# Does the rotation of galaxies precess?

Does the axis of a galaxy as a whole "wobble" the same way that the Earth does? If so do we have any idea how much or how fast the Milky Way is doing so?

Precession is a general property of rotating systems under forces from external sources. For instance, Earth's precession is due to gravitational force from Sun and Moon. Since the galaxy is very far away from any other gravitational source, I don't see a reason for its precession.

Edit: as pointed out in the comment below (@RobJeffries), if it is not sufficiently far from massive system, than it will possibly precess…

The whole galactic disk is made up of many individual stars and the interstellar gas between them. The whole thing doesn't tilt or wobble as a solid object would.

Other parts of the Galaxy do precess though. The central supermassive black hole might and therefore the jest coming out of it might move around like the axis of the Earth's rotation does.

Also, galactic disks do wobble but not like a solid disk, more like a series of rings - the orbits are different at different distances from the centre. These can change very slowly and parts of the disk can be as a different angle than other parts.

This is generally not well understood because we can't see the disks very well, being so far away, and they would change very slowly.

To expand a bit on FJC's answer: galactic disks are not rigid bodies, and so we should not expect them to rigidly precess.

Most disk galaxies do show warps at large radii (including our own), in both the interstellar gas and the stars. From observations of warps in gas disks, we know that the azimuthal angle of the warp often changes with radius: i.e., the tilt of the disk changes as a function of radius.[*] Since the angular rotation speed changes as a function of radius,[**] this suggests the tilt could precess at different rates for different radii.

Unfortunately, since there isn't really a consensus on what causes or maintains warps, it's very hard to go from observing a warp to predicting what the underlying precession rates might or might not be. (Although one can argue that the precession rate can't vary too much with radius, otherwise the warp would rapidly "wind up" and lose its coherent warped shape, and we wouldn't see warps as often as we do.)

See here for some discussion of the theoretical issues.

[*] This is a kind of precession, although it's a change in the orientation of the rotational axis with radius rather than a change with time (as in the Earth's precession).

[**] Even if the circular velocity is approximately constant, as it often is in the outer regions of galaxies, the increase in the circumference as a function of increasing radius means the angular velocity goes down.

## Astronomers spot largest rotation in the universe

Artist’s concept of a cosmic filament – a strand in the cosmic web, containing galaxies and dark matter – stretching from one galaxy cluster to another. Fantastically, astronomers now say these vast filaments spin in space. Image via AIP/ A. Khalatyan/ J. Fohlmeister.

### Filaments of the cosmic web

Astronomers at the Leibniz Institute for Astrophysics Potsdam, in collaboration with scientists in China and Estonia, said on June 14, 2021, that they’ve discovered a rotation – a spin – on an enormous scale never seen before. They made the discovery by mapping the motion of galaxies in huge filaments or strands of what’s called the cosmic web. They were looking at the universe on the grandest scale, in which there are great filaments made of galaxies, separated by giant voids. And they found that these long tendrils of galaxies and matter, forming the vast cosmic filaments of the cosmic web, rotate on the scale of hundreds of millions of light-years.

It’s the largest rotation in the universe, these astronomers said.

You know how ice skaters spin faster as they pull in their arms? Scientists describe that faster spin as due to conservation of angular momentum. These astronomers said their results:

… signify that angular momentum can be generated on unprecedented scales.

The study was published on June 14, 2021, in the peer-reviewed journal Nature Astronomy.

### Largest rotation in the universe

So the cosmic filaments are essentially galaxy-packed bridges. And you might ask, from where to where? Astronomers say that vast clusters of galaxies lie at the nodes, or connection points, of the cosmic web. A cosmic filament made of galaxies – now known to be spinning – spans the vast distant between clusters of galaxies.

Noam Libeskind at the Leibniz Institute for Astrophysics Potsdam, initiator of the project, said that galaxies:

… move on helixes or corkscrew like orbits, circling around the middle of the filament while travelling along it. Such a spin has never been seen before on such enormous scales, and the implication is that there must be an as yet unknown physical mechanism responsible for torquing these objects.

He also described the filaments themselves as “thin cylinders:”

… similar in dimension to pencils, hundreds of millions of light-years long, but just a few million light-years in diameter.

These fantastic tendrils of matter rotate. On these scales, the galaxies within them are themselves just specks of dust.

Here’s a real image of a strand in the cosmic web, released in April 2021. This image looks in the direction of our constellation Fornax the Furnace, to a time 2 billion years after the Big Bang. Each point of light is a galaxy. You can see a filament between the galaxies, tracing the path of the cosmic web. Read more about this image. Image (c) ESO / NASA/ Roland Bacon et al.

### How do we know?

As Noam Libeskind said above, the galaxies in the filaments funnel on corkscrew paths into the clusters at their ends. Thus, to us on Earth, the light of the funneling galaxies appears red-shifted when moving away from us, and blue-shifted when moving toward us. Astronomers can measure a shift like that.

These astronomers measured red and blue shifts using existing data in the Sloan Digital Sky Survey, which began collecting data in 2000. Peng Wang of the Leibniz Institute for Astrophysics Potsdam explained:

By mapping the motion of galaxies in these huge cosmic superhighways using the Sloan Digital Sky survey – a survey of hundreds of thousands of galaxies – we found a remarkable property of these filaments: they spin.

### What does it mean?

In hindsight, it’s logical to think that the filaments would spin. After all, there must have been a period during which – as the early universe expanded outward from the Big Bang, and as galaxies began to form – the galaxies for some reason pulled themselves into these vast filaments, creating the cosmic web in the first place. And, as they did so, it’s easy to think of the filaments spinning up, like ice skaters pulling in their arms.

In fact, it was earlier work by theorist Mark Neyrinck that caused these astronomers to analyze the Sloan Digital Sky Survey data. Libeskind said:

It’s fantastic to see this confirmation that intergalactic filaments rotate in the real universe, as well as in computer simulation.

The scientists still wonder, though, why do they spin? Or perhaps it’s better to ask the question as how. How is the angular momentum generated? What made the galaxies pull themselves together into filaments? Why does the universe appear as a cosmic web at all?

