Distant and Weird Galaxies


What's covered here:

One thing that people started to do when they figured out that those fuzzy things out there were actually separate, distant galaxies was to get as much information about them as possible. Now this isn't an easy thing to do, especially when you consider just how far away these things are. There are only certain things that can be observed with galaxies, and one of those things is the spectrum of the galaxy. Now different parts of a galaxy are moving in different directions and at different speeds, so astronomers thought that there would just be a bunch of random velocities observed in the spectrum, but that wasn't the case. They started noticing that along with the random velocities there was an overall general motion of entire galaxies through space. This would be sort of like watching how people move on a jet plane. If you looked at any given time there would be people moving up and down the aisles (and waiting with crossed legs to use the bathrooms), but if someone were to ask you how fast those people were actually moving, you would say something like a couple of hundreds of miles an hour. Their small individual motions are pretty measly compared to the plane's velocity - the same is true with galaxies. They can have some fairly high velocities involving the stuff within them but those velocities can be pretty small compared to the overall motion of the galaxy through space.

Now there is another type of velocity involved with galaxies, and these are the velocities that galaxies have when they are moving around one another, like in a cluster or binary pair. This motion can be seen as sort of random, since some galaxies in clusters would be moving towards us as they orbit around other galaxies in their cluster, while others are moving sideways to our line of sight and others are moving away from us. One galaxy that is moving towards us is the Andromeda Galaxy. As mentioned in the previous set of notes, that galaxy will actually run into ours sometime in the future - I don't think our insurance has coverage for such an event. Galaxies move around one another in clusters at velocities that can be 100s of km/s - pretty fast.

We know that these motions are occurring, since we can see the evidence of such motions in the spectra of galaxies. If you were to make a survey of thousands of galaxies, what sort of velocities would you expect to see? Would you expect to see all sorts of different velocities due to the random motions of galaxies in their clusters? After all, galaxies are just moving around in various random directions, aren't they? Aren't some going to be moving toward us while others are moving away? It should be pretty random, right? Am I ever going to stop asking questions and get to the point?

Here's the weird thing - when you look at thousands of galaxies, you see pretty much one type of motion - and that is motion away from us. It looks like pretty much every distant galaxy out there is moving away from us! All of their spectra show large redshifts (remember, velocity away from an observer shifts the spectral features to longer, or redder, wavelengths). Now not every single galaxy has a redshifted spectrum, but the number of blueshifted spectra seen in galaxies is pretty sparse, and that is mainly for galaxies that are near us. Apart from these few galaxies, pretty much all the others are moving away from us.

Now this is a really bizarre thing - but what does it mean, and what does it tell us about galaxies and the Universe? Just measuring the spectra and getting the velocities is only one part of the answer. It took some careful detective work to find the other piece of information to figure out what was going on, and that piece of information had to do with the distances of the galaxies. One fellow noticed that there was actually a trend in the velocity values with the distance values. That fellow was Edwin Hubble; you remember him, the guy who figured out that galaxies were separate, distant objects. He used his methods of obtaining distances to galaxies (by using Standard Candles) and combined that information with velocity data (obtained from the spectra of galaxies). Just how did he combine that data? - in a graph, of course.

If you plot up this data, you find that the greater the distance a galaxy is from us, the faster it is moving away from us. Put another way, distant galaxies are receding from us at great speeds. The greater the distance, the greater the speed. This diagram, which was first constructed by Edwin  Hubble is known, surprisingly, as a Hubble Diagram.

Figure 1. A Hubble Diagram, simply a plot of galaxy velocity versus galaxy distance. This diagram is based upon distances found using Supernovae. The slope of the line gives the value for the Hubble Constant, Ho.

Now this diagram will only work on distant galaxies - nearby galaxies don't show this sort of trend since they are moving randomly around in their clusters - sometimes toward and sometimes away from us. For distant galaxies there is a direct correlation between the speed a galaxy is moving away from us and how far away it is from us. There must be a formula that relates these two things, distance and velocity. Of course there is! The data appear to fall along a straight line that can be drawn through the data, so the line represents the slope of the formula, and it can relate the two quantities. Guess what we call this formula? You guessed it; it's known as the Hubble Law.

Mathematically, the formula is written out as
v = Hod
where, v=velocity (in km/s), d=distance (in Mpc), and Ho is the slope of the formula and appears to be a constant, which we'll call the Goober constant - no, that's silly, we better stick with the trend here and call it the Hubble constant (its units are km/s/Mpc). This is a pretty simple formula, eh?

