed lines in the drawings) that we choose. If all these ellipses of stars rotate at the same rate, we get a static, coherent pattern. If all the major axes (or long axes) of these ellipses are aligned, our stellar disk will have a constant, rotating oval distortion. If instead, the major axes of these ellipses in any single snap shot of the disk appear to rotate counterclockwise with increasing guiding center radius, then we have a spiral wave of the type we find in disk galaxies. 4 Here again (and still bigger on the next slide) is the spiral pattern that was shown earlier. You should think about this diagram as follows. Imagine that each of the elliptical curves that make up this diagram is a rotating ellipse of stars, of just the sort that was explained at such length in the previous set of slides. Also, imagine that somehow, all these orbits are coordinated so that all these elliptical arrangements of stars rotate clockwise at the same speed. Then it is clear that where the ellipses come close together, the local density of stars will increase. These regions trace a spiral pattern, and it is this pattern that we would expect to see when we observe the light emitted by all these stars through a telescope. Note, by the way, that the stars travel through the spiral pattern going from the inner edge of each spiral arm through to the outer edge. The following slide shows the result of a self-consistent solution for a spiral density disturbance in a disk galaxy. The challenge for the theory of galactic spiral structure is to find a set of such elliptical stellar orbits that trace out a spiral such that the gravitational perturbation created by the new density enhancements in <a href="/keyword/spiral-arms/" >spiral arms</a> in the disk produces the precise gravitational perturbation which will in turn create these same elliptical orbits. This challenge is one of self consistency. We can easily show that if we have a spiral gravitational perturbation, we will get a spiral response from the disk stars. And we can show that a spiral density enhancement of the disk stars will give rise to a spiral gravitational perturbation. The trick is to find the magic conditions under which the spiral density enhancement is just the one that arises in response to the gravitational perturbation that it itself induces. Lin and Shu demonstrated that this trick can be performed, and in a decade of careful work, observers showed that galaxies out there are actually doing it. The self-consistent nature of the spiral density distribution shown in the previous slide, of course, comes out of considerable mathematical analysis. Nevertheless, we can see that it is reasonable, because it is consistent with the behavior of stars responding to gravitational forces in the normal way. Remember that a star approaches a spiral arm from its inner edge. The extra gravitational attraction of the spiral arm's density enhancement then tends to pull the star outward toward the arm. The outward motion toward the spiral arm causes the star to orbit a bit more slowly, in order to conserve its angular momentum about the galactic center. This slowing down in its orbital motion causes the star to linger within the spiral arm. It then moves back inward, and this motion carries it along a path that tends to be nearly along the spiral arm, again reinforcing the spiral arm's density enhancement (look again at the slide 4 slides back). 5 Numerical experiments carried out on the largest computers available in the 1960s, which were less powerful than your laptop machine, investigated whether an initially rigidly rotating system of stars would &quot;all by itself&quot; and without any coaxing develop a spiral structure. Spirals were found to be transient features in such systems, but not persistent ones. The next slides show a typical example, in which a spiral forms but does not last. The final stellar system develops instead an oval distortion, not a spiral structure. We now believe that the reason for these &quot;failures&quot; is that these stellar systems that were simulated did not possess massive halo components, like our galaxy does. Some however, still believe that spiral structure in galaxies is a transient phenomenon that results from gravitational interaction with nearby galaxies, not from an inherent instability of stellar disks. We have said that the coming together of the various elliptical orbits of stars in the region of a spiral arm produces an enhancement in the density of stars in the arm. It is very important to recognize a fundamental difference between the behavior of stars and of gas under the same gravitational influences. Stars essentially never collide. Each individual star responds to the combined gravitational forces of a great many stars. Unless they are born together, stars in the galactic disk essentially never get close enough to each other for the gravitational interaction from a close approach to change the direction of their orbits significantly. Thus if the orbits of stars intersect, the stars on these orbits simply continue along those orbits unaffected. However, gas clouds do indeed collide, and when they do, they do not bounce. That is, their collisions are highly inelastic. When gas clouds collide, the gas becomes highly compressed by shock waves that dissipate the kinetic energy of the clouds' relative