As we look further out into space, we see objects as they appeared further in the past. You have to remember that light travels at one speed, so you can't see something until the light from that object gets to your eyeballs. It is sort of like waiting for the post office to deliver a message. You can't read the message until it arrives. If the distance is large, the delay in the message is greater. If you were to look at the Sun in the sky, you would not be looking at where the Sun currently is, but where it was about seven minutes ago, since that is how long it takes for light to go from the Sun to the Earth. You could also say that the Sun is seven light minutes away. Alpha Centauri is four light years away, so we are seeing it as it looked four years ago when we look at it in the night sky. The center of our galaxy, Sgr A*, is about 25,000 light years away, so if there were a great big explosion in the middle of our galaxy today, we wouldn't see it until about 25,000 years from now. When you start to look at galaxies, the time delay is even worse. Even our nearest large neighbor, the Andromeda galaxy, is about 2.25 million light years away.
When we look at galaxies that are billions of light years away, we are seeing them as they looked billions of years ago. They are so far away and the light from them takes so long to get here that we don't see them as they look today, but as they looked billions of years ago, when they were younger objects. As we look further and further out, the objects we see are younger and younger. If you remember how young stars were sometimes rather active and did strange things then you may not be surprised to learn that young galaxies are also rather peculiar. Many of these young galaxies are quite unusual - even more unusual then young stars. They go through phases where they will emit abnormally large amounts of light, often at unusual wavelengths, such as radio or x-ray. Sometimes these galaxies also have unusual colors, spectra, and shapes.
We generally throw all of these unusual galaxies into one main group, those that are labeled as Active Galaxies. Sometimes only the centers of these galaxies are visible, in part because they are so abnormally bright. In those cases, we may just refer to them as Active Galactic Nuclei. Either way, these are amongst the strangest things out there. Actually, about 10% of all galaxies are active, so you can't ignore them. I suppose you could, but that wouldn't be very polite. Here are some examples -
Figure 1. The spectrum of a BL Lac galaxy is compared to that for a normal galaxy. In the normal galaxy there is a mix of absorption lines (from mainly stars) and emission lines (from hot gas clouds). In the BL Lac galaxy there are neither of these things, which makes it difficult to determine the chemical nature, velocity and distance of these galaxies. Based on the image from Bill Keel's slide set.
BL Lac Galaxies, which are also called blazars, were misnamed. The first one discovered was given a star name (BL Lac), in particular a name associated with stars that change brightness. This was a mistake, since we know that these are actually galaxies with very bright central regions, so bright that they look like a bright star when seen at great distances. It took some careful studies of their spectra and detailed telescopic studies to show that these are actually the bright cores of galaxies. Once that was figured out, astronomers started to notice all of their bizarre features.
One of the annoying features of blazars is that because they don't have any spectral features that can be seen clearly (no emission or absorption lines), it is difficult to determine their velocities or (by using Hubble's Law) their distances. Actually, blazars are kind of rare when compared to some of these other strange galaxies.
Seyfert galaxies are the next strange beasts in the astronomical zoo. There are actually two types of these. Like BL Lac, Seyferts show very abnormally bright cores. However, unlike BL Lac galaxies, these have emission lines (hurray!). One aspect of the emission lines is how they can appear as either broad or narrow - this is one of the things that caused astronomers to divide them into two categories. Now why would an emission line look narrow or broad? Don't they all look the same? - not really.
Figure 3. Spectra for the two types of Seyfert galaxies. Notice how the emission lines for the Type I Seyferts are wider than that for Type II Seyferts. Based on the image from Bill Keel's slide set.
Emission lines are produced by hot gas. If the hot gas is moving at a pretty good rate of speed, then the Doppler effect comes into play - the emission lines could be shifted to different wavelengths. If the hot gas is moving at many different speeds, some fast, some slow, some toward, and some away, then the Doppler effect would cause the spectral features to appear at a wider range of wavelengths than if it was just sitting still and being boring. Therefore (I like that word; it sounds so final), the presence of broad emission lines tells us that there are a lot of different velocities associated with the gas that is producing the emission lines and that these motions are large and varied in direction. Perhaps this is a clue as to what is going on in these objects...
Here are some gory details about the two types of Seyferts -
First blazers, now Seyferts - quite different, but are they really that different? You'll see...