View larger. | This image – from a 2020 study – is computer-generated. It suggests that the distribution of dark matter in the universe, along with ordinary matter, takes the form of a cosmic web. Read more about this image. Image via J. Wang/ S. Bose/ CfA.

Bottom line: Astronomers have found the largest rotation in the universe by analyzing red and blue shifts in galaxies. The galaxies compose strands or filaments in the cosmic web. Those filaments are now believed to be spinning.

## Galaxy Rotation

[/caption]
Look across the Universe, and you’ll see that almost everything is rotating. The Earth rotates on its axis as it orbits the Sun. And the Sun itself is rotating. As you can probably guess, we even have galaxy rotation with our Milky Way galaxy.

Our galaxy is rotating incredibly slowly, however. It takes the Sun 220 million years to complete a single orbit around the galaxy. In the 4.6 billion years that the Sun and planets have been here, they’ve only rotated around the center of the galaxy about 20 times.

We know that galaxy rotation is happening because the Milky Way is a flattened disk, in the same way that the Solar System is a flattened disk. The centrifugal force from the rotation flattens out the galactic disk. All stars in the galactic disk follow roughly circular orbits around the center of the galaxy. Stars in the halo can have much different orbits and speeds.

The calculation of the high rotational speed of the galaxy led to the discovery of dark matter. If our galaxy contained just the matter we can see – planets, gas, etc – the galaxy rotation should cause it to spin apart. Instead, there’s much more mass holding the galaxy together. In fact, astronomers have calculated that the total mass of the galaxy is probably 10 times greater than the sum of all the stars in it. 90% of this is invisible dark matter, holding the galaxy rotation together. And only 10% is the regular matter that we can see. Our galaxy really has a mass of more than 1 trillion suns, and extends out more than 600,000 light-years a third of the distance to the nearby Andromeda galaxy.

All the galaxies we can see are rotating. It’s this rotational force that counteracts the inward pull of gravity from all the galaxies. If galaxies didn’t rotate, they’d collapse inward and just join the supermassive black holes at the hearts of galaxies.

We have written many articles about galaxies for Universe Today. Here’s another article about the rotation of the Milky Way.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

## Corotation circle

The corotation circle is the circle around the galactic center of a spiral galaxy, where the stars move at the same speed as the spiral arms. The radius of this circle is called the corotation radius. Inside the circle the stars move faster and outside they move slower than the spiral arms.

The Sun is located near the corotation circle of the Milky Way. [1] [2]

The corotation circle can be used to probe dark matter in a galaxy. In barred spiral galaxies, such as the Milky Way, the stars in the bar rotate faster than the stars in the spiral arms, as they are closer to the centre of the galaxy. Calculations have shown that sufficiently massive dark matter haloes slow the rotation, causing the corotation radius to be greater than 1.4 times the length of the bar. [3]

Most measurements [ citation needed ] have found that the corotation radius is always less than 1.4 times the bar length, leading to the conclusion that dark matter does not significantly influence galactic rotation.

However, a 2017 study found that the arms of galaxies rotate more slowly than previously thought, implying that dark matter does sometimes influence the rotation of a galaxy even when the corotation radius is less than 1.4 times the bar length. [4] [5]

## Masses of Galaxies

Astronomers can measure the rotation speed in spiral galaxies by obtaining spectra of either stars or gas, and looking for wavelength shifts produced by the Doppler effect. Remember that the faster something is moving toward or away from us, the greater the shift of the lines in its spectrum. Kepler’s law, together with such observations of the part of the Andromeda galaxy that is bright in visible light, for example, show it to have a galactic mass of about 4 × 10 11 MSun (enough material to make 400 billion stars like the Sun).

The total mass of the Andromeda galaxy is greater than this, however, because we have not included the mass of the material that lies beyond its visible edge. Fortunately, there is a handful of objects—such as isolated stars, star clusters, and satellite galaxies—beyond the visible edge that allows astronomers to estimate how much additional matter is hidden out there. Recent studies show that the amount of dark matter beyond the visible edge of Andromeda may be as large as the mass of the bright portion of the galaxy. Indeed, using Kepler’s third law and the velocities of its satellite galaxies, the Andromeda galaxy is estimated to have a mass closer to 1.4 × 10 12 MSun. The mass of the Milky Way Galaxy is estimated to be 8.5 × 10 11 MSun, and so our Milky Way is turning out to be somewhat smaller than Andromeda.

Elliptical galaxies do not rotate in a systematic way, so we cannot determine a rotational velocity therefore, we must use a slightly different technique to measure their mass. Their stars are still orbiting the galactic center, but not in the organized way that characterizes spirals. Since elliptical galaxies contain stars that are billions of years old, we can assume that the galaxies themselves are not flying apart. Therefore, if we can measure the various speeds with which the stars are moving in their orbits around the center of the galaxy, we can calculate how much mass the galaxy must contain in order to hold the stars within it.

In practice, the spectrum of a galaxy is a composite of the spectra of its many stars, whose different motions produce different Doppler shifts (some red, some blue). The result is that the lines we observe from the entire galaxy contain the combination of many Doppler shifts. When some stars provide blueshifts and others provide redshifts, they create a wider or broader absorption or emission feature than would the same lines in a hypothetical galaxy in which the stars had no orbital motion. Astronomers call this phenomenon line broadening. The amount by which each line broadens indicates the range of speeds at which the stars are moving with respect to the center of the galaxy. The range of speeds depends, in turn, on the force of gravity that holds the stars within the galaxies. With information about the speeds, it is possible to calculate the mass of an elliptical galaxy.

Table 1 summarizes the range of masses (and other properties) of the various types of galaxies. Interestingly enough, the most and least massive galaxies are ellipticals. On average, irregular galaxies have less mass than spirals.

Table 1: Characteristics of the Different Types of Galaxies
Characteristic Spirals Ellipticals Irregulars
Mass (MSun) 10 9 to 10 12 10 5 to 10 13 10 8 to 10 11
Diameter (thousands of light-years) 15 to 150 3 to >700 3 to 30
Luminosity (LSun) 10 8 to 10 11 10 6 to 10 11 10 7 to 2 × 10 9
Populations of stars Old and young Old Old and young
Interstellar matter Gas and dust Almost no dust little gas Much gas some have little dust, some much dust
Mass-to-light ratio in the visible part 2 to 10 10 to 20 1 to 10
Mass-to-light ratio for total galaxy 100 100 ?