motion into heat. This process can be compared to collisions of cars on a highway. When two cars collide head on, they do not bounce but instead both come to rest, and all the kinetic energy that the two cars had disappears. This energy is used up in compacting each of the two cars and in heating up the materials from which the cars are made in the process. Like the cars, two gas clouds colliding head on dissipate all their energy of relative motion in compressing the gas in the clouds and in heating it up. Much of this heat is immediately radiated away as light, since the gas clouds are not likely to be dense enough, even after the collision, to stop this light from escaping into space. After large gas clouds collide and merge, their newly strengthened gravity can overwhelm their thermal pressure forces and cause the clouds to collapse, resulting in the formation of new stars. 6 Therefore, even if the enhancement of the density of stars in a spiral arm is not large, because intersecting stellar orbits go right through each other, the enhancement of the density of the gas can be enormous. The following slide shows the result of a computation of the response to a spiral density disturbance in a disk galaxy of the gaseous component of the disk (see the curve labeled &quot;8&quot;), the stellar component of the disk (the curve labeled &quot;32&quot;), and the halo component of the galaxy (the curve labeled &quot;128&quot;). The gas density increases by an order of magnitude upon entering the spiral arm, while the density of stars increases by less than 20%. The most dramatic observational evidence for a sudden compression of the gas entering a spiral arm of a disk galaxy came from radio observations with the Westerbork telescope in Holland during the early 1970s. The Westerbork telescope was the first with sufficient angular resolution and sensitivity to observe the sudden increase in radio emission associated with the compression of the gas and the magnetic fields threading through it. Westerbork observations could distinguish emission centered on the dust lanes that line the inner edges of the <a href="/keyword/spiral-arms/" >spiral arms</a> from emission centered instead upon the regions just downstream (outward in radius) where the formation of new stars occurs. The best such evidence came from observations of the spiral galaxy M51. An optical picture of this galaxy, overlaid with the contours of the radio emission (so-called &quot;radio continuum emission&quot;) is shown, followed by purely optical images of this fascinating galaxy. The great disparity in the compression of the gas and of the stars upon entering a spiral arm, and the fact that both enter from the inner edge of the spiral arm and exit from the outer edge, leads to the following general morphological picture of spiral arm structure. Dust lanes, marking the locations of strong, sudden gas compression, extend along the inner edges of <a href="/keyword/spiral-arms/" >spiral arms</a> . The inner edges are the location of the entry of gas and stars into the spiral arm region. The compression of the gas clouds leads to gravitational collapse of some of them, followed by the formation of star clusters whose light is dominated by that from very massive, blue stars. These stars light up the star forming regions and ionize the surrounding gas. Because it takes a few million years for this star formation to happen, the bright blue stars and their surrounding ionized nebulae are located downstream from the prominent dust lanes, that is, just outside them, tracing out the spiral pattern &quot;like beads on a string.&quot; 7 The following color picture (in visible light) of M51 exhibits all the standard spiral arm morphological features we have discussed. The blue &quot;beads on a string,&quot; the large regions of new star formation, that trace the <a href="/keyword/spiral-arms/" >spiral arms</a> are especially prominent and are located just outside the very clearly defined dust lanes (which are also traced out by the ridges of peak radio continuum emission). From the picture it is also clear that there is a strong gravitational interaction between M51 and its close companion galaxy. This companion galaxy is actually located at the end of one of the <a href="/keyword/spiral-arms/" >spiral arms</a> . It is quite possible that the spiral structure in M51 is so beautiful and well defined because of this interaction with the companion. In fact, there are those who believe that such an interaction with a nearby galaxy is a necessary condition for the generation of a prominent two-armed spiral pattern in a disk galaxy. &quot;Whirlpool&quot; Galaxy, type Sc, in Canes Venatici (M-51, NGC 5194/5195). KPNO # 02102, 4-meter photograph Hubble Reveals the Heart of the Whirlpool Galaxy: New images from NASA's Hubble Space Telescope are helping researchers view in unprecedented detail the <a href="/keyword/spiral-arms/" >spiral arms</a> and dust clouds of a nearby galaxy, which are the birth sites of massive and luminous stars. The Whirlpool galaxy, M51, has been one of the most photogenic galaxies in amateur and professional astronomy. Easily photographed and viewed by smaller telescopes, this celestial beauty is studied extensively in a range of wavelengths by large ground- and space-based observatories. This Hubble composite im...