That's enough of the weird galaxies; now for the galaxies that just look really weird when we see them in different ways. This sort of has to do with the different types of technologies that have come along over time. After World War II, radio telescope technology really exploded on to the scene (not literally; you know what I mean). As the technology got better, our views of some things changed. What looked like normal galaxies with visible light telescopes looked really strange with radio telescopes. In some cases, galaxies were producing more light at radio wavelengths than at the visible light wavelengths. Galaxies that do this or do other weird things at radio wavelengths are called Radio Galaxies. What we've got here are galaxies that at visible wavelengths look like regular galaxies, but pull out your radio telescope and you got a completely different, bizarre galaxy.
Figure 4. A typical radio galaxy. The galaxy itself is not much larger than the dot that represents its location in the image. The sizes of the lobes are much greater than the size of the galaxy they come from. Also visible are the jets that come from the galaxy's core toward the lobes. Images from NRAO.
Generally, when a galaxy has some sort of strange radio light coming from it, it tends to come in two forms - either there is a strong source of light from the core, which isn't so exciting, or it is spewing out radio emitting gas in various directions. This is sort of what people think of when they think of radio galaxies. In the case of the big-time spewage, there is usually some sort of structure that shows the motion of material from the core, generally in a jet type of structure (sort of like that bipolar outflow you have with accretion disks around black holes). The stuff that is getting spewed will pile up over time, generally into big lobes. A classic view of a radio galaxy shows a double lobe structure. These lobes are huge compared to the galaxies that are spitting them out - they can be millions of light-years in size (galaxies tend to only be only thousands of light years in size). These great big lobes are nothing more exciting than radio emitting hydrogen gas. It's just that there is a lot of it out there!
Cen A is a classic example of a Radio Galaxy. In the visible light view, it looks like a galaxy with a large dusty disk, a little bit unusual but not too bizarre. With the radio telescope, you get a totally different picture. Notice how the lobes come out of the disk - they are actually perpendicular to it.
Figure 5. Cen A, as seen in multiple wavelengths. On the left is a visible light image. In the center is a radio (orange) and visible composite. On the right is a composite of visible, microwave (orange) and x-ray (blue) light. These images help astronomers to understand the core of Cen A and the jet features. You can read about the images, view movies and learn about what astronomers think is happening at the NASA website. Left image from Capella Observatory. Middle image from Capella Observatory (optical), with radio data from Ilana Feain, Tim Cornwell, and Ron Ekers (CSIRO/ATNF), R. Morganti (ASTRON), and N. Junkes (MPIfR). Right image from ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray).
Another good example of a radio galaxy is M87, an elliptical with a very strong jet extending from it. M87 is relatively close, so the jet is seen even in some visible light images. The jet is about 1800 pc long (about 6000 light-years). Hubble Space Telescope observations of the core indicate motions of material around 550 km/s about the center. By using Kepler's laws we can estimate the mass of the core to be about 2.5 billion solar masses. For reference, the mass at the center of our galaxy is a few million solar masses. That is a big difference!
Figure 6. Images of M87, an elliptical galaxy with a jet. The view at the left shows the regular telescopic view (image from NOAO/AURA/NSF). The top right view is a radio telescope image, which mainly shows the large jet coming out from the center of the galaxy (image from NRAO/NSF). The lower right image is from the Hubble space telescope and it shows the jet in more detail. Data from the Hubble telescope were used to measure the velocity of material near the center of the galaxy and estimate the mass of the black hole in the center. Hubble image from NASA and John Biretta (STScI/JHU).
Radio galaxies can come in a variety of shapes - they can be spirals or ellipticals, though it seems that the most extreme ones tend to be ellipticals.
Now I suppose you think that these bizarre objects are out there at very great distances. Actually, some of these strange radio galaxies are relatively close. The really weird galaxies, the ones that astronomers have still not figured out, are amongst the most distant objects out there. Let's take a look at these really extreme galaxies...
Quasars were originally thought to be stars with very strange spectra. If you look at a quasar you usually see a point of light - just like what you see when you look at a normal star in our galaxy. Remember, galaxies traditionally look like fuzzy blobs, not points of light. Quasars (short for quasi-stellar radio source) stood out from normal stars because they had weird spectra. First of all, they have emission lines, which is unusual for stars. Not only that, the spectral features did not correspond to any normal spectral features. When you look at a star, the spectrum usually has normal elements in it, like hydrogen, carbon, iron, etc. The wavelengths where the emission lines were seen didn't correspond to any of these things. They were just weird.
