20. Galaxy Interaction and Motion

Transcript: In cosmology galaxies are used as markers of expanding space. In addition to the overall cosmic expansion gravity has caused matter to clump or cluster on many different scales. In addition to the gravitational attraction that caused galaxies to collapse and form in the first place gravity has also caused galaxies to aggregate together or congregate in regions of space. The distribution of galaxies in three dimensions in the universe overall is called large scale structure.

Transcript: Most of the techniques for measuring distance in astronomy depend on well understood properties of stars or entire galaxies. However, if astronomers believe they’ve measured the current expansion rate of the universe then redshift itself can be used to indicate distance. Redshift is defined as the fractional wavelength shift caused by the expansion of the universe on the light of a galaxy. At low redshift it’s also equal to the recession velocity divided by the velocity of light. When this is combined with the Hubble relation you get the expression distance equals redshift times the velocity of light divided by the Hubble constant, so in knowing the Hubble constant and the redshift the distance can be calculated. Redshift maps directly to distance in a linear way at low redshift. For example, if the redshift of a galaxy is one percent, z = 0.01, then the distance is calculated to be just over forty megaparsecs or about a hundred and forty million lightyears.

Transcript: When a massive star dies the supernova that results can rival an entire galaxy in brightness, so it can be seen to a very large distance, a billion lightyears or more. When a single massive star dies it does not do so in a well regulated way, but the supernovae that result from a binary system are standard bombs that can be used as distance indicators. Basically the mass from the companion spoons slowly on to a white dwarf until it pushes it over the Chandrasekhar limit, 1.4 solar masses. At that point explosive nucleosynthesis occurs, carbon and oxygen are fused up to silicon and then nickel, and then nickel 56 decays to cobalt 56 and then to stable iron 56. This type of supernova explosion called a Type Ia is well regulated enough to have a variation of only ten percent from one explosion to the next and so is the best distance indicator to use in cosmology.

Transcript: A clever technique for measuring the distance of galaxies that applies to galaxies with smooth light distributions like ellipticals is called surface brightness fluctuations. The idea of surface brightness is that it is the flux per unit area in a galaxy. The important thing to realize is that the observed flux of a galaxy goes down as the distance squared, inverse square law, but in any fixed area of a detector like a CCD detector the number of stars enclosed by a pixel of the detector increases as the distance squared. The product of these two, flux and number of stars enclosed, is a constant. That means the surface brightness of galaxies is constant from one distance to another. But as the galaxy becomes more distant the variation in the flux from one pixel to the next goes down because the number of stars enclosed in a pixel goes up, and the RMS variation or the square root of the number is a smaller fraction. Thus the distance indicator involves the fact that the fluctuations in surface brightness from one pixel to another of a CCD detector are proportional to the distance, and this can be used to measure the relative distance of elliptical galaxies.

Transcript: One of the best distance indicators for elliptical galaxies is the Faber-Jackson relation, named after the two astronomers who discovered it. The Faber-Jackson relation relates the range of stellar velocities or their velocity dispersion in the nucleus in an elliptical galaxy with the size of the galaxy. The width of the stellar absorption features from a spectrum of the elliptical galaxy is used to give the velocity dispersion, and the size of the galaxy comes from an image. Astronomers must be careful to measure the size of galaxies always at a same fixed percentage of their total brightness since galaxies do not have sharp edges. The velocity dispersion is an indicator of mass which in turn relates to luminosity. Ellipticals are old stellar populations which behave in a relatively simple way, and this is the physical basis for the Faber-Jackson relation. Unfortunately, they have no Cepheid variables, so this technique can not calibrated with a local distance scale.

Transcript: An important distance indicator for spiral galaxies is called the Tully-Fisher relation, named after the two astronomers that discovered it. Observationally the luminosity of spiral galaxies is correlated with their rotation speed of their gas disks. There’s a physical basis for this distance indicator because the rotation of the gas disk is an indicator of mass, and if the mass to light ratio of the spiral galaxy is relatively constant then it’s also an indicator of luminosity. The rotation velocity is easy to measure with a single dish radio telescope simply looking at the width of the twenty-one centimeter line of neutral hydrogen. The global properties of the gas are being measured, and this property can be observed at distances far beyond those where Cepheid variables can be resolved. Intermediate inclination spirals must be used because face-on galaxies show no Doppler shift in the disk, and edge-on galaxies often have the obscuring effects of dust to complicate the measurement. The precision of this indicator is about fifteen percent per galaxy.

