9. Galaxies and Black Holes

A galaxy typically has 100 billion stars spread over 100 kly (kilo/thousand light years).

Most galaxies are shaped liked footballs and are called elliptical galaxies. Some are more elongated than others. Some very big ellipticals can harbor as much as 1 trillion stars, but they are a minority. Elliptical galaxies do not have much interstellar gas, therefore they cannot form that many new stars and they are old.

Spiral galaxies, on the other hand, have a lot of interstellar gas and many internal structures, spiral arms being the most prominent. They can form new stars continually. They are not necessarily younger than the ellipticals, but they still have enough gas to remain active. Spiral galaxies are divided into three types depending on the shape of the central region and the compactness of the spiral arms, as shown in the diagram.

There is a third main class of galaxies called the irregulars. Most galaxies in the universe are irregular. They are smaller than spirals and ellipticals and, hence, are sometimes also called dwarf galaxies. They typically has enough interstellar gas to form stars, but the gas is nor organized in any definite shape like the spirals.

Many irregular or dwarf galaxies are found close to big spiral and elliptical galaxies, sometimes orbiting around the bigger galaxies like satellites. Streams of gas bridging them with their parents suggest they might be leftovers from the formation of the parent galaxies or from violent mergers and close encounters of multiple galaxies.

Our Milky Way has many companion dwarf irregular galaxies, the most prominent among them are the Large and the Small Magellanic Clouds, named after the Portuguese explorer Ferdinand Magellan.

Before delving into the birth and evolution of galaxies, we have to familiarize ourselves with one last type of galaxies which is very special, the active galaxies. Almost every galaxy has a supermassive black hole (millions of times heavier than the sun) at its center. In case of active galaxies, the central black hole swallows (accretes) gas and stars from its surroundings along a circular flat disk called the accretion disk.

The black hole takes in material from the equatorial disk, uses some of it in order to get bigger and heavier and throw up the excess material via two jets ejected from its two poles. The artist’s impression below shows how such an active galaxy would look like with its equatorial accretion disk and polar jets if they could be visualized at all.

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Unfortunately, we cannot see active galaxies in such details, mainly because all of them are extremely far away, but also because they do not emit so brightly at our visible wavelengths. Remember that if a galaxy is a billion light years away, we see it as it was roughly (although not exactly) a billion years ago. So we see active galaxies as they looked like a long time ago; many of them might have become inactive in the meantime.

The following diagram shows almost all the known quasars, another name for the brightest active galaxies. Each dot is a quasar here. Our location is at the very center of the diagram and from this vantage point we have been able to observe only toward the directions indicated by the two cones; it does not mean there are no galaxies in other directions—there are quasars in every direction.

Now the distance of the quasars are indicated along the top border of the cones. Notice the density of the dots and you will realize that most quasars are around 12 billion ly away and as you travel closer to earth, the total number of quasars decreases. This tells us that all galaxies formed around 10–13 billion years ago and no new galaxy formed in the last 10 billion years. The quasars that are, for example, 5 billion ly away did not really form 5 billion years ago; they formed much earlier during the heyday of galaxies, the galactic age. This diagram lets us assign a time-range for the galactic age.

Look carefully toward the center of the diagram and you will notice a gap near the center where we do not see any quasar. Most inactive galaxies are actually located within that gap, meaning most of them are very close to us. Inactive galaxies are closer and more recent whereas active galaxies are distant and ancient. This immediately tells us that the ancient universe was much more violent, turbulent and chaotic than our current local universe. You need a lot of chaos and violence for getting so many active galaxies as you see around 12 billion years ago in the diagram. And this will nicely lead us to the next section where the formation and evolution of galaxies are discussed.

How did the galaxies form in the first few billion years of our history? The answer lies in the image of the universe around a million years after the big bang as shown below. During that time the universe was nothing but a homogeneous and almost isotropic conglomeration of hydrogen and helium gas. But if it was completely isotropic (the same in all directions), its picture would not look like what you see below.

You see a lot anisotropies, differences in density and temperature. Here the color indicates temperature, bluer regions are colder and denser whereas the redder ones are more hot and dilute. Pay attention to the blue dots because these tiny temperature anisotropies or overdensities gave rise to all the galaxies we know. But exactly, how?

