This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Strange Galaxies

by Dr Jamie Love Creative Commons Licence 1997 - 2011

We live in an ordinary galaxy and I'm glad we do. Things here are fairly settled. Stars come and go at a normal rate and we travel around our spiral galaxy in a "safe" environment. However, there are some strange galaxies out there and, although you won't be studying them as an amateur astronomer, you should know a little about them because they are so interesting and an active research topic in astronomy. Besides, some are visible with only a little magnification (but what makes them strange is not visible).

What kinds of galaxies are strange?

There are really two types of what I call "strange galaxies" - starburst galaxies and active galaxies. Let's start with starburst galaxies because they are not too strange.

A starburst galaxy is a galaxy undergoing a massive burst of star formation. They are characterized by having an infrared luminosity much higher than their optical luminosity. In other words, starburst galaxies produce more heat than light.
Infrared is the part of the light spectrum "below the red" in the sense that infrared waves (or particles) carry less energy than red waves (or particles).

We can see red light but infrared light is invisible to us. Instead, we humans sense infrared as "heat". Our skin is covered with "infrared sensors" but we cannot form an image of the object giving off the infrared because our skin doesn't have the ability to focus the energy into an image. Therefore, we can sense the direction, but not the shape, of an infrared source. [Some animals, such as rattlesnakes, can "see" into the infrared but they don't use their eyes. They use a pair of sensors in the nose to get a good "image" of an infrared object.]

Infrared energy, or simply "heat", is produced by many different things. It's the leftover energy given off by machines (due to friction in their parts), by many animals (as part of their biochemistry) and by just about everything that makes any kind of energy. For example, a light bulb gives off a lot of light in the optical range, including red light, but it also gives off heat - infrared light. Fluorescent lights and most lasers are designed to be much more efficient with their energy and do not waste it so they don't produce infrared. Those special lights are an exception. Most things that give off some light also give off infrared.

Here's a star's spectrum that I showed you several months ago when I taught you about the Maxwell-Boltzmann distribution. The wavelength is along the X-axis (horizontal) and runs from low energy on the left (red) to high energy on the right (violet). The height (Y-axis) represents the amount of energy at that wavelength. This image shows only the optical range but there is a lot of energy given off in the infrared. That's why the surface of a star is hot! Recall that high-energy gamma rays move upward from the center of a star towards its surface. That energy bounces around and gets spread into the lower energy parts of the spectrum. By the time it emerges from the star's photosphere, much of that energy has been distributed into the optical and infrared range. Keep that thought in mind as we go through this lesson.

Very little infrared light passes through our atmosphere. You may be surprised to learn that most of the heat at the Earth's surface is actually produced at the surface - but not from the Earth itself (although a small amount of our heat is from volcanoes and other forms of geothermal energy). Most of our planet's warmth is due to optical energy, which easily passes through our atmosphere, then bounces off the soil and water and is converted to infrared (in a process not unlike the way high energy is distributed down to lower energy as it moves up from the inside of the Sun's surface).

If infrared is invisible and doesn't even get through our atmosphere, how can astronomers "see" it (in a starburst galaxy, for example)?

Technology is the answer. Instruments can sense infrared light and focus it into an image. Rockets can put those instruments above the atmosphere. The first "infrared astronomy" was done using remote telescopes on balloons but in the early 1980s the Infrared Astronomical Satellite (IRAS) surveyed the whole sky from Earth orbit. IRAS found many infrared-bright galaxies (and some other interesting objects, too).

OK, but why are starburst galaxies putting out so much more infrared?

Because they're making stars! New stars are still enveloped in a dense cloud of gas from the condensing nebula. When these new stars first form they must shine through all that extra "dust". The high-energy parts of their spectrum are absorbed by the surrounding materials and re-emitted at lower energies - mostly infrared energies. Indeed, up to 50 times as much energy escapes as infrared light than in the optical (visible) parts of the spectrum.

Note that not ALL of the energy is in the infrared, just most of it, so a starburst galaxy can still be seen at optical wavelengths. M82 is an example of a starburst galaxy. Obviously, it's visible - otherwise Messier would not have seen it and given it a number!

In fact, due to the way the energy is redistributed, during collisions with materials, the final visible color of a starburst galaxy is a bit on the red side - like a red giant. (And for similar reasons. The energy has been "cooled" to a lower energy before being "released" into space.)

So, starburst galaxies are often "dim" in the optical range but "bright" in the infrared range. That means the "luminosity" of a starburst galaxies can be deceiving. Indeed, when total luminosity is calculated - taking into account ALL the energy in ALL parts of the spectrum (visible and invisible) - we find that starburst galaxies are one of the most luminous galaxies known!