Transcript: Beyond a distance of about twenty megaparsecs, or sixty or seventy million lightyears, it becomes difficult to use individual stars as distance indicators. Cepheid variables are hopelessly blurred in the summed light of billions of stars in the distant galaxy, and even supernovae, which indeed can be seen above the light of an individual galaxy, may be imbedded in dusty regions. In addition, there are multiple types of supernovae, and with out high quality spectroscopic information it’s not always easy to pick the one that is the precise and reliable distance indicator. Thus astronomers use global properties of galaxies to estimate distances which is okay as long as they are calibrated by a technique with well understood physics in the nearby regions, such as Cepheids or supernovae. Unfortunately the two most obvious properties of galaxies are poor distance indicators. Apparent brightness is a bad estimator of distance because galaxies come in such a wide range of luminosities, and apparent size is a bad estimator of distances because galaxies range by over two orders of magnitude in their true physical sizes.

Transcript: In the nearby universe astronomers primarily use stars as distance indicators. Cepheid variables which were classically used by Hubble to demonstrate the distance to the nebulae and the universal expansion are still used. As luminous stars with well understood physics they can be found locally in the Milky Way where their distances are tethered by parallax and main sequence fitting, but they are bright enough to be observed out to distances of twenty megaparsecs or over fifty million lightyears, a region which encompasses dozens of galaxies. Beyond this distance observation of Cepheids is limited by the crowding of the stellar fields and by their faintness. Supernovae can be observed to much larger distances, easily to five megaparsecs, and individual examples have been found two or three times further than this. However, they are rare; only one supernova occurs in every fifty years per galaxy on average. So they cannot reliably be found in any particular galaxy, and there hasn’t been a supernova in the Milky Way to tether the distance indicator for several centuries.

Transcript: The Hubble constant sets the current expansion rate of the universe and gives an indication of its size and age. The best currently measured value of the Hubble constant comes from a heroic project done with the Hubble Space Telescope over a number of years. The Hubble Space Telescope project was based on observations of Cepheid variables as the distance indicator because they represent well understood physics that can be applied across large distances in space. Several dozen galaxies were observed out to a distance of fifty to sixty million lightyears. The observations involved multiple epochs of single galaxies to pick out the variable stars. This work had to be done with the Hubble Space Telescope because of the crowded stellar regions making it difficult to disentangle the variable stars in crowded fields. A lot of it was done in the infrared to minimize the effects of dust obscuration and reddening. The result of this project was a measurement of the Hubble constant of about seventy kilometers per second per megaparsec with an accuracy of ten percent. Although ten percent doesn’t sound like very high accuracy this enormous project showed that it would be very difficult to measure the expansion rate with higher precision.

Transcript: If we assume that the cosmic expansion has been uniform we can use the current value of the Hubble constant to get an estimate of the edge of the universe. Velocity is the same as distance times time, and the Hubble relation states that the velocity is equal to the Hubble constant times the distance. Using these two relations we can get that the age of the universe is equal to one divided by the Hubble constant, the reciprocal of the Hubble constant. Substituting the value of seventy kilometers per second per megaparsec for the Hubble constant gives an age of fifteen billion years. This in fact is an upper bound on the age of the universe because the gravity of galaxies acting on each other and all the mass in the universe has acted to decelerate the expansion over the history of the universe. Thus the true age is less then the age calculated from a smooth constant expansion.

Transcript: For galaxies velocity is proportional to distance, and the constant in proportionality in this linear relationship is called the Hubble constant given by the large letter H and the subscript zero. It’s defined therefore as velocity divided by distance, and in astronomers units it has units of kilometers per second per megaparsec. Modern measurements place the best value of the Hubble constant at around H0 of seventy kilometers per second per megaparsec. This means, for example, that a galaxy moving with a recession velocity of seven hundred kilometers per second is at a typical distance of ten megaparsecs or thirty-three million lightyears whereas a galaxy moving with a recession velocity of seven thousand kilometers per second is at a distance of a hundred megaparsecs, and so on. The Hubble constant gives the expansion rate of the universe. A higher value implies a younger, smaller universe, and a lower value would imply an older, larger universe.

