3 The lives of galaxies

Transcription

1 Discovering Astronomy : Galaxies and Cosmology 24 3 The lives of galaxies In this section, we look at how galaxies formed and evolved, and likewise how the large scale pattern of galaxies formed. But before we can understand those issues, we need to understand the mysterious dark matter, as it plays a key role. 3.1 The dark universe Lecture 5 : Cosmic Perspective 22.1, 22.2 What are galaxies made of? We can do a reasonably accurate census of the contents of the solar neighbourhood, i.e. the region within a few tens of parsecs of the Sun. The result, by fraction of mass, is : 37% stars 13% failed or dead stars (brown dwarfs, white dwarfs, neutron stars) 50% gas However, if we average over the whole of a typical galaxy, we get a rather different story : 90% dark matter 8% stars 2% gas How we do know about this dark matter, and what is it? Two ways of estimating mass. (1) Measure the amount of light L from a galaxy. Assume that on average this light is coming from stars quite like the Sun. The Sun has mass-to-light ratio = M /L. Then our estimate of the mass is M = L. This should give us the mass in stars. [Actually of course there is a mixture of stars of different types. Detailed calculations show that the best multiplying factor to use is about =2 ] (2) Measure rotation speed V of a galaxy. We measure rotation speed at distance R around the galaxy, just like we did for the Milky Way. Then we use V 2 = GM/R, for the velocity expected according to Newton s laws. This should give us the total gravitating mass. However material only feels a pull from material inside radius R; pulls from material further out cancels out. So this method gives us the mass within radius R: Do these two methods give us the same answer? Measuring rotation. M(< R)=RV 2 /G

2 Discovering Astronomy : Galaxies and Cosmology 25 l Figure 30: Examples of real measured rotation curves for various galaxies We rely once again on our trusty friend the Doppler effect. Using spectral features (e.g. stellar absorption lines, ionised gas emission lines, or neutral hydrogen radio emission) we measure the Doppler shift in different parts of the galaxy, as indicated in Fig. 29. If we see a blueshift on one side and a redshift on the other side, this is a sign of rotation. If the shift in wavelength is then the rotation velocity must be V rot = c /. We can do this measurement at a series of different radial distances, and see how rotation velocity changes with radius, V (R). This is known as the rotation curve. Rotation curves have now been measured for many spiral galaxies, and a consistent pattern emerges, as shown in Fig. 30. In the inner parts of galaxies, rotation velocity starts off low and rises as you move outwards, then it flattens off and stays flat out to very large distances from the centre. Is this what we expect? Figure 29: Measuring rotation from the Doppler shift of spectral features. Types of rotation curve. (1) A solid body, rotating with a fixed angular rotation speed, would show velocity following V / R, increasing outwards. (2) Material orbiting a central point mass, like the planets orbiting the Sun, would have V / 1/R 1/2. (3) Galaxies don t do either : they show V / const. This is because the matter is not all in a central point, as in the solar system case - the material is distributed. But we can measure how the light at least is distributed - does that help us to get a better prediction for galaxy rotation curves? Observed vs predicted rotation curves. First, we measure the light profile of a galaxy from an image. As described above, this can enable us to estimate the mass in stars within each radius, M (< R). If we assume that those stars constitute all the relevant mass, then we can predict the rotation within that same radius as V pred (R) =(GM (<R)/R) 1/2. The typical result of this exercise (see Fig. 31 is that it agrees well in the inner parts, but falls short by a factor 3 in the outer parts. This means that at those outer radii, the total enclosed mass is about 9 times bigger than the visible stelllar mass - the remainder of the mass is something dark. Figure 31: Comparing predicted and observed rotation curves for the galaxy NGC 3998.

3 Discovering Astronomy : Galaxies and Cosmology 26 Figure 32: Left : Illustrating the orbits of stars in Elliptical galaxies. Right : Effect on the width of absorption lines seen in the spectrum Dark Matter halos. For a typical spiral galaxy, 90% of its mass is dark matter. The luminous matter is more concentrated than the total gravitating matter. In the central parts, including the radius in the Milky Way where the Sun resides, dark matter is present but negligible. Most of the dark matter is in an extended halo. Fig. 31 shows how you can get a much better fit if you include such an extended dark halo in the calculation. Finding dark matter in elliptical galaxies. Elliptical galaxies do not rotate. The stars are on random orbits, like the stars in the Milky Way bulge and halo. Some individual stars may be on circular orbits, but in many different directions; other stars are on highly elliptical orbits, or more complicated orbits. The result is no net rotation. However the typical stellar velocity is still given by V 2 = GM/R. How do we measure this typical random velocity? Line velocity broadening. Fig. 32 shows an absorption line seen in a galaxy spectrum at some radius R. The light we see comes from all the stars in the same line of sight, looking through the galaxy. Because of the random motions, some of these stars are moving away from us and stretch the line to the red; and some are moving towards us and stretch the line to the blue. This results in net velocity broadening. If we see a line of width then the spread of velocities is V /. This tells us the typical random velocity at R, and then we estimate M(< R) R V 2 /G. The result is that, just as with spiral galaxies, in ellipticals we find M total 10 M stars. Dark matter in clusters. Clusters of galaxies contain hundreds to thousands of galaxies. These are moving around at random, rather like the stars in an elliptical galaxy. We can use this to estimate the total mass of the whole cluster. We measure V for each individual galaxy from the Doppler shift as normal, and subtract the mean V for the cluster, and then calculate the spread V around that mean. Finally, we measure the size R of the cluster, and calculate the mass as M R V 2 /G. Cluster masses. The result is even more dark matter than we already found in the halos of galaxies. A good example is the Coma cluster. It has size R 1.5Mpc and the spread of velocities is V 1700km s 1. This gives M clus M. Coma has 1000 galaxies with stars each. So we have : Stellar mass : M Summed halo mass : M Total cluster mass : M In other words, while individual galaxies are 90% dark matter, clusters are 99% dark matter. Rich clusters are the most massive objects known.