## OBSERVING ROTATION CURVES OF DISK GALAXIES

To begin, we will consider spiral galaxies. Spiral galaxies (like the Milky Way Galaxy) are large systems that typically have three distinct components (see Figure 8.8): The first is the flat disk that contains stars, gas, and dust, and that is most prominent when we look at these galaxies in visible light. You will not be surprised to learn that this is called the disk component. The second component is a sphere-shaped collection of stars near the galaxy center. It is called the bulge, or sometimes the central bulge. Finally, there is a much larger sphere-shaped collection of stars and star clusters extending out at least as far as the disk, called the halo. The halo is much larger in extent than the bulge, but there are not very many stars in it compared to the number of stars in the bulge or disk. The number of stars is so low, in fact, that galaxy halos are nearly invisible. We tend to look right through them.

All of this material is held together by the mutual gravitational attraction of all the material within the galaxy.

Figure 8.8: Illustration of a spiral, or &ldquodisk&rdquo, galaxy. Note the bulge, disk, and halo of the galaxy. Credit: NASA/SSU/Aurore Simonnet

How do we measure the orbital speeds of the stars and gas within a spiral galaxy? First note that because the bulges and disks of spiral galaxies are so much brighter than their halos, we mainly concentrate on measuring the orbital speeds of the stars and gas in those components of spiral galaxies.

The stars and gas in the disks of spiral galaxies tend to rotate around the center of the galaxy. If the galaxy is &ldquoface-on&rdquo from our vantage point, we will not be able to observe its rotation. On the other hand, if the galaxy is &ldquoedge-on,&rdquo we can measure the rotation through redshift and blueshift. For an edge-on, rotating spiral galaxy, we will observe redshift on the side of the galaxy that is rotating away from us (the light will appear to be &ldquostretched out,&rdquo and will appear to have longer, redder wavelengths). We will observe blueshift on the side of the galaxy that is rotating toward us (the light will appear &ldquoscrunched&rdquo and will appear to have shorter, bluer wavelengths). See Figures 8.9 and 8.10 for more on how we observe rotation in galaxies.

Figure 8.9: (top) We cannot measure rotation in spiral galaxies that are seen face-on. (bottom) We can measure rotation in spiral galaxies that are seen edge-on. One side of the galaxy will appear redshifted, and the other side will appear blueshifted. Note that the color shift in this image has been exaggerated. The real redshift and blueshift we observe in a rotating galaxy is very small and can only be measured by taking a spectrum . Credit: NASA/ SSU / Aurore Simonnet

One might think astronomers would study the stars in galaxies to learn about the motion within galaxies. Stars are bright, and there are many of them throughout a galaxy. For very nearby galaxies, we can measure the motions of stars. However, gas is also spread throughout the disk of a spiral galaxy. This gas provides a bright emission spectrum at specific optical wavelengths, as well as bright emission at a particular radio wavelength (21 cm). At these wavelengths, the gas within a spiral galaxy actually emits more light than stars. Also, if we take spectra at different distances out from the center of the galaxy, we will have measurements that can tell us about the motions of the gas at different radii from the galaxy&rsquos center. We can do this by placing a slit over the galaxy that blocks out all the light except from a thin strip along the length of the galaxy. This way, we can measure motions only from the light that makes it through the slit, as shown in Figure 8.10.

Figure 8.10: Placing the slit from a spectrograph so that it allows only light from a particular region of a galaxy is a way to limit the measurements to a region of interest. At top, an artist&rsquos illustration of a galaxy is shown, with a rectangular region highlighted. This rectangular region shows the portion of a galaxy&rsquos light that would be observed through a slit. This light would then be dispersed into different wavelengths to show the galaxy&rsquos spectrum. The middle of the diagram shows what the resulting spectrum of the galaxy would look like. The top of the spectrum shows the shorter-wavelength (bluer) light from the galaxy, and the bottom of the spectrum shows the longer-wavelength (redder) light from the galaxy. The vertical stripe down the middle is light from stars in the galaxy the stars emit light at all wavelengths measured here. The wiggly horizontal lines are light from the gas in the galaxy. If the galaxy were not moving, these gas emission lines would be perfectly horizontal, only glowing at one wavelength. However, because the galaxy is rotating, the emission lines are blueshifted on the left and redshifted on the right, resulting in the wiggly appearance. These observed blueshifts and redshifts can be converted into a measured rotation curve for the galaxy, seen at the bottom of the figure. Credit: NASA/ SSU / Aurore Simonnet

For nearby galaxies astronomers must take many individual measurements if they want to make a complete rotation curve, or velocity-distance plot. Nearby galaxies are fairly large in the sky, often larger than the field of view of a telescope. As a result of their large apparent size it is not possible to view them in their entirety with a single exposure. Instead, separate observations must be made across the galaxy, each at a different distance from its center. This is a time consuming process.

Perhaps paradoxically, motions are easier to measure in distant galaxies. When a galaxy is distant enough to lie completely within the telescope/camera field of view, then a spectrograph slit can be laid down to coincide with the long axis of the galaxy, allowing a complete map of velocities for the entire length of the system to be collected at once.

Whichever method astronomers use, the end result is the sort of spectrum shown in Figure 8.10 above. It shows the rotation velocity of a disk galaxy versus distance from the center of the galaxy. Such plots are rotation curves, just like the rotation curves you graphed and studied earlier in the chapter.

##### ROTATION CURVE OF A SPIRAL GALAXY

In this activity, you will obtain a rotation curve for a spiral galaxy by moving a slider over the image of the galaxy.

To move the slider, click and drag the line under the red &ldquobutton.&rdquo Move the slider to each location where you would like to make a measurement, then click the red button. (Alternatively you can drag the slider with the button, and when you release the button, it will trigger a measurement.)

Choose at least five points on each side of the galaxy&rsquos center. At each point, the calculations are done to find the rotation speed of the gas in the galaxy. This is done by comparing the measured value of the H-alpha emission line to its laboratory value and applying the Doppler relation to the shift.

