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Galaxy Formation 101

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In The Beginning

If you read Cosmology 101, you know that the Big Bang produced large amounts of hydrogen, some helium, and a small amount of lithium. Actually, it's likely that the Big Bang produced about as much anti-matter as matter, and it has been a puzzle why the matter and anti-matter didn't just mutually annihilate. Recent studies have indicated a type of quantum asymmetry that might explain why a residual amount of matter in excess of the anti-matter was produced.

Anyway, the result was a universe with vast clouds of hydrogen. The initial question about that Big Bang was why there were clouds, and not just a single uniform cloud. That is, what caused any clumping? It seemed, from the equations, that a uniform distribution should have been created.

Primordial Universe

One answer promoted was that the primordial distribution of material (first disconnected quarks, then particle soup, and finally the first few elements) was not homogeneous because of quantum fluctuations in the Big Bang itself. Earth-based radio telescope examinations of the cosmic background radiation reveal no such fluctuations.

The COBE space probe, able to measure much smaller fluctuations in the background radiation, detected the quantum fluctuations hard sought. The incredible difficulties of getting the COBE built and launched are detailed in Wrinkles in Time, by George Smoot and Keay Davidson.

It is, then, the quantum fluctuations inherent in the Big Bang itself that led to an uneven distribution of hydrogen. The image depicts such an uneven distribution of gas. The uneven distribution allowed the mutual gravitational attraction to operate on the unevenness, further clumping the gas.

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The Game is Afoot

This image illustrates a further clumping created by the initial distribution, and the mutual attraction of gravity between the atoms. Depending upon the primordial clumping, clumps of gas became more condensed, and thin areas of distribution began to disappear.

Also beginning to be apparent in this image is a slight angular motion. It is virtually impossible for the initial clumps to have had no angular momentum.

Recall how a spinning ice skater spins faster has she pulls in her arms? That's from the conservation of angular momentum law. Angular momentum is a function of the distribution of mass and the angular rate. If the mass moves closer to the center of rotation (which it will in the gas clouds from the gravitational attraction), the angular rate will increase to conserve the momentum. It's the same effect, by the way, that leads to neutron stars spinning at hundreds of times a second.



The Plot Thickens (and so does the soup)

This image shows the further collapsing of the distributions of gas and the increasing angular rate.

So why is the rotation around some invisible point in the center?

Good question. Maybe it never was like that. Then again, current ideas on Dark Matter suggest it may have been.

Whatever the solution regarding dark matter turns out to be, it is now recognized that most (perhaps all) galaxies have massive black holes at their centers. Did the black holes coalesce from the hydrogen, were black holes part of the Big Bang's creation, or does dark matter have something to do with the early formation of black holes?

That answer is still illusive. The image depicts the hypothetical situation in which a black hole already exists in a region of hydrogen. If that's the way it happened, the swirling hydrogen would continue to collect and shrink toward the black hole, spinning more rapidly as a result. With the current view, it's a good guess of how things frequently happened.



The Shape Begins to Form

The further shrinking of the swirling hydrogen begins to take shape. What shape? It could vary, but the characteristic pinwheel shape of a galaxy is a common result.

We're talking galaxy formation here. Hydrogen is pulled toward the center of mass, and inhomogeneities within the hydrogen cause local collapses that lead to star formation.

Near the center, the voracious black hole begins to strip apart matter, creating high energy x-rays and gamma rays. In this phase, the proto-galaxy is incredibly bright, visible through Earth-based telescopes from billions of light years away. The initial objects spotted a few decades ago were given the name Quasars.

Now we know that the quasar is a galaxy being born, at the phase when the black hole in the center is gorging itself on abundant material and spewing out an enormous amount of energy.



I See It Now

Eventually, the material settles into a stable configuration. The voracious appetite of the black hole produces so much violence around the vicinity of the event horizon that an evacuated zone develops, pushing away other material. The shape of a galaxy is recognizable now, and star formation (and planet formation) in the orbiting gas continues.

So the history is clear, we have it before us. Or -- do we?

There are still problems with this view of galaxy formation. It first seemed that the mutual attraction of gas in the early clumps wouldn't have had time to form galaxies this early in the universe's development.

How long is that, by the way? About 13 billion years. Given the energy and rapid expansion of the Big Bang, it's difficult to explain how galaxies would exist in the abundance they display so early in the history of the universe.

The fact that black holes play a part is helpful in some ways, but the rapid clumping still isn't explained.

The fact of misbehaving orbital velocities of the material throughout the galaxy is another problem. In the typical galaxy, material further from the galactic center doesn't move as slow as a simple gravitational model would dictate, even with the existence of the black hole taken into account.

Ah, it was never meant to be easy. Only fascinating.





The pictures on this page were created (however crudely) with the Gimp. The Gimp fortunately had enough tools to give the spinning and collapsing effect I was seeking. The Gimp, by the way, is freely available to Windows, Mac, and Linux users (I happen to favor the Linux version).

Another book that gives a description of galaxy formation, the problems with it, and how it is coupled to our knowledge of the microwave background radiation is Steven Weinberg's The First Three Minutes. If you're so inclined, I suggest first reading this book, then Smoot's book about the COBE satellite. Weinberg's book describes the necessity for a satellite view, which hadn't happened at the time of his book.