Not only is this formula pretty simple, but is probably one of the most important formulas ever written. THIS IS REALLY, REALLY GREAT!!!!  Why? There are three reasons.

1. If you remember the discussion of Standard Candles, you know that there are some problems in finding accurate distances. The Hubble Law provides astronomers with another method for finding distances - so long as you know what the value of Ho is. How do we get Ho? You need data from galaxies, in particular their distances and velocities. Just take this data and plot it up like Hubble did, draw a line through it and you'll get a value for the Hubble Constant, since it is equal to the slope of the line. That sounds easy; get velocities and distances of galaxies so we can use the Hubble Law to find distances to galaxies. Wait a minute, if we want to use the Hubble Law to determine distances, we need to first have distances to determine what Ho is before we can use it to find distances - that's silly. This doesn't make sense. How can we find distances with the Hubble Law if we first need to have distances to get the constant? That's sort of the problem with the whole thing.

Astronomers can try many times to determine the value of the Hubble Constant using distances measured with Standard Candles and velocities obtained from galaxy spectra, but they rarely get the same value for the constant. This has been one of the biggest problems in astronomy since Hubble first did this. Each time someone tries to determine the value of the Hubble constant, they get a value that doesn't agree with anyone else's value, so there has usually not been much agreement on what value to use for the Hubble Constant. One of the reasons that astronomers don't agree with one another about the value for the Hubble Constant is that they use different methods or assumptions in getting distances, so they will get different values for Ho. Astronomers who do this experiment over and over even change their own ideas as to what the value of Ho is.

That's just dandy; how are we supposed to know what the value of the Hubble Constant is? Actually, things have gotten better in the past few years. Astronomers who have been using the Hubble Telescope (which was not built by Edwin Hubble, it was just named for him) have been able to measure distances using Cepheids and Supernovae in distant galaxies. Actually, several different groups of astronomers were doing this, so of course they ended up with different values of the Hubble Constant. However when you look at all of the data it tends to converge to a value of around 70 km/s/Mpc. This is actually pretty good; it used to be that the values for the Hubble Constant that people calculated were widely divergent by large amounts. Typically, if you wanted to use the Hubble Law to get distances, you would pick one of these values or something in the middle, like 65 km/s/Mpc, and just go from there. Most astronomers today use a value of 65-70 km/s/Mpc.

It is sort of a good thing that the Hubble Telescope was able to do what it was built for, namely to help figure out what the value for the Hubble Constant is fairly accurately. Now it's onto another reason why the Hubble Law is so cool.

2. What exactly is Ho? What does it represent? According to the Hubble Law, Ho=Velocity/Distance = 1/Time. It is related to a time. What does this time represent? The time value which Ho represents is the time it took for the Universe to expand to its current size, or another way of saying it is that it is a rough estimate for the age of the Universe - good gravy, that's one of those basic questions of life, the universe and everything. That's sort of an important number - not as important as your student ID number or your weight, but it is one of those numbers that have been on people's minds for a long time.

3. The last reason why the Hubble Law is so valuable is why it even exists at all. That's a rather interesting thing - why does it exist? Why are those more distant galaxies moving away from us at greater velocities? The Hubble Law exists because of what the Universe is doing - it is Expanding Uniformly! If the Universe weren't expanding or moving in some way, there would be no trend in the velocities. Hubble wouldn't have been able to make that nifty plot of his if the Universe were sitting still. We'll look at this aspect more closely later.

The Hubble Constant is a pretty important thing. Why can't we figure out what its value is simply? It is easy, isn't it? No, it is not easy at all, but is a rather complex step-by-step process. There are many places to mess up in the process.

To determine the scale of the Universe, you must first start closer at home. You need to determine the distance to the Sun, the AU. Why? The concept of parallax is based upon our orbit about the Sun, so the size of a parsec depends upon the size of the AU. Once we know how big an AU is, you can start to find the distances to nearby stars (using the parallax method), then the distances to more distant stars (using the properties of the nearby stars and methods such as spectroscopic parallax), then the distances to nearby galaxies (using various Standard Candles, which are based upon the characteristics of those stars that you looked at previously in your own galaxy), then the distances to more distant galaxies (using the characteristics of the nearby galaxies) and so on.

As you go further out, more assumptions, more uncertainties, more inconsistencies between various scientists, more guess work, and more shaky ground pops up. No wonder those silly astronomers can't agree; it's too much of a mess to begin with!!!!