##### ROTATION CURVE MATCHING ACTIVITY

Match the images of the galaxies on the left with the rotation curves shown on the right.

For each galaxy image, the relative velocities at three different locations are indicated by the length of the arrows.

Click and drag each image to the square next to the rotation curve that provides the best match. When you have made a correct match, you will see a green check mark in the upper right corner of the galaxy image.

## Galaxy Formation and Evolution

Since Hubble's work on galaxies in the 1920s, astronomers have continued to observe more and more galaxies at larger and larger distances from us. The goal of much of this work has been to determine the mechanism by which galaxies form and how they evolve. For example, one of the major undertakings of the astronomical community in recent years has been the Sloan Digital Sky Survey. If you read the "The SDSS Science Legacy," they say that their team was responsible for:

Systematic characterization of the galaxy population: By providing high quality images, distances, and stellar masses and ages of hundreds of thousands of galaxies, the SDSS transformed the study of galaxy properties and the correlations among them into a precise statistical science, yielding powerful insights into the physical processes that govern galaxy formation.

When we studied stars, we saw that using star clusters, which contain stars in various stages of evolution but all of the same age, astronomers were able to construct and verify a model for the evolution of stars. In order to do the same for galaxies, you would like the same set of information. You would like to find galaxies of different ages so we can see how galaxies change over time. We can certainly use our own Milky Way and the Local Group galaxies as examples of old galaxies, but we need a sample of young galaxies for comparison. To find young galaxies, what we need to do is identify very distant galaxies. The reason distant galaxies = young galaxies is the finite speed of light. If you observe an object 1 million light years away, you are not seeing it as it is today. The light you see today left the object 1 million years ago. This phenomenon is called the lookback time. So, if you want to find a galaxy 5 billion years younger than the Milky Way, you should search for galaxies 5 billion light years away. Then, you can compare those galaxies to the ones you find 10 billion light years away, because those will appear as they were 10 billion years ago.

The phenomenon of lookback time is crucial to the study of galaxy formation and evolution. We can directly observe how galaxies appeared when they were forming if we can find galaxies at very large lookback times. In recent years, astronomers have been using the technique of observing deep fields (like the Hubble Deep Field you've seen previously, and more recently, the Hubble Ultra Deep Field) to pursue the most distant galaxies in the universe. The question that these deep fields have helped answer is: “How did galaxies look billions of years ago?” The answer appears to be that when galaxies were young, they looked very irregular. Galaxies with spiral arms like the Milky Way did not appear until about 10 billion years ago. We think that galaxies apparently formed from the bottom up that is, more than 10 billion years ago, small, irregularly shaped sub-galaxies appear to have collided and merged, leading to the formation of the large spiral and barred spiral galaxies that we see today. Although the Milky Way continues to form new stars today, the star formation rates in these subgalaxies were much higher. Observations suggest that the peak of star formation occurred about 8 billion years ago. Below is an image of several very distant, and therefore very young, galaxies observed in the Hubble Ultra Deep Field. Compare these to the images of nearby galaxies that you have seen previously.

By comparing local galaxies to distant galaxies and supplementing these observations with numerical simulations of the early universe, astronomers believe that galaxies form in roughly the following way:

1. The first objects are sub-galaxy sized "pieces."
2. Several of these pieces coalesce to form a larger mass object.
3. The gas in the larger galaxy can collapse, increasing the rotation speed of the galaxy.
4. Stars will rapidly form inside this disk, and their orbits will sort into the familiar spiral structure.
5. Disk galaxies will continue to evolve by the various interaction processes we saw previously, and major mergers will create elliptical galaxies.

In this general prescription for the evolution of galaxies, we did not fit AGN into the scenario. The AGN phase appears to be a short phase in the overall lifetime of a galaxy, and it occurs when the SMBH in the core of that galaxy has enough fuel to power the enormous luminosities these objects emit. Again, using lookback time, we see that quasars are most numerous about 10 billion years ago. So, the quasar phase appears to be an early phase that perhaps most galaxies went through before settling down as normal galaxies.

## 28.3 The Distribution of Galaxies in Space

In the preceding section, we emphasized the role of mergers in shaping the evolution of galaxies. In order to collide, galaxies must be fairly close together. To estimate how often collisions occur and how they affect galaxy evolution, astronomers need to know how galaxies are distributed in space and over cosmic time. Are most of them isolated from one another or do they congregate in groups? If they congregate, how large are the groups and how and when did they form? And how, in general, are galaxies and their groups arranged in the cosmos? Are there as many in one direction of the sky as in any other, for example? How did galaxies get to be arranged the way we find them today?

Edwin Hubble found answers to some of these questions only a few years after he first showed that the spiral nebulae were galaxies and not part of our Milky Way. As he examined galaxies all over the sky, Hubble made two discoveries that turned out to be crucial for studies of the evolution of the universe.

### The Cosmological Principle

Hubble made his observations with what were then the world’s largest telescopes—the 100-inch and 60-inch reflectors on Mount Wilson. These telescopes have small fields of view: they can see only a small part of the heavens at a time. To photograph the entire sky with the 100-inch telescope, for example, would have taken longer than a human lifetime. So instead, Hubble sampled the sky in many regions, much as Herschel did with his star gauging (see The Architecture of the Galaxy). In the 1930s, Hubble photographed 1283 sample areas, and on each print, he carefully counted the numbers of galaxy images (Figure 28.13).

The first discovery Hubble made from his survey was that the number of galaxies visible in each area of the sky is about the same. (Strictly speaking, this is true only if the light from distant galaxies is not absorbed by dust in our own Galaxy, but Hubble made corrections for this absorption.) He also found that the numbers of galaxies increase with faintness, as we would expect if the density of galaxies is about the same at all distances from us.

To understand what we mean, imagine you are taking snapshots in a crowded stadium during a sold-out concert. The people sitting near you look big, so only a few of them will fit into a photo. But if you focus on the people sitting in seats way on the other side of the stadium, they look so small that many more will fit into your picture. If all parts of the stadium have the same seat arrangements, then as you look farther and farther away, your photo will get more and more crowded with people. In the same way, as Hubble looked at fainter and fainter galaxies, he saw more and more of them.