Our own Milky Way Galaxy has a total luminosity equal to 25 billion Suns. Astronomers often use the abbreviation Lsun to represent the luminosity of our Sun so the total luminosity of the Milky Way Galaxy is 25,000,000,000 Lsun. A starburst galaxy can have a total luminosity of 100,000,000,000,000 Lsun. That's 4,000 times more luminous as the Milky Way!

When working with large numbers, scientists like to use scientific notation, which is a way to represent the number in base ten exponetials. Using scientific notation, our galaxy has a total luminosity of 2.5 X 1010 Lsun and a starburst galaxy has a total luminosity of 1014 Lsun.

In a starburst galaxy the star formation occurs in a region a few thousand light-years in size. This is a much larger area than you would expect to be involved in "normal" star formation (and also much larger than the area involved in active galaxies, which we will discuss shortly). Most importantly, in a starburst galaxy the stars are formed at a rate that cannot be sustained for very long. That is, if stars continued to be formed at the current (fast) rate, starbursting would have used up all the starting materials in that galaxy long ago. Therefore, starbursting must be a short-lived phenomenon.

You will recall that star formation involves the contraction of huge amounts of gas and dust. Eventually enough mass is squeezed into a small enough space that the heat and pressure cause the material to undergo nuclear fusion and "a star is born". Star formation in our Galaxy is a slow steady process - about one "solar mass equivalent" of interstellar gas and dust is turned into a star each year in the Milky Way. That is just an average number and, like most things involving statistics, there is some variation. It's not like a new star is formed in our Galaxy exactly each year (like a birthday gift)! All I mean is that it's a slow and steady process. In a starbursting galaxy, however, there is a huge amount of star formation - about 100 new stars a year! If that had been the rate of star formation for the billions of years that the starburst galaxy had been around, it would have run out of dust and gas (to make new stars) long ago. Instead, a starburst galaxy undergoes star formation for a brief period of time - just a few million years.

So, what causes them to starburst?

We aren't completely sure. Starbursting might occur for different reasons and by different means. The slow and steady rate of star formation in our own galaxy is due to the fairly even distribution of materials, various ages of stars (so recent novas will distribute materials for new stars) and is helped by the density waves that go around spiral galaxies.

We see some starbursting when galaxies collide (as you learned last month) but those are exceptional cases. Most starburst galaxies are not undergoing collision. Instead, it appears that the burst in star formation occurs due to an increase in local density of star forming materials within that galaxy. This might be caused by local fluctuations in the materials. In spiral galaxies this could be caused by unusual density waves travelling in the disk. Inside elliptical galaxies there may be a series of collisions that perpetuate a chain reaction of stars colliding. When stars collide we would expect them to produce a new, brighter star and there would be a cloud of materials around it due to the disruption that occurs before, during and after the collision.

Sounds like a starburst galaxy is an active galaxy.

Well, yes, but don't confuse them with an "active galaxy".

By definition, an active galaxy emits unusually large amounts of energy from a very compact source - much more compact then the thousands of light-year radius of star formation that occurs in a starburst galaxy. Also, a starburst galaxy can be emitting the energy from just about any part of its "body" but an active galaxy emits its energy only from its center or nucleus. As a matter of fact, an alternative name for them is active galactic nucleus or AGN.

Most of the light in a normal galaxy comes from the stars and that light is distributed throughout the galaxy, with most of the light in the visible part of the spectrum. By contrast, most of the light in an active galaxy comes from gas at the center and most of that light is in the radio wavelengths of the spectrum.

Radio waves are even lower in energy than infrared but don't let that confuse you. Each photon is of low energy but there are so many photons that the TOTAL energy output of an AGN is huge!

Take a look at this graph and convince yourself that an AGN produces large amounts of (invisible) low-energy light as radiowaves.

There are several different kinds of AGNs and we will spend the rest of this lesson learning about them.

But where does all this energy come from? And how is this "gas" giving off light and radio waves?

Those are the questions that astronomers are now trying to answer and they now think they have a good explanation. A massive black hole seems to be the source of energy! Gas accreting onto the black hole radiates energy as it falls towards the event horizon.

The black hole at the center of an AGN is more massive than the black holes you learned about earlier. An ordinary black hole is formed when a large star collapses into a non-fusing sphere of more than three solar masses. The super massive black hole at the center of an active galaxy has a mass between 1 million and ten billion solar masses! Such black holes would have a Schwartzchild radius between 3 million and 30 billion kilometers! These are very big black holes. It would take about 10 seconds for light to travel 3 million kilometers and an entire day to cross 30 billion kilometers! [Of course, inside the back hole that light would be trapped forever anyway, but my point is that we are talking about GIGANTIC black holes here.]