Transcript: The Hubble relation, a linear relationship between recession velocity of galaxies and their distance from the Milky Way, is a cornerstone of modern cosmology. It’s sometimes called the Hubble law, but it’s not a law of physics in the way that Newton’s laws are laws of physics. There’s nothing in physics that says that the universe should behave this way. Hubble relation is a purely observational result, but it’s a major clue on how the interpret the universe we live in. The two quantities that are plotted are quite different in their nature. The redshift of a galaxy is relatively easy to measure. It’s the relative wavelength shift of spectral features in the galaxy itself which is equivalent to the recession velocity divided by the velocity of light, as long as that recession velocity is small compared to the speed of light. Redshift is denoted by the small letter z. The other quantity, distance, is much harder to measure. It involves the use of a distance indicator. Hubble’s original relationship has been extended by factors of twenty or thirty since his work in 1929 using in particular the distance indicator of supernovae rather then Cepheid variables. The modern Hubble relationship has been shown to be linear out to distances of five hundred megaparsecs, or over one and a half billion lightyears, and recession velocities of thirty thousand kilometers per second, or a tenth the speed of light.

Transcript: The Hubble relation is a linear relationship between the distance of a galaxy from the Milky Way and its recession velocity or redshift. In this situation the more distant galaxies are moving away from the Milky Way faster. The Milky Way has no privileged position in this expansion because a viewer on another galaxy would see exactly the same thing. This relationship suggests that at some time in the distant past all the galaxies were in one place. We can trace the expansion back. Imagine running a movie backwards. The more distant galaxies are moving away faster so tracing the expansion back there was a time when every galaxy was in one place at the origin of the expansion. It’s an origin in time only and not space because the entirety of space has been expanding to create the universe of the current large size.

Transcript: If all the galaxies are moving away from us and the more distant galaxies are moving away faster, does this not indicate that we are at the center of the universal expansion? No it does not because we observe a distant galaxy to be moving away from us; reverse the situation and an observer on that galaxy would observe us to be moving away from them. Draw dots on a balloon, and as you expand the balloon by inflating it you would see that the distance between any two dots on the balloon will increase. This is the Hubble relation. There’s no way to choose between one dot or one galaxy and any other galaxy. There’s no preferred sense and no sense of any galaxy being the center of the expansion.

Transcript: The cosmological interpretation of galaxy redshifts leads to the idea of the expanding universe. This is a sufficiently dramatic implication that astronomers have from time to time wondered whether it’s the correct interpretation. Occasionally objects of high and low redshift are seen close together on the sky or even with an apparent connection between them, and the implied luminosities of the highest redshift objects are extraordinary. For these and other reasons it’s worth understanding why astronomers believe that redshifts are cosmological in origin. In practical terms redshift of galaxies is correlated both with their apparent brightness and their angular size which is strong supporting evidence that the redshift is in fact a measure of distance. Armed with this evidence astronomers have become convinced that redshifts are a distance indicator that they can use to place galaxies in cosmological space.

Transcript: The redshift of galaxies is not caused by their motion with respect to a medium as in the example of the Doppler shift. It’s caused by the expansion of space itself. This is called the cosmological redshift. Imagine a balloon that you’re blowing up, and on it you’ve drawn a waving line to represent a light wave. As you inflate the balloon, a universal expansion, the wavelength of the wave on the balloon will increase, move to redder wavelengths. This example of a redshift which would apply to a wave drawn anywhere on the balloon’s surface shows that it’s the medium itself, the universal fabric of space-time, that is expanding, and this is the cause of the redshift of galaxies.

Transcript: When Hubble first published his linear relationship between recession velocity and distance for galaxies he was cautious about interpreting it in terms of universal expansion. In fact it’s easy to be confused between two different types of redshift. One familiar type of shift is the Doppler shift. This is true for waves traveling in any medium where the velocity or Doppler shift depends on the velocity of the source of the waves relative to some stationary point in the medium. We’re familiar with the Doppler shift in terms of sound waves or in terms of the detection of extrasolar planets. The situation in cosmology is somewhat different because there is no fixed medium, and the redshift is caused by the universal expansion itself.