Hubble’s findings are enormously important, for they indicate that the universe is both isotropic and homogeneous —it looks the same in all directions, and a large volume of space at any given redshift or distance is much like any other volume at that redshift. If that is so, it does not matter what section of the universe we observe (as long as it’s a sizable portion): any section will look the same as any other.

Hubble’s results—and many more that have followed in the nearly 100 years since then—imply not only that the universe is about the same everywhere (apart from changes with time) but also that aside from small-scale local differences, the part we can see around us is representative of the whole. The idea that the universe is the same everywhere is called the cosmological principle and is the starting assumption for nearly all theories that describe the entire universe (see The Big Bang).

Without the cosmological principle, we could make no progress at all in studying the universe. Suppose our own local neighborhood were unusual in some way. Then we could no more understand what the universe is like than if we were marooned on a warm south-sea island without outside communication and were trying to understand the geography of Earth. From our limited island vantage point, we could not know that some parts of the planet are covered with snow and ice, or that large continents exist with a much greater variety of terrain than that found on our island.

Hubble merely counted the numbers of galaxies in various directions without knowing how far away most of them were. With modern instruments, astronomers have measured the velocities and distances of hundreds of thousands of galaxies, and so built up a meaningful picture of the large-scale structure of the universe. In the rest of this section, we describe what we know about the distribution of galaxies, beginning with those that are nearby.

### The Local Group

The region of the universe for which we have the most detailed information is, as you would expect, our own local neighborhood. It turns out that the Milky Way Galaxy is a member of a small group of galaxies called, not too imaginatively, the Local Group . It is spread over about 3 million light-years and contains 60 or so members. There are three large spiral galaxies (our own, the Andromeda galaxy, and M33), two intermediate ellipticals, and many dwarf ellipticals and irregular galaxies.

New members of the Local Group are still being discovered. We mentioned in The Milky Way Galaxy a dwarf galaxy only about 80,000 light-years from Earth and about 50,000 light-years from the center of the galaxy that was discovered in 1994 in the constellation of Sagittarius. (This dwarf is actually venturing too close to the much larger Milky Way and will eventually be consumed by it.)

Many of the recent discoveries have been made possible by the new generation of automated, sensitive, wide-field surveys, such as the Sloan Digital Sky Survey, that map the positions of millions of stars across most of the visible sky. By digging into the data with sophisticated computer programs, astronomers have turned up numerous tiny, faint dwarf galaxies that are all but invisible to the eye even in those deep telescopic images. These new findings may help solve a long-standing problem: the prevailing theories of how galaxies form predicted that there should be more dwarf galaxies around big galaxies like the Milky Way than had been observed—and only now do we have the tools to find these faint and tiny galaxies and begin to compare the numbers of them with theoretical predictions.

You can read more about the Sloan survey and its dramatic results. And check out this brief animation of a flight through the arrangement of the galaxies as revealed by the survey.

Several new dwarf galaxies have also been found near the Andromeda galaxy. Such dwarf galaxies are difficult to find because they typically contain relatively few stars, and it is hard to distinguish them from the foreground stars in our own Milky Way.

Figure 28.14 is a rough sketch showing where the brighter members of the Local Group are located. The average of the motions of all the galaxies in the Local Group indicates that its total mass is about 4 × 10 12 MSun, and at least half of this mass is contained in the two giant spirals—the Andromeda galaxy and the Milky Way Galaxy . And bear in mind that a substantial amount of the mass in the Local Group is in the form of dark matter.

### Neighboring Groups and Clusters

Small galaxy groups like ours are hard to notice at larger distances. However, there are much more substantial groups called galaxy clusters that are easier to spot even many millions of light-years away. Such clusters are described as poor or rich depending on how many galaxies they contain. Rich clusters have thousands or even tens of thousands of galaxies, although many of the galaxies are quite faint and hard to detect.

The nearest moderately rich galaxy cluster is called the Virgo Cluster , after the constellation in which it is seen. It is about 50 million light-years away and contains thousands of members, of which a few are shown in Figure 28.15. The giant elliptical (and very active) galaxy M87, which you came to know and love in the chapter on Active Galaxies, Quasars, and Supermassive Black Holes, belongs to the Virgo Cluster.

A good example of a cluster that is much larger than the Virgo complex is the Coma cluster , with a diameter of at least 10 million light-years (Figure 28.16). A little over 300 million light-years distant, this cluster is centered on two giant ellipticals whose luminosities equal about 400 billion Suns each. Thousands of galaxies have been observed in Coma, but the galaxies we see are almost certainly only part of what is really there. Dwarf galaxies are too faint to be seen at the distance of Coma, but we expect they are part of this cluster just as they are part of nearer ones. If so, then Coma likely contains tens of thousands of galaxies. The total mass of this cluster is about 4 × 10 15 MSun (enough mass to make 4 million billion stars like the Sun).

Let’s pause here for a moment of perspective. We are now discussing numbers by which even astronomers sometimes feel overwhelmed. The Coma cluster may have 10, 20, or 30 thousand galaxies, and each galaxy has billions and billions of stars. If you were traveling at the speed of light, it would still take you more than 10 million years (longer than the history of the human species) to cross this giant swarm of galaxies. And if you lived on a planet on the outskirts of one of these galaxies, many other members of the cluster would be close enough to be noteworthy sights in your nighttime sky.

Really rich clusters such as Coma usually have a high concentration of galaxies near the center. We can see giant elliptical galaxies in these central regions but few, if any, spiral galaxies. The spirals that do exist generally occur on the outskirts of clusters.

We might say that ellipticals are highly “social”: they are often found in groups and very much enjoy “hanging out” with other ellipticals in crowded situations. It is precisely in such crowds that collisions are most likely and, as we discussed earlier, we think that most large ellipticals are built through mergers of smaller galaxies.

Spirals, on the other hand, are more “shy”: they are more likely to be found in poor clusters or on the edges of rich clusters where collisions are less likely to disrupt the spiral arms or strip out the gas needed for continued star formation.