Figure 7. On the left, an image of a typical quasar (the object in the middle with the two lines pointing to it) as seen from the ground. Notice how much this looks like the stars that are also in this image and are actually within our galaxy. To the right is a collection of images of quasars taken by the Hubble Space Telescope. In this picture it is possible to see hints of the galactic structure around the quasars. Image on the left from Bill Keel's collection, image on the right credits: John Bahcall (Institute for Advanced Study, Princeton) Mike Disney (University of Wales) and NASA.
In 1963, Maarten Schmidt was looking at the spectra of some of these quasars and noticed that the spacing of the emission features was about the same as that for hydrogen emission features, but they were well off their normal wavelengths. Maarten did the calculations and found that the spectra for the quasars would be what you would get if the features were redshifted to longer wavelengths due to the motion of the object being about 45,000 km/s = 15% the speed of light! This is a pretty high velocity for a star! After some more study, Maarten figured out that the object he was looking at (which has the really romantic name of 3C 273) was actually fuzzy, meaning that it wasn't a star but a galaxy with a really bright core. To put this in perspective, you should know that 3C 273 is one of the slower quasars out there!
If we use the Hubble law, we find that these incredibly large velocities (many of them good fractions of the speed of light) appear to correspond to incredibly large distances - amongst the greatest distances measured!
We need to have word of CAUTION here about the redshifts. The redshifts of quasars tend to be pretty large, so values of redshifts (the symbol for redshift is z = / ) of two or three are not unusual. This means that the light has been shifted to two or three times its current value (if normal wavelength for a spectral feature is 1200Å, and the redshift, z, is three, then the observed spectral feature is seen at a wavelength value of 1200 + 3x1200 = 4800Å ). The formula for Doppler shift is z=/ = v/c
Does this mean that a redshift (z) of two, implies a velocity of two times the speed of light?
No. When such high velocities are observed you need to use a different redshift formula, one that is based upon Special Relativity. We have to use this formula since nothing can go greater than the speed of light, c. The Relativistic Redshift formula allows values for redshifts (z) greater than one, but velocities are always less than c.
The Relativistic Redshift formula is
Quasars are among the most distant objects seen and they have some rather extreme characteristics -
Figure 8. Spectrum of the currently highest redshifted quasar. The Lyman Alpha feature (large bump) is supposed to be at a wavelength of 1216 Å, which on the scale in the spectrum would normally be at 0.1216 if the quasar wasn't moving! Click on the image to see the full size version. Most of the light from this quasar is only visible at infrared wavelengths - it is moving that fast. Image from D. J. Mortlock et al, (2011).
Figure 9. The Lyman Alpha Forest is shown in the spectrum of the quasar on the bottom. The nearer quasar's light is not absorbed by many intervening gas clouds, so its spectrum doesn't have as many absorption features as the more distant quasar. Based on the image from Bill Keel's slide set.
Now we have strange galaxies which go from being the most extreme
(quasars) to being just a little quirky (radio galaxies). Are they all
drastically different, or are they all just variations of one common set
up? It might be easier to deal with them if there is just one model or
concept that can explain all of these weird things - that way we don't
have to come up with all sorts of different exotic ideas and figure out
why there would even be different exotic models. Sometimes in science,
it's better to search for a single solution than for many diverse
solutions. Often the simplest one is correct. The basic upshot is that
astronomers think that there is one model that can explain all of these
strange galaxies - from the strange spectral features and the unusual
energy sources to the crazy structures that are seen. The type of active
galaxy that is observed may only depend upon the observer's point of
view. Here is what the model is all about.
Figure 10. The basic model for Active Galaxies. It consists of a massive black hole (located in the center), an accretion disk around the black hole, a bunch of high velocity clouds (yellowish ones around the disk), low velocity clouds (bluish ones located above and below the accretion disk), a dusty torus (cut open here so you can see the stuff in the middle), and jets going perpendicular from the center (cut off in this view). The relative sizes of objects are not correct here, but are just shown so that you can get an idea of where everything is. Of course, the colors are completely unrealistic.
The model is made up of the following components
How does this explain everything? First of all, not all strange galaxies will have each component in equal amounts - some may not have very significant black holes, or jets, or things like that. That is something that should be remembered.
How do the different active galaxies show up? Let's say you are looking along the edge of the disk, which will be pretty boring, since the dusty torus blocks all the action in the center. You aren't seeing any of the high velocities or high-energy action in the center. All you are seeing is the dusty torus, which would be a good IR source, and the low velocity clouds away from the center. This is a good way to explain the appearance of Seyfert Type II galaxies.
Now let's look at it from an angle. Now we can see the action in the middle, including all those high velocity clouds. This will produce broad emission lines. You are also seeing stuff that is a source of high energy light, like UV, x-rays or maybe even gamma-rays, so now you're seeing the stuff that we associate with Type I Seyferts.