Transcript: Lets imagine an expanding universe, one in which the distance between galaxies is increasing with time. It’s hard to imagine an expanding universe, but there are analogies that can help. Cut a rubber band and then stretch the rubber band with dots on the rubber bans to mark the galaxies, and as it stretches in one dimension the distance between all the dots will increase. This is a one dimensional analogy for cosmic expansion. Equally, galaxies can be marked on a two dimensional rubber sheet, and as the sheet is expanded in two dimensions the distance between all points will increase. A three dimensional analogy might be a raisin loaf baking in the oven where the raisins represent galaxies, and as the loaf expands the distance between all raisins expands and increases with time. In an expanding universe the distance between any two galaxies increases, and more widely separated galaxies increase their distance at a larger rate. This is called the Hubble expansion. It’s a linear relationship between radial velocity or recession velocity and distance, also sometimes called the Hubble law.

Transcript: Consider a universe of fixed size where the galaxies are milling around randomly. They might change their position over time, but the average distance between each galaxy and all of its neighbors does not change. If we observed these galaxies by measuring spectra and redshifts we would find on average that half of the galaxies had redshifts and half had blueshifts, and the result would not depend on the distance to the galaxies because the galaxies in any region of space are behaving similarly. This is not what Hubble observed.

Transcript: Hubble had previously shown that many of the spiral nebulae were in fact distant systems of stars remote from the Milky Way. He then combined the distances he obtained from the Cepheid variable technique with Slipher’s redshifts and some that he measured himself to produce an amazing new result. In 1929 his study of galaxies showed that most galaxies had a redshift that was proportional to their distance from the Milky Way galaxy. This implied that galaxies were all moving away from the Milky Way, and the more distant ones were moving away the fastest. The implication of this result was a universal expansion, and this was the birth of the idea of the expanding universe.

Transcript: In 1912 Vesto Slipher working at the Lowell Observatory began a project to observe spectra of spiral nebulae. He was working under the direction of Percival Lowell who became known later for his speculation about the canals on Mars. Slipher detected rotation in the nebulae that he studied. With the Doppler Effect he was able to show that some parts of the nebulae were moving towards us and other parts moving away from us indicating rotation, but in another version of the Doppler Effect he got a surprise. The galaxies overall had a systematic red shift with respect to the Milky Way galaxy. Twenty-one out of twenty-five of the galaxies he observed were shifted in their spectra to the red by up to one thousand kilometers per second. Interpreted as a Doppler shift this meant that the galaxies were moving away from us with speeds up to a million miles an hour. At the time the result was extremely puzzling because it was ten years before Hubble would demonstrate that the spiral nebulae were actually distant stellar systems.

Transcript: When a large primordial gas cloud has a large amount of rotation or angular momentum the collapse will occur preferentially along the rotation axis leading to a disc-like formation. The disc will subsequently shear, and gas pockets will collapse within the disc to create star formation. This is the basic morphology of the disc of a spiral galaxy, but spiral galaxies are complex and have multiple stellar populations. Spirals have almost certainly formed throughout the history of the universe, and we know that some of their stellar populations must have been assembled early on because the stars in bulges and halos are mostly old. However subsequently gas and stars are added to spiral galaxies due to interactions and mergers and gas infall from the region within galaxies.

Transcript: Some elliptical galaxies form relatively early in the universe, seven to eleven billion years ago. Giant gas clouds with a small amount of rotation began to collapse. Remember that most of the mass of these gas clouds was dark matter. As the gas collapses stars form from the gas and stay on their elliptical orbits rarely interacting. Thus the shape of the galaxy is frozen as an elliptical shape. With little gas left over for star formation the stars passively evolve becoming older and redder with time, and in the present day we see an elliptical. In other situations ellipticals were formed by the successive merger and collisions of larger galaxies initially containing gas, but when the gas is used up or swept away in a cluster region the stars redden and become older and once again we see the shape of an elliptical galaxy.