### Astronomy Basics

#### Gravitational Lensing

As we saw in Black Holes and Curved Spacetime, spacetime is more strongly curved in regions where the gravitational field is strong. Light passing very near a concentration of matter appears to follow a curved path. In the case of starlight passing close to the Sun, we measure the position of the distant star to be slightly different from its true position.

Now let’s consider the case of light from a distant galaxy or quasar that passes near a concentration of matter such as a cluster of galaxies on its journey to our telescopes. According to general relativity, the light path may be bent in a variety of ways as a result we can observe distorted and even multiple images (Figure 28.17).

Gravitational lenses can produce not only double images, as shown in Figure 28.17, but also multiple images, arcs, or rings. The first gravitational lens discovered, in 1979, showed two images of the same distant object. Eventually, astronomers used the Hubble Space Telescope to capture remarkable images of the effects of gravitational lenses. One example is shown in Figure 28.18.

General relativity predicts that the light from a distant object may also be amplified by the lensing effect, thereby making otherwise invisible objects bright enough to detect. This is particularly useful for probing the earliest stages of galaxy formation, when the universe was young. Figure 28.19 shows an example of a very distant faint galaxy that we can study in detail only because its light path passes through a large concentration of massive galaxies and we now see a brighter image of it.

We should note that the visible mass in a galaxy is not the only possible gravitational lens. Dark matter can also reveal itself by producing this effect. Astronomers are using lensed images from all over the sky to learn more about where dark matter is located and how much of it exists.

You can use the Gravitational Lensing Simulator to explore how the distance and mass of a cluster of galaxies affects the offset of lensed images of a very distant galaxy. Instructions are available by clicking on Help.

### Superclusters and Voids

After astronomers discovered clusters of galaxies, they naturally wondered whether there were still larger structures in the universe. Do clusters of galaxies gather together? To answer this question, we must be able to map large parts of the universe in three dimensions. We must know not only the position of each galaxy on the sky (that’s two dimensions) but also its distance from us (the third dimension).

This means we must be able to measure the redshift of each galaxy in our map. Taking a spectrum of each individual galaxy to do this is a much more time-consuming task than simply counting galaxies seen in different directions on the sky, as Hubble did. Today, astronomers have found ways to get the spectra of many galaxies in the same field of view (sometimes hundreds or even thousands at a time) to cut down the time it takes to finish their three-dimensional maps. Larger telescopes are also able to measure the redshifts—and therefore the distances—of much more distant galaxies and (again) to do so much more quickly than previously possible.

Another challenge astronomers faced in deciding how to go about constructing a map of the universe is similar to that confronted by the first team of explorers in a huge, uncharted territory on Earth. Since there is only one band of explorers and an enormous amount of land, they have to make choices about where to go first. One strategy might be to strike out in a straight line in order to get a sense of the terrain. They might, for example, cross some mostly empty prairies and then hit a dense forest. As they make their way through the forest, they learn how thick it is in the direction they are traveling, but not its width to their left or right. Then a river crosses their path as they wade across, they can measure its width but learn nothing about its length. Still, as they go on in their straight line, they begin to get some sense of what the landscape is like and can make at least part of a map. Other explorers, striking out in other directions, will someday help fill in the remaining parts of that map.

Astronomers have traditionally had to make the same sort of choices. We cannot explore the universe in every direction to infinite “depth” or sensitivity: there are far too many galaxies and far too few telescopes to do the job. But we can pick a single direction or a small slice of the sky and start mapping the galaxies. Margaret Geller, the late John Huchra, and their students at the Harvard-Smithsonian Center for Astrophysics pioneered this technique, and several other groups have extended their work to cover larger volumes of space.

### Voyagers in Astronomy

#### Margaret Geller: Cosmic Surveyor

Born in 1947, Margaret Geller is the daughter of a chemist who encouraged her interest in science and helped her visualize the three-dimensional structure of molecules as a child. (It was a skill that would later come in very handy for visualizing the three-dimensional structure of the universe.) She remembers being bored in elementary school, but she was encouraged to read on her own by her parents. Her recollections also include subtle messages from teachers that mathematics (her strong early interest) was not a field for girls, but she did not allow herself to be deterred.

Geller obtained a BA in physics from the University of California at Berkeley and became the second woman to receive a PhD in physics from Princeton. There, while working with James Peebles, one of the world’s leading cosmologists, she became interested in problems relating to the large-scale structure of the universe. In 1980, she accepted a research position at the Harvard-Smithsonian Center for Astrophysics, one of the nation’s most dynamic institutions for astronomy research. She saw that to make progress in understanding how galaxies and clusters are organized, a far more intensive series of surveys was required. Although it would not bear fruit for many years, Geller and her collaborators began the long, arduous task of mapping the galaxies (Figure 28.20).

Her team was fortunate to be given access to a telescope that could be dedicated to their project, the 60-inch reflector on Mount Hopkins, near Tucson, Arizona, where they and their assistants took spectra to determine galaxy distances. To get a slice of the universe, they pointed their telescope at a predetermined position in the sky and then let the rotation of Earth bring new galaxies into their field of view. In this way, they measured the positions and redshifts of over 18,000 galaxies and made a wide range of interesting maps to display their data. Their surveys now include “slices” in both the Northern and Southern Hemispheres.

As news of her important work spread beyond the community of astronomers, Geller received a MacArthur Foundation Fellowship in 1990. These fellowships, popularly called “genius awards,” are designed to recognize truly creative work in a wide range of fields. Geller continues to have a strong interest in visualization and has (with filmmaker Boyd Estus) made several award-winning videos explaining her work to nonscientists (one is titled So Many Galaxies . . . So Little Time). She has appeared on a variety of national news and documentary programs, including the MacNeil/Lehrer NewsHour, The Astronomers, and The Infinite Voyage. Energetic and outspoken, she has given talks on her work to many audiences around the country, and works hard to find ways to explain the significance of her pioneering surveys to the public.

“It’s exciting to discover something that nobody’s seen before. [To be] one of the first three people to ever see that slice of the universe [was] sort of being like Columbus. . . . Nobody expected such a striking pattern!”—Margaret Geller

Find out more about Geller and Huchra’s work (including interviews with Geller) in this 4-minute NOVA video. You can also learn more about their conclusions and additional research it led to.

