Galaxy Formation

A galaxy is the region of space in which stars are born and die, and distant galaxies are the luminous beacons that enable us to probe the distant universe. Our galaxy, the Milky Way, is one of couple of billions of such systems in the observable universe. How galaxies were formed represents a central theme in modern cosmology.

Galaxies began as distinct clouds of primordial gas, hydrogen and helium. Even before galaxies condensed into a number distinct clouds, infinitesimal density fluctuations were present in the Expanding Universe. These originated at early epochs in an inflationary phase transition from a universe that initially was relatively uniform. Fluctuations