Transcript: Any successful theory of galaxy formation must explain certain fundamental differences between the types of galaxies. The bulges of spiral galaxies and elliptical galaxies contain mostly old, mostly red stars. They have a nearly spherical space distribution, and they are slowly rotating stellar systems. By contrast the discs of spiral galaxies are filled with mostly young stars, have a disc-like configuration, and are relatively rapidly rotating. The fact that things are not simple is illustrated by the Milky Way itself which contains very old stars and globular clusters, ten or eleven billion years old, but many young stars just formed. For example in the Orion Nebula stars have formed almost within the history of human existence. The process of galaxy formation goes from the situation of a protogalaxy, a gas cloud early in the universe before any stars are formed, to the galaxies we see around us in the local universe. We know the galaxies did not all form at once early in the universe; they have been formed continuously over the entire history of the universe.

Transcript: The ubiquitous attractive force of gravity can cause galaxies to merge and even coalesce into a single object. Computer simulations have been used to show that when two equal size spiral galaxies merge they fling off tails of stars hundreds of thousands of lightyears into space. After several orbital times, however, the stars coalesce into a single object and relax into a more regular distribution. In other simulations after a series of major merges it’s possible to create an object that looks like an elliptical galaxy from small pieces none of which were elliptical galaxies. Thus astronomers believe that some elliptical galaxies are the byproducts of major mergers early in the universe. Another way that mergers occur is through cannibalism where a large primary galaxy successively absorbs small dwarf companions. Each of these mergers creates a slight trigger in the star formation rate, but once again the stars are subsequently absorbed and the spiral galaxy will retain its regular shape.

Transcript: Galaxies in the universe are typically widely separated in space by about ten to twenty times their own diameters. Thus direct collisions of galaxies are rare, but they can occur. Imagine you took a handful of sand and someone else took a handful of sand, and you flung them at each other. The sand particles are like the stars. Most of the stars when galaxies collide will pass by each other as if they weren’t there feeling only the tidal interaction of their gravity. While the stars themselves do not collide, galaxies are also filled with a material that’s more like water. So now imagine two buckets of water thrown towards each other. The interstellar medium and the gas and the dust in a galaxy is a thick enough medium that when two galaxies collide this gas gets compressed and heated as the kinetic energy in the collision is turned into heat energy. The excited gas becomes hot, glows, and the compression can trigger star formation and sometimes activity in the nuclei of the galaxies. Thus galaxy collisions, even though rare, account for some of the most spectacular phenomena in galaxies.

Transcript: Most galaxies are well separated in space, but often gravities can interact. Gravity is an inverse square law, and so the long arm of gravity means that galaxies that are separated by millions of lightyears can still interact gravitationally. This is called a tidal interaction because the presence of one galaxy can distort the shape of another galaxy. These distortions or evidence of the interaction manifest as tails or arcs, and sometimes the interaction can cause gas or other material to move towards the center of a galaxy and trigger nuclear activity. Not every galaxy interacts with another galaxy, but astronomers have found that roughly ten percent of all galaxies show evidence of interactions, distortions, or other peculiarities in their shapes or morphologies.

Transcript: Galaxies are complex systems composed of tens or hundreds of billions of stars, gas, dust, and large amounts of dark matter. Understanding galaxies in detail is very difficult. Astronomers have developed powerful techniques based on computers to better understand the behavior of galaxies. To simulate a galaxy inside a computer the computational space is filled with particles that represent stars. There’s not sufficient computing power for a single particle to represent a single star, so the resolution of such simulations is not as high as the resolution of individual stars in a galaxy. The particles then interact according to the laws of gravity against a backdrop of dark matter. Also in the simulations gas can be included, and the laws of hydrodynamics have to be coded into the computer to make the relationships between the gas and the stars operate properly. This procedure is called n-body simulations, and when it’s added to the behavior of gas it’s called hydrocode. These computational techniques have accelerated our understanding of the complex world of galaxies and their interactions.

Transcript: One of the most profound discoveries in astronomy in the past few decades has been the fact that most galaxies have most of their mass in the form of dark matter which is to say material that does not shine in any part of the electromagnetic spectrum, so it cannot be composed of normal stars or even sub-stellar objects or gas and dust. Dark matter is ubiquitous in galaxies. Its presence is indicated by rotation curves for spiral galaxies and velocity dispersion measurements in elliptical galaxies. Every type of galaxies that we’ve studied, including dwarfs and irregulars, are dominated by dark matter. Most of the mass of the universe is in the form of this material whose fundamental nature is still mysterious but is almost certainly in the form of microscopic, subatomic particles.