The largest universe mapping project to date is the Sloan Digital Sky Survey (see the Making Connections feature box Astronomy and Technology: The Sloan Digital Sky Survey at the end of this section). A plot of the distribution of galaxies mapped by the Sloan survey is shown in Figure 28.21. To the surprise of astronomers, maps like the one in the figure showed that clusters of galaxies are not arranged uniformly throughout the universe, but are found in huge filamentary superclusters that look like great arcs of inkblots splattered across a page. The Local Group is part of a supercluster we call the Virgo Supercluster because it also includes the giant Virgo cluster of galaxies. The superclusters resemble an irregularly torn sheet of paper or a pancake in shape—they can extend for hundreds of millions of light-years in two dimensions, but are only 10 to 20 million light-years thick in the third dimension. Detailed study of some of these structures shows that their masses are a few times 10 16 MSun, which is 10,000 times more massive than the Milky Way Galaxy.

Check out this animated visualization of large-scale structure from the Sloan survey.

Separating the filaments and sheets in a supercluster are voids , which look like huge empty bubbles walled in by the great arcs of galaxies. They have typical diameters of 150 million light-years, with the clusters of galaxies concentrated along their walls. The whole arrangement of filaments and voids reminds us of a sponge, the inside of a honeycomb, or a hunk of Swiss cheese with very large holes. If you take a good slice or cross-section through any of these, you will see something that looks roughly like Figure 28.21.

Before these voids were discovered, most astronomers would probably have predicted that the regions between giant clusters of galaxies were filled with many small groups of galaxies, or even with isolated individual galaxies. Careful searches within these voids have found few galaxies of any kind. Apparently, 90 percent of the galaxies occupy less than 10 percent of the volume of space.

### Example 28.1

#### Galaxy Distribution

Let’s do a quick calculation to see why this is so.

Suppose that you have completed a survey of all the galaxies within 30 million light-years and you now want to survey to 60 million light-years. What volume of space is covered by your second survey? How much larger is this volume than the volume of your first survey? Remember that the volume of a sphere, V, is given by the formula V = 4/3πR 3 , where R is the radius of the sphere.

#### Solution

The total volume covered is (4/3)π × (90 million light-years) 3 = 3.05 × 10 24 light-years 3 . The survey reaches 3 times as far in distance, so it will cover a volume that is 3 3 = 27 times larger.

Even larger, more sensitive telescopes and surveys are currently being designed and built to peer farther and farther out in space and back in time. The new 50-meter Large Millimeter Telescope in Mexico and the Atacama Large Millimeter Array in Chile can detect far-infrared and millimeter-wave radiation from massive starbursting galaxies at redshifts and thus distances more than 90% of the way back to the Big Bang. These cannot be observed with visible light because their star formation regions are wrapped in clouds of thick dust. And in 2021, the 6.5-meter-diameter James Webb Space Telescope is scheduled to launch. It will be the first new major visible light and near-infrared telescope in space since Hubble was launched more than 25 years earlier. One of the major goals of this telescope is to observe directly the light of the first galaxies and even the first stars to shine, less than half a billion years after the Big Bang.

At this point, if you have been thinking about our discussions of the expanding universe in Galaxies, you may be wondering what exactly in Figure 28.21 is expanding. We know that the galaxies and clusters of galaxies are held together by their gravity and do not expand as the universe does. However, the voids do grow larger and the filaments move farther apart as space stretches (see The Big Bang).

### Making Connections

#### Astronomy and Technology: The Sloan Digital Sky Survey

In Edwin Hubble’s day, spectra of galaxies had to be taken one at a time. The faint light of a distant galaxy gathered by a large telescope was put through a slit, and then a spectrometer (also called a spectrograph) was used to separate the colors and record the spectrum. This was a laborious process, ill suited to the demands of making large-scale maps that require the redshifts of many thousands of galaxies.

But new technology has come to the rescue of astronomers who seek three-dimensional maps of the universe of galaxies. One ambitious survey of the sky was produced using a special telescope, camera, and spectrograph atop the Sacramento Mountains of New Mexico. Called the Sloan Digital Sky Survey (SDSS), after the foundation that provided a large part of the funding, the program used a 2.5-meter telescope (about the same aperture as the Hubble) as a wide-angle astronomical camera. During a mapping program lasting more than ten years, astronomers used the SDSS’s 30 charge-coupled devices (CCDs)—sensitive electronic light detectors similar to those used in many digital cameras and cell phones—to take images of over 500 million objects and spectra of over 3 million, covering more than one-quarter of the celestial sphere. Like many large projects in modern science, the Sloan Survey involved scientists and engineers from many different institutions, ranging from universities to national laboratories.

Every clear night for more than a decade, astronomers used the instrument to make images recording the position and brightness of celestial objects in long strips of the sky. The information in each strip was digitally recorded and preserved for future generations. When the seeing (recall this term from Astronomical Instruments) was only adequate, the telescope was used for taking spectra of galaxies and quasars—but it did so for up to 640 objects at a time.

The key to the success of the project was a series of optical fibers, thin tubes of flexible glass that can transmit light from a source to the CCD that then records the spectrum. After taking images of a part of the sky and identifying which objects are galaxies, project scientists drilled an aluminum plate with holes for attaching fibers at the location of each galaxy. The telescope was then pointed at the right section of the sky, and the fibers led the light of each galaxy to the spectrometer for individual recording (Figure 28.22).

About an hour was sufficient for each set of spectra, and the pre-drilled aluminum plates could be switched quickly. Thus, it was possible to take as many as 5000 spectra in one night (provided the weather was good enough).

The galaxy survey led to a more comprehensive map of the sky than has ever before been possible, allowing astronomers to test their ideas about large-scale structure and the evolution of galaxies against an impressive array of real data.

The information recorded by the Sloan Survey staggers the imagination. The data came in at 8 megabytes per second (this means 8 million individual numbers or characters every second). Over the course of the project, scientists recorded over 15 terabytes, or 15 thousand billion bytes, which they estimate is comparable to the information contained in the Library of Congress. Organizing and sorting this volume of data and extracting the useful scientific results it contains is a formidable challenge, even in our information age. Like many other fields, astronomy has now entered an era of “Big Data,” requiring supercomputers and advanced computer algorithms to sift through all those terabytes of data efficiently.