4 Discovering Astronomy : Galaxies and Cosmology 27 Figure 33: Left : Galaxies and hot gas in the cluster Abell The whiteish light is the visible light image. The purplish light is the X-ray emission. Right : gravitational lensing by dark matter in the cluster Abell The light from background galaxies is distoprted into arcs. Dark matter in clusters Take 2 : hot gas. Smooth X-ray emission shows the presence of hot gas filling the space between the galaxies. (See Fig. 33.) It is very hot : T 108 K. At this temperature, gas will be highly ionised : the electrons are stripped from the Hydrogen atoms, so that electrons and protons move separately. The gas temperature tells us how fast the protons are moving. Thermal physics says that at temperature T the speed of protons is vp = (3kT /mp )1/2. (Here mp = kg is the mass of the proton, and k = J K 1 is Boltzmann s constant.) For the Coma cluster T = K so vp = 1492 km s 1. Note that this is quite similar to the speed of a typical galaxy. This is not a coincidence. It is because the protons follow v 2 = GM/R in just the same way as the galaxies. So the gas temperature also tells us the total gravitating mass... and gives us the same answer. Gravitational lensing by dark matter. Gravity bends light. Light from background galaxies gets distorted as it passes through the gravitational field of the massive cluster. Because the cluster is symmetrical, this results in stretched-out arcs of light. (See Fig. 33.) This gives us another way of estimating the mass but also allows the dark matter to be mapped. What is Dark Matter? Is it Gas? Well, gas isn t really dark. Hot gas shines in spectral emission lines. Very hot gas shines in X-rays. Even very cold gas emits easily detectable radio waves. What is Dark Matter? Compact objects? Dead stars leave behind compact remnants - white dwarfs, neutrons stars, and black holes. We know such objects exist in the disk of the Milky Way. Perhaps the halo of our Galaxy has large numbers of them? Such hypothetical objects are known as Massive Compact Halo Objects or MACHOs. They might be detected indirectly, because they can make background stars temporarily brighten, as illustrated in Fig. 34. If an invisible MACHO passes in front of a distant star, the gravitational lensing effect will make the star temporarily get brighter. This effect has been seen but not as often as it should have been if all dark matter is mae of MACHOs. So it seems this is not the answer. Figure 34: As a background star passes be- hind a MACHO, its light is focused by gravitational lensing and so it gets temporarily brighter.

5 Discovering Astronomy : Galaxies and Cosmology 28 What is Dark Matter? Particles pervading space? We know in fact that dark matter can be any kind of ordinary matter. In later lectures we will see that Helium and other elements are made during the Big Bang by nuclear fusion; the amount made depends on the density of the Universe. The density of dark matter that we see implies that much more heavy elements should have been made than we see. Ordinary atomic matter is also called baryonic matter. So we need a non-baryonic particle. Are there any? What is Dark Matter? Neutrinos? Neutrinos don t contribute to the creation of elements. They are also very hard to detect. Traditionally they were thought to have zero mass, but now they are thought to have a tiny but non-zero mass. If there are lots of them, could they make the dark matter? As we will see in the next section, the dark matter gradually clumps during the history of the universe, allowing galaxies to form. As neutrinos are light, they would move around very quickly, making hot dark matter. Computer simulations show that this gives the wrong pattern of clustering. What is Dark Matter? We are left with Cold Dark Matter. We need a non-baryonic particle that is massive so that it moves slowly ( cold ). It also needs to interact poorly with ordinary matter, so that it plays no part in element creation - it is a Weakly Interacting Massive Particle (WIMP). No such particle is definitely known to exist. But several have been proposed to exist as part of particle physics theories. So whether they do exist is doubly important! Looking for Dark Matter. It is possible that dark matter particles could be directly detected by their scattering effect on ordinary matter. But this behaviour is mimicked by cosmic rays, so experiments have to be placed underground where cosmic rays don t reach. A handful of underground experiments are underway worldwide. There are no positive results yet, but watch this space...