Gravity is the answer. The regions where hydrogen gas had higher density were more susceptible to inward gravitational pull. Starting from a few million years after the big bang, these dense regions or clouds began to collapse under their own gravity. The rotating and collapsing clouds were gradually turned into galaxies. Compare this with the atmosphere of earth. The atmosphere is made of air and it contains a lot of clouds. Sometimes the clouds, wind and air are turned into hurricanes which look similar to spiral galaxies.

Let us follow the metamorphosis of an overdense hydrogen patch into a galaxy step by step:

1. An overdense hydrogen patch of the universe develops into a dense cloud millions of light years across in a hundred million years.
2. Through chance, the cloud becomes just enough dense that it begins to collapse toward its center because its inward gravitational pull wins against the universal pressure of outward expansion.
3. The collapsing cloud is fragmented into a lot of smaller clouds and an irregular galaxy is formed from each of the smaller clouds.
4. The irregular dwarf galaxies gradually collide and merge with each other to form bigger elliptical and spiral galaxies.

This scenario is called the hierarchical clustering theory (pictorially shown above). It has one big problem. Formation of stars is very easy to explain using the collapse and fragmentation scenario, but it does not work very well at such large scales as galaxies. Two solutions can be sought. First, maybe the extremely turbulent early period of the universe helped by creating swirling eddies akin to the eddies created by water falling into a sink. Second, an altogether different type of matter (called dark matter) might have worked as a kind of gravitational scaffolding to sustain the collapsing clouds and steer them toward the path of becoming galaxies.

Whichever scenario is true, trillions of galaxies were created somehow and they are currently organized into a humongous cosmic web a part of which can be seen above in a snapshot from a computer simulation. Here each dot is a galaxy, the nodes are clusters of thousands of galaxies, and the filaments harbor galaxies in lesser number as well. The voids between the filaments are devoid of galaxies; they do not have enough gas to initiate gravitational collapse.

This video gives you a glimpse of how such a simulation is created.

However the galaxies originally formed, they must have gone through a lot of changes. We have already seen that the first galaxies were very turbulent and active, they ate material from a plate and ejected their waste via polar jets. This action was powered by black holes. The question is, did the black holes formed first or the galaxies? A chicken-and-egg problem.

Black holes are dead stars (as will be explained in the stellar age). If stars formed after and within the galaxies, then black holes must have formed after the formation of galaxies as well. This is the ‘outside-in’ scenario where the galaxies form first and inside them dead stars keep eating each other until they become supermassive and occupy the central place of the galaxies. This might be true as there is a tight relation between the mass of the central bulge (central region of gas) of a galaxy and its supermassive black hole.

What about the ellipticals and spirals? How did they form from the primordial irregulars (if this really was the case)?

Look at the solar system and you will see that smaller objects are irregular in shape and larger objects are nicely spherical. The same could be true for galaxies. Maybe the first galaxies were small and irregular because they did not have enough mass for gravity to sculpt a spherical shape. But as more and more irregular galaxies merged, the mass increased and the giant galaxy gradually became elliptical in shape. Or, even better, maybe the irregulars became loose spirals first and then the spirals became more and more tight before turning into ellipticals.

The problem is that this picture is not correct. If this was true, all irregular and loose spiral galaxies would be young and ellipticals would be old. But there are a lot of old irregulars and spirals and young ellipticals in our vicinity.

In reality, the evolution of galaxies is somewhat like the evolution of different biological species on earth. Humans and chimpanzees did not evolve from each other, but both of them evolved from a single ancestral species which no longer exists. Similarly, irregular, spiral and elliptical galaxies did not evolve from each other, but evolved from a single ancestral galaxy.

The picture gets even more complicated when we see that any galaxy can be turned into any other galaxy through various mechanisms. The diagram above shows how two spirals merge to become an elliptical. And how a spiral galaxy interacts with a dwarf irregular and becomes an even more enhanced spiral.

And, even more interesting, we see that an elliptical galaxy can turn into a spiral because of its interaction with a dwarf galaxy that is passing it by. The dwarf displaces some gas from the center of the elliptical and this displaced gas spirals toward the galactic center. In a few billion years this process could convert the elliptical into a spiral.