One very successful solution to the challenge of dealing with such large datasets is to turn to “citizen science,” or crowd-sourcing, an approach the SDSS helped pioneer. The human eye is very good at recognizing subtle differences among shapes, such as between two different spiral galaxies, while computers often fail at such tasks. When Sloan project astronomers wanted to catalog the shapes of some of the millions of galaxies in their new images, they launched the “Galaxy Zoo” project: volunteers around the world were given a short training course online, then were provided with a few dozen galaxy images to classify by eye. The project was wildly successful, resulting in over 40 million galaxy classifications by more than 100,000 volunteers and the discovery of whole new types of galaxies.

Learn more about how you can be part of the project of classifying galaxies in this citizen science effort. This program is part of a whole series of “citizen science” projects that enable people in all walks of life to be part of the research that professional astronomers (and scholars in a growing number of fields) need help with.

## ROTATION OF OUR GALAXY

The Sun, for instance, a fairly typical disk star, is orbiting with a speed
v = 220 km/sec = 0.000225 parsecs/year.
The radius of the Sun's orbit around the galactic center is
a = 8000 parsecs.
The circumference of the Sun's orbit is then
2 pi a = 50,300 parsecs.
The orbital period of the Sun thus turns out to be
P = 2 pi a / v = (50,300)/(0.000225) = 220,000,000 years.

It takes the sun 220 million years to circle once around the center of our galaxy. During the 4.6 billion years that the Sun has been in existence, it has gone around the center just over 20 times.

#### (2) The high orbital speed of stars shows that our galaxy contains dark matter.

Each star in the disk is on a very nearly circular orbit, anchored by all the mass enclosed within its orbit, whether it's luminous or not. Thus, the amount of mass within a star's orbit can be determined from Kepler's Third Law:

where M = mass inside star's orbit (in solar masses)
M* = mass of the star (in solar masses)
a = radius of the star's orbit (in AU)
P = orbital period of star (in years)

A few clarifying words:
In the above equation, M is the total mass in a sphere of radius a, centered on the galactic center. (The mass outside the sphere doesn't have any net effect on the star's orbit).
Since the mass M includes the mass of the supermassive black hole at the galactic center, M is guaranteed to be much much greater than M*, the mass of a single star.

For the Sun's orbit:
a = 8000 parsecs = 1.65 billion A.U.
P = 220 million years

THEREFORE (get out your calculators if you want to check these numbers), the mass inside the Sun's orbit is M = a 3 / P 2 = 90 billion Msun.

Ninety billion solar masses is a lot of stuff, but this just represents the mass inside the Sun's orbit. What's the TOTAL mass of our galaxy, out to its very farthest edge? Most luminous matter (stars, gas, and dust) lies within 15,000 parsecs of the galactic center. Therefore, if luminous matter were the only matter present in our galaxy, the orbital speeds of stars and gas clouds would decrease beyond 15,000 parsecs, just as the orbital speeds of planets in the Solar System decrease as you go outward from Mercury to Pluto. BUT (and this is a big but!) orbital speeds of star are constant, or actually slightly rising, as you go more than 15,000 parsecs from the galactic center. Those few lonely stars stars and gas clouds at a distance of 25,000 parsecs are zipping around at 300 kilometers per second. There must be a great deal of dark matter in the outer regions of our galaxy in order to keep these high speed stars from escaping.

The exact extent of the dark halo around our galaxy is poorly known. The high orbital speeds of globular clusters indicate that the dark halo may extend as far as 200,000 parsecs from the center of our galaxy (that's nearly a third of the distance to our neighbor, the Andromeda Galaxy). The total mass of our galaxy, in that case, is 1 Trillion solar masses, of which 90 percent is dark rather than luminous.

What you see is more than what you get!

#### (3) The dark matter consists partly of MACHOs, partly of WIMPs.

Neutrinos are elementary particles. They snub other elementary particles such as electrons, neutrons, and protons, very rarely interacting with them. Neutrinos also snub photons, very rarely absorbing, scattering, or emitting them. In other words, since they rarely emit light, neutrinos are dark matter. The main drawback to neutrinos as dark matter is that an individual neutrino has very little mass. The exact mass of a neutrino is so tiny it hasn't yet been measured accurately. However, the upper limits on neutrino mass tell us that it takes at least 4 billion neutrinos to equal the mass of a single proton. Neutrinos partially make up for their low mass by the fact that they are extremely numerous in total, neutrinos may contribute a few percent of the dark matter. Candidate 2: the MACHO

MACHO is a (rather contrived) acronym for MAssive Compact Halo Object. MACHOs are dim, dense objects with masses comparable to, or somewhat smaller than, the Sun. For example, brown dwarfs, if they exist in the halo, would qualify as MACHOs, as would old cold white dwarfs, and isolated black holes. MACHOs can be detected because they act as gravitational lenses, briefly amplifying light from distant stars as they pass in front of them. Obsessive-compulsive astronomers have carefully monitored the apparent brightness of millions of stars in the Magellanic Clouds, waiting for MACHOs to pass in front of them. The result of their watching and waiting is an estimate of the number of MACHOs in the halo. The verdict: about half the dark matter in the halo is made of MACHOs. Candidate 3: the WIMP

WIMP is an acronym for Weakly Interacting Massive Particle. (The name MACHO was, in fact, first proposed as a humorous riposte to the name WIMP.) WIMPs are elementary particles proposed by the theory of particle physics. They have not, however, been seen yet in laboratory experiments, so their existence should still be regarded as hypothetical. WIMPs resemble neutrinos, in that they rarely interact with other particles, including photons. Their main difference from neutrinos is that, as their name implies, they are massive. One WIMP is equal in mass to as much as 10,000 protons (or 40 trillion neutrinos). WIMPs are thought to contribute about half the dark matter. (If the non-MACHO half of the dark matter doesn't consist of WIMPs, then it must be made of something even stranger!)

Thus, the question WIMP or MACHO?'' probably has the answer, Some of each''.