As material, mostly gas, spirals towards the event horizon it bumps into other materials and they emit X-rays! These X-rays are produced whenever electrons smash into something and there are plenty of materials to smash into as you approach the event horizon. This type of X-ray production is referred to as bremsstrahlung which is German for "braking radiation". That's because these X-rays are emitted by the bombarding electrons as they are suddenly slowed down, braked, upon impact. [Exactly why and how X-rays are produced when electrons brake is explained by a combination of quantum mechanics and relativity, but we won't go into those details.]

X-rays are another form of light but they are very high energy, so they would be at the opposite end of the spectrum that we've been talking about. Let's take a look at the high-energy end of the spectrum.

At the far end of the visible spectrum is violet - the highest energy wavelength that we can see. Just above it is the ultraviolet. The ultraviolet has more energy than the violet so we cannot see it but it's still there and it can do us damage. Ultraviolet light, or simply "UV", can cause sunburns and skin cancer. Most of the UV from our Sun is blocked out by the layer of ozone at the top of the Earth's atmosphere, but some parts of the UV slip through. Beyond the UV we enter the part of the spectrum that is home to the X-rays. Obviously, X-rays are higher in energy than UV. Just to complete the spectrum, you should know that beyond the X-rays lies the highest energy part of the spectrum and that is where gamma rays belong. You may recall that gamma rays are produced by the nuclear fusion that occurs at the center of our Sun. [Indeed, "gamma ray bursts" are a sign that something has undergone a burst of nuclear fusion.] You will also recall that those gamma rays do not come bolting out from the Sun's center - instead they interact with materials along the way and in doing so they lose energy, producing many low-energy photons in the process. Perhaps you can guess what happens to the X-rays produced around a black hole.

Ah, collisions cause the X-rays to get changed into lower energy parts of the spectrum?

Right!

X-rays are reflected off of the surrounding materials and some of the energy is absorbed and re-emitted at lower energies. The outcome of all this is similar to what we have been talking about earlier. A spectrum is produced that includes a variety of wavelengths including light in the visible range as well as the infrared.

And even further, into the radiowaves! That's why AGNs produce so much radiowaves. Right?

Ah, wrong. (But that was a good guess. ) The radiowaves produced by AGNs are created by synchrotron emission. Here's how that works.

As the materials fall towards the event horizon, it gets ionized. (The electrons are stripped away by all the energy.) Ionized materials carry a charge (by definition) and, therefore, can be moved around by magnetic fields. A black hole has a huge magnetic field and it accelerates the ions to nearly the speed of light! The magnetic field pulls and twists the paths of these high-speed ions. All this tugging of ions causes them to emit radio waves. [The details of exactly how that happens are tied up in some complicated physics involving quantum mechanics, relativity and nuclear energy but we won't go into that.] Light, including radio waves, emitted by charged particles moving near the speed of light is called synchrotron radiation.

This is different from the kind of energy discussed earlier (involving heat). Unlike infrared radiation (like that produced in abundance by starburst galaxies), radio waves easily penetrate our atmosphere and can be detected by Earth-based radio receivers. AGNs produce huge amounts of radiowaves by synchrotron emission.

The details of AGNs are still being worked out but we can divide AGNs into different types based upon some of the observed details.

OK. What types of active galaxies (AGNs) exist?

Most of the energy of radio galaxies is emitted as radio waves. (So they are well named. ) Radio galaxies are said to have a very high "radio to visible" ratio. But that does NOT mean they are ONLY emitting in the radio wave part of the spectrum. They are often visible (M87 is an example) and many of them have jets of gas extending from the nucleus. These gas jets are produced by the powerful magnetic field hurling gas away just before it reaches the event horizon. The amount of energy released from a radio galaxy, as radio waves, can be as much as ten times the total energy output of the Milky Way.
[Recall that the total luminosity of the Milky Way Galaxy is 25,000,000,000 Lsun so a radio galaxy has a total luminosity of 250,000,000,000 Lsun but most of that is in the radio wavelengths.]
Apparently synchrotron radiation is responsible for most of the output of a radio galaxy and that is why they are so "radio bright".

Most radio galaxies are giant elliptical galaxies and display variations in light output on a scale of a few days.

Contrast radio galaxies with Seyfert galaxies - named after Carl Seyfert who first described them in 1943. Seyfert galaxies are another kind of AGN but they are spiral galaxies with very bright, compact centers. Some have jets and a few (about 10%) emit radio waves but most of the energy emitted from Seyfert galaxies is in the infrared! That means the high-energy X-rays (produced by bremsstrahlung radiation as materials smash into each other as they spiral towards the event horizon) get bounced around by a lot of materials before escaping into intergalactic space. That makes sense because we expect to have a lot of dust in a spiral galaxy. However, Seyfert galaxies also emit a lot of X-rays and ultraviolet so clearly some of that energy is getting through.