Distant Galaxies

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 2. 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.


Figure 3. The changing brightness of BL Lac is illustrated over the years. This shows the change in the radio intensity of the galaxy. Based on the image from Bill Keel's slide set.

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 4. 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 5. 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 6. 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 7. 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

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 8. 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 1200, and the redshift, z, is three, then the observed spectral feature is seen at a wavelength value of 1200 + 3x1200 = 4800 ). 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 9. Spectra of the currently highest redshifted quasar and some that are nearly as fast. The Lyman Alpha feature (large bump) is supposed to be at a wavelength of 1216 ! Click on the image to see the  full size version.  Image from Sloan Digital Sky Survey. Image credit: Donald Schneider and Xiaohui Fan, SDSS Collaboration.

Figure 10. 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 11. 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 not realistic.

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 Hubble Space Telescope and the Chandra x-ray telescope 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.

Getting back to the very massive black holes, you might remember that the radio galaxy M87 (Figure 7) 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 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.

Figure 12. 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.

Gravitational Lensing

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 13. 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 14. 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 15. 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. 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.

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 16 shows the galaxy cluster and the evidence for the dark matter.


Figure 16. 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).

Most Distant Objects

What is the most distant object? If there is one thing that can be said about astronomy, records don't last long. On March 1, 2004, a group of European astronomers using the VLT discovered an object (named Abell 1835 IR1916, or "Abby" to its friends) with a redshift estimated to be around 10. This object's light comes from a time when the age of the Universe was only about 480 million years old, only about 3% its current age. Unfortunately, there is a problem with this data, as other astronomers were not able to verify the discovery. So they don't get the record. Oddly enough, only 8 days later, the folks at Hubble Space Telescope released an image known as the Hubble Ultra Deep Field (HUDF), which was obtained by pointing the telescope at one part of the sky for an equivalent of 11 days. The image shown below shows a range of galaxies, some of which may be from the earliest moments in the Universe. The Hubble folks think that some of the little specs that you are seeing below are from objects with redshifts possibly as high as 12! If this is the case, these would be amongst the first objects formed in the Universe, since they would be originating from a time when the Universe was only 370 million years old. Again, it should be pointed out that neither of these actually has been confirmed, so they don't get to go into the record books just yet. For your information, the most distant galaxy that has been confirmed to be distant (based on its redshift) so far is UDFy-38135539 with a redshift of 8.55. The discovery of this object was initially announced in 2009 and that was followed by measurements of its spectra, which were announced in 2010. The image is courtesy of NASA, ESA, G. Illingworth (UCO/Lick Observatory and University of California, Santa Cruz) and the HUDF09 Team. The observations of this object are pretty clear cut and it is the current record holder. This means that the light we see today left it when the Universe was only about 4% of its current age - only 600 million years after the Big Bang (or 13.1 billion years ago). Of course it is likely that someone will break the record sometime in the future. It is rather interesting that the most distant known object is a galaxy - and not a quasar. The most distant quasar currently known is ULAS J1120+0641, which has a redshift of only 7.085. Clearly not as distant as UDFy-blah-blah-blah, but still it is way out there!

I should mention that in April 2009 a gamma-ray burst (named GRB 090429B) occurred which was estimated to originate from an object with a redshift greater than 9, perhaps even 9.4, which would make it the most distant object. Unfortunately gamma-ray bursts don't last very long, so it is very difficult to measure it later with other telescopes to find the source and accurately measure the redshift of the source. You can learn more about the discovery here.

So is GRB 090429B the most distant object? Maybe. There is still a chance that its redshift could be lower than the value announced in the press, since several assumptions were used to determine the redshift. Remember, just because one scientist observed something and says it is one thing doesn't make it true - all work needs to be checked and verified before it is accepted. In fact there is another galaxy which may have a redshift as large as 10.3, but since that has not be confirmed through other means, it doesn't get to be included in my notes (yet).

So the record for the most distant object can change in the future, and with bigger and bigger telescopes, it most likely will change. If you want to look in the direction of the most distant galaxy known (UDFy-etc), you would have to look during the winter towards the southern horizon, down and to the right of Orion, in the direction of the faint constellation Fornax. The most distant gamma-ray burster came from a source in Canes Venatici (below the handle of the Big Dipper).



Figure 17. The Hubble Ultra-Deep Field.  This image, which is less than 1/10 the diameter of a Full Moon, 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, S. Beckwith (STScI) and the HUDF Team.

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.


Now that you've read this section, you should be able to answer these questions....