Another direction you can look is straight down the throat of one of those jets. Now you'd think that this would show you all sorts of stuff, but actually it doesn't, since the bright core is not really easy to see since it is behind the material being shot out of the core in the jet. You are really just seeing most of the stuff that is associated with the jet that is coming toward you. That material is from a jet, which is produced by the strong magnetic field of the accretion disk, so you would see non-thermal light from this source as well as highly polarized light. This is what you'd get if you were to look at a BL Lac (Blazar).
You can also see from most of these directions the ingredients for a radio galaxy, though that would depend upon how much radio-emitting material is being ejected by the jets. A quasar could be observed along the same viewing angles as the Type I Seyfert views or some of the other angles, but since quasars are so distant you might not be able to detect evidence of all of these other features. A summary of these views in the active galaxy model is available for viewing here.
Does this model really make sense? In a way it does. First of all, in recent studies by the WISE IR and the Chandra x-ray telescopes indicate that massive black holes appear to be present in the cores of most galaxies. In fact it appears that black holes are quite common and very active in most galaxies, and remember, you can determine their masses by seeing how objects move around them (good old Kepler's laws). Here is a news item about such a black hole, one that is only a few million solar masses in a distant galaxy. In fact, it seems that it would be unusual for a galaxy not to have a massive black hole in its core! Recent observations with all the main space telescopes have shown evidence of a star being torn about by a massive black hole in the core of a galaxy. Just read about it in this link.
If you remember the material about radio telescopes, you will recall that they expell material at great distances from their cores. What if something is in the way? Apparently that has happened. The so-called "Death Star Galaxy" is blasting material from its core in the direction of a nearby, small galaxy, which is causing all sorts of disruption to the small galaxy. You can read about it at the Chandra website. There are three videos that go with this - one of a model, one of the real data, and one showing how they relate to one another.
One interesting aspect of these likely black holes is that it appears that black holes tend to be either small (few dozen solar masses) or very large (millions or billions of solar masses). There are very few instances of "in-between" black holes - those that would have perhaps a few thousand solar masses worth of material. Recently observations of the cores of globular clusters indicate that perhaps there could be some "in-between" black holes in them. So far the smallest black hole detected in the core of a galaxy was "only" 50,000 times the mass of the Sun. Hardly worth mentioning...
Getting back to the very massive black holes, you might remember that the radio galaxy M87 (Figure 6) is likely to have a huge black hole in its core, and there are many galaxies with visible disks in their cores, jets, and the things described in the above model for an active galaxy. So why aren't there more active galaxies out there? It is likely that when the galaxies were first forming, that they were initially quasars. At that time, they were sucking in huge amounts of material into the accretion disk and black hole, so that when we look at them now, we see them as very distant, active, high energy objects. However, this won't last forever; eventually the party has to shut down and the mechanism that produces all of this energy will fade away as less and less material is sucked in. If you don't feed the black hole, you don't get the fireworks show. Over time a quasar will fade away, possibly going through a Seyfert phase and then finally ending up as just a boring galaxy. This helps to explain why we only see quasars at great distances (which corresponds to looking further into the past, when galaxies were younger). Actually, you don't have to put a huge amount of material into these things to get all of the observed energy that comes out of them. Maybe only about 1 solar mass of stuff per year needs to be fed into the black hole to provide the observed energy output.
Galaxies can also undergo other changes that will alter their fates, such as mergers. In fact, elliptical galaxies may have gone through quite a few steps to reach their final fate. Observations by the Hubble Space Telescope appear to show a range of steps for the evolution of ellipticals, from formation and sudden star formation, to eventual mergers. Over time the mass, color and appearance of a galaxy can change significantly given the right conditions.
Figure 11. Views of NGC 4261. On the left is a composite of the visible light image (whitish blob) and radio telescope image (yellow lobes). On the right is an up close view of the core from the Hubble Space Telescope. Notice that this is pretty much a disk structure located at the center of the galaxy, which is in line with the model for Active Galaxies described above. Image credit: Hubble Space Telescope.