About 2% of large spiral galaxies are Seyfert galaxies and it is suspected that Seyfert galaxies represent a transitory (temporary) period in the life of a spiral galaxy. If that is true then all spiral galaxies, including our own, spend 2% of their time as Seyferts. The nucleus of a Seyfert galaxy, which may be only a few thousand Astronomical Units wide, is 0.1 to 10 times as luminous as our entire Milky Way. The luminosity of a Seyfert galaxy can vary over the course of weeks.

One kind of AGN displays a combination of traits of the previous two types. BL Lacertae galaxies are a type of elliptical galaxy emitting large amount of radio waves by synchrotron radiation (like a radio galaxy) but they also have a very bright nucleus and a continuous spectrum produced by bremsstrahlung radiation that has "cooled" to lower energies by collisions (like a Seyfert galaxy). They show short periods of variability but they differ from radio or Seyfert galaxies in that BL Lacertae galaxies are highly red shifted. That means they are moving away from us (or we are moving away from them) at a very fast speed!

Does that make them the fastest moving objects in the universe?

No. However, that brings us to the first kind of AGN ever identified. Quasars are the fastest galaxies in the universe and they are very important in astronomy research right now.

Quasars were first identified as "radio stars" - stars emitting lots of radio waves. We now know what is causing that emission (and so do you if you've understood this lesson). The optical spectrum of these objects is very "strange" and at first no one could identify the absorption lines. Some people were not convinced these things were stars at all, so they were called "quasi-stellar" objects. In 1960 Marten Schmidt explained that the strange absorption spectrum was simply a highly-shifted spectrum. These "quasi-stellar" objects are moving away from us very fast. We now call them "quasars" and they are very important in our understanding of the universe. (Next month we will return to quasars as I teach you some cosmology - the study of the size, shape, origin and fate of the universe!)

Quasars have a total luminosity between a hundred and a thousand times that of a normal galaxy like our own. The total luminosity of the Milky Way Galaxy is 25,000,000,000 Lsun or 2.5 X1010Lsun so a quasar puts out somewhere between 2,500,000,000,000 Lsun and 25,000,000,000,000 Lsun energy! That's 2.5 X 1012Lsun or 2.5 X 1013Lsun. Pretty impressive but remember that a starburst galaxy can have a total luminosity of 100,000,000,000,000 Lsun or 1 X 1014Lsun! So starburst galaxies, which are NOT AGNs, are a wee bit more energetic than a big quasar. However, starburst galaxies are very short-lived, but there is no reason to suspect that a quasar can last only a few million years. That's because the energy is produced in a totally different way in an AGN. There is no star formation going on in an AGN and little, if any, bremsstrahlung or synchrotron radiation is produced by a starburst galaxy.

This is confusing! Why do you make these comparisons?

Because YOU SHOULD!

Science isn't about memorizing a lot of facts. It's about understanding how things differ and what makes them different. (Science is a lot of other things too, but it's definitely not memorizing lots of details.) If you have been taking notes you will understand that AGNs and starburst galaxies are very energetic but they are also very different. Different physics is involved and different observations are made

I said that we believe that quasars are not a short-lived phenomenon (unlike starburst galaxies) but that doesn't mean they are constant in their energy output. They can dim slightly or brighten slightly. A quasar can vary its energy output in very short periods - days or a couple weeks. Some quasars have jets of materials and some of those jets appear to be moving away from the quasar's core at speeds greater than the speed of light!

But that's impossible!

Right!

Notice I said they "appear to be". Careful observations and calculations of the jets' motion, along with a little help from geometry, prove that the superluminal ("faster than light") motion of these jets is an illusion. [Einstein's special theory of relativity is NOT violated.]

Quasars are very strange galaxies.

Yes they are, but the Hubble telescope shows that quasars appear to have normal galactic structure. (In a twisted sort of way - that makes quasars even more strange! )

We are not sure why quasars produce more energy than any other type of AGN. Some astrophysicists suggest that there is some kind of very violent core activity going on in a quasar and they propose that quasars represent a very early stage in a galaxy's evolution. Perhaps our own Milky Way Galaxy was once a quasar.

Next month I will tell you more about quasars, especially about their red shift. You will discover that quasars are very distant and very old. And they are an important clue to understanding our universe. So, please understand this important lesson (especially about quasars).




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.