How was the Milky Way born?
The moment of creation has fascinated philosophers and scientists throughout history. In the distant past, how did our Milky Way Galaxy form?
The creation of our Galaxy is something of a conundrum. "It is a big mystery in astronomy and cosmology," says Professor Bob Nichol, University of Portsmouth. "As yet, there is not a compelling theory that beats all others. We have seen galaxy evolution, but we have not yet witnessed galaxy formation, and until we do, we are going to be scratching our heads."
It is thought that a combination of events led to the formation of the Milky Way in its present form. First there was a monolithic collapse. After the big bang, dark matter and ordinary matter were in equilibrium and spread evenly throughout the Universe. As the Universe expanded, regions of very slightly lower densities and very slightly higher densities, or 'overdensities', were fonned. "As the Universe evolved, dark matter and gas flowed into the 'overdensities' and these became the sites of galaxy formation.
As the density increased, the ordinary matter cooled and began to form clumps, which then joined other clumps to make bigger clumps. Eventually the cooling is enough for an overall collapse of the gas, from a roughly spherical shape, to give the disc, bulge and stellar halo. However, it is also possible that the central bulge and the outer galactic halo are not created together. In some observed galaxies, they seem to have a different origin, the outer galactic halo was the accumulation of a lot of very small things, unlike the bulge, which Was an accumulation of a small number of large things. This is Where other events comee in, which are described in the cold dark matter dominated cosmologies. These show that galaxies are built up over time through the steady accretion of smaller dwarf galaxies, bringing with them gas, stars and dark matter. Other stellar matter is also continuously pulled into the gravity well of our Galaxy, thus further enlarging the galactic halo.
The more theories that are investigated the more questions are generated. A few that are currently being researched include:
- Which models and assumptions are correct? In order to make more accurate models and simulations, the basic assumptions about dark matter, gas physics, star formation and the feedback of energy
into interstellar gas must be accurate.
- Where are all the dwarf galaxies and why are the stars in dwarfs not like the stars in our Galaxy? Simulations predict that there should be thousands of dwarf galaxy companions to the Way, but
only a few dozen have so far been found.
If the Milky Way was partly formed from the accretion of dwarfs, then the chemistry of stars in the dwarfs should resemble those in the Way, but they don't.
- How did the different parts of our Galaxy form? Is the thick disc just the thin disc warmed up by the interaction with dwarfs, or is it the remnants of cannibalised dwarfs, or is it something else?