There seeems to be all sorts of support for the unified model described above to explain active galaxies, so obviously we've solved it, right? NO! Just because a theory can explain what is going on doesn't mean it is necessarily correct. In 2014, astronomers using the WISE infrared satellite surveyed over 170,000 active galaxies with black holes - some obstructued from view by gas, some unobstructed. The problem was that there wasn't a random distribution of the galaxies in terms of how obstructed they were. If there was only one reason to explain how well you can see the centers of these galaxies (your viewing angle), then there should be a random distribution of the degree of obstruction, but there wasn't. This indicates that something else is influencing how well we can see the centers, more than just our viewing angle. You can learn more about this study here. Does this mean we have to scrap the theory? No, it just means that we are missing something else that can influence our view of active galaxies, so more work needs to be done!
Another thing that is seen occasionally with quasars and other distant galaxies is one of the effects of Generally Relativity, the distortion of space due to massive objects. Let's say you have a bunch of galaxies in a cluster. You then have a lot of mass, and since mass distorts space, you have a bunch of distorted space in the area of the cluster. Anything that has to travel through a cluster of galaxies, like light from very distant objects, will have to deal with the spatial distortions that pop up due to the galaxy cluster. Under some special circumstances, you don't even need to have a bunch of galaxies, but just a well placed large galaxy in front of another distant object, like a quasar. This distortion is just like the distortion seen in the starlight that travels close to the sun. In this case, since the light isn't coming from a star but from a distant galaxy, you don't have a single point of light but rather a larger blob to screw up. The effects of this distortion can be really nifty.
Figure 12. The set up for gravitational lensing. The light from the distant quasar passes near the large galaxy and the path is bent. An observer on the Earth may think that there are actually two quasars since they see light from the same quasar coming from two slightly different directions.
At times the distortion will cause us to see not one but two images of the same quasar. Sometimes we see more images of the same quasar; in several cases up to four images of the same quasar have appeared in various spatial distortion events. There is one image that shows five images of the same quasar being distorted (you can see images of that here). When you have something doing these things to the light from a distant object, then you have what is known as a Gravitational Lens. A gravitational lens can do more than just make multiple images appear. There are several instances where a large concentration of galaxies distorts space like a fun-house mirror distorts your view of the world around you. The images of the more distant galaxies aren't actually reproduced in this case but are smeared out into unusual shapes.
Figure 13. The Einstein Cross. The four bright sources around the larger light source are actually four images of the same quasar that has been split up into four different images. The large light source is the core of the galaxy that is distorting the light from the distant quasar. Image from Bill Keel's slide set.
Very distant objects are more likely to have their light screwed up,
so we can look at some of these distant objects more closely to see just
how far away they are. This sort of cuts down on the "searching for a
needle in a haystack" for distant objects, since we know they won't be
as likely to be distorted if they are nearby. This also is one of the ways
we know that quasars are very far away - their light would not likely
be distorted if it didn't have to travel so far (there was a time when
quite a few astronomers thought that quasars can't really be so
distant, but gravitational lensing kind of shot their arguments down).
Actually, some of the most distant objects ever discovered were found
through the study of a gravitational lens system. In one case, the
redshift (z) for the distorted galaxy was measured at 4.92, which
corresponds to a velocity of about 94% the speed of light.
Figure 14. A cluster of galaxies distorting the light from even more distant galaxies. There are all sorts of streaky images of galaxies shown here around the large cluster. This is one of the best examples of gravitational lensing, especially since it is happening on such a large scale. Credits: NASA, A. Fruchter and the ERO Team (STScI, ST-ECF).
Gravitational lensing is easily visible in large concentrations of mass like a giant cluster of galaxies. However it is also possible to see a similar effect on a much smaller scale. Astronomers look for indications of dark matter in our own galaxy by trying to see the influence that unseen but massive objects have on the light from more distant stars behind them. The unseen objects will cause the light from distant stars to become brighter for a short period of time in a well defined manner. This happens due to the warping of space that causes the light from a distant object to be concentrated so that the light source looks abnormally bright. However to see just one of these "lensing events" astronomers have to monitor literally millions of stars to see if any of them do get brighter. This is further complicated by the fact that there are stars that normally do change their brightnesses (remember Cepheids and RR Lyrae?). So those normal variable stars have to be taken into account when people are searching for the small scale gravitational lensing effects of dark matter. Several of the current searches for dark matter, particularly the MACHO searches, use this method to find the unseen objects.
Recently astronomers using the Hubble Space Telescope has observed such a gravitational brightening effect on the light of a distant Ia supernova - remember, these were the ones that have well defined brightnesses, so when one of these appears too bright, that requires a closer examination. You can read about the discovery here. Another study by the Chandra x-ray telescope shows both the multiple images of a quasar and the brightening of it. This study (which can be seen here) also was able to measure the rotation of the core of the quasar, which is likely a black hole.
Gravitational lensing is also visible at other wavelengths, including gamma-rays. Observations of a gamma ray burst in a distant galaxy resulted in two images of the outburst due to gravitational lensing. However the two outburst didn't happen ("burst") at the same time. If light has to travel along different paths to get to you, the time for that journey will depend upon the length of the path, which will depend upon how the objects are lined up. You can play around with this simulation to show how the outburst can be seen days apart depending upon the alignment of the objects to you.
In 2007, a report from the Hubble Space Telescope indicated that gravitational lensing was used to infer the existence of a large distribution of dark matter in a distant galaxy cluster. You can read about the discovery by following this link. In this case, the distortions appear to happen at a pretty good distance away from the actual location of the cluster, indicating that dark matter can extend quite a ways out from the galaxy or galaxy cluster that it is located in. Remember, we can't actually see dark matter, but we can certainly detect it by its influence on matter and light. Figure 15 shows the galaxy cluster and the evidence for the dark matter.
Figure 15. The effects of dark matter on an image of a galaxy cluster. The blue smears are actually images of more distant galaxies, whose light is smeared by the present of dark matter. If you click on the image, you'll see an animation that shows the distribution of dark matter that the astronomers were able to derive based upon the distortion of galaxies. Credits: NASA, ESA, M. J. Lee, and H. Ford (Johns Hopkins University).
What is the most distant object? If there is one thing that can be said about astronomy, records don't last long, so that there is a good chance that during the time you are in this course, these records will be broken. So here goes - currently the candidate for the most distant object, which is thought to be a proto-galaxy (a galaxy in the process of forming) is UDFj-39546284 (what a lovely name). Any ways, this object has a redshift of 11.9, which indicates that the light from this object left it 13.42 billion years ago. Based on our current estimates for the age of the Universe at about 13.8 billion years, the light we see today came from an era when the Universe was at most only 3% its current age. There is a bit of debate about the actual distance to this object, since its distance isn't based upon the spectra but upon how red it appears. So this is really only a possibility, not a slam-dunk certainty.
What is the distance to the object, which has a verified distance - something based upon a spectra? Actually that object is also the most distant galaxy. The most distant confirmed galaxy is at a redshift of z=8.68, and goes by the name of EGSY8p7. This object was discovered in July 2015 at the Keck Observatory in Hawaii, and its light was altered by gravitational lensing, which also provides a method for measuring distances.
Other phenomena are also seen at great distances, such as supernova or gamma-ray bursts. Generally these distance breakers will eventually run out since we can only see objects out to a certain distance (you'll learn why that is the case in the next sets of notes). So don't expect major revisions to these numbers.
How is it possible for us to find more distant objects? We could do this by building larger telescopes,
but that requires a lot of money. We can also use years of data and build up images by gathering together
the light. Remember, a lot of modern
data is digital. So you can combine multiple pictures of the same region of the sky together to get a better and
better image as more and more light is gathered together. In June 2014 the folks at the Hubble Space Telescope released the latest version of the
Hubble Ultra Deep Field,
an image that is based upon observations by several telescopes of one small part of the sky during a time that spanned 10 years and
required about 600 hours of observations - that's like staring at something for 25 days straight! An image of this
view is shown below, and if you click on it, you'll see a larger image. In this small region of sky you can see
thousands of little specks - each of those is a distant galaxy, probably around 10,000 total galaxies.
How much of the sky does this picture cover? Take a penny out of your
pocket and hold that penny 280 meters away, nearly 3 football fields away, and the size of the penny at that distance
is how much sky is in this picture. Imagine what is visible in all the other directions of the sky!
Figure 16. The Hubble Ultra-Deep Field, 2014 version. This image contains about 10,000 galaxies. Some of them are thought to have redshifts greater than 10. If you click on the image, you'll get a view at a larger version. In this larger view you should some small red objects which are very distant faint galaxies with redshifts near 6. Image from NASA, ESA, H. Teplitz, and M. Rafelski (IPAC/Caltech), A. Koekermoer (STScI), R. Windhorst (Arizona St. Univ.), and Z. Levay (STScI).
Images such as the HUDF and all of the redshift surveys help astronomers understand the evolution of not only galaxies but also the Universe. They are able to see how galaxies at great distances have some rather interesting characteristics; amongst these is the existence of massive black holes early on (which supports the quasar model discussed above). Not only do we see this with visible light telescopes, but also x-ray telescopes such as Chandra reveal galaxy evolution and black hole growth. So I guess we should move onto the next step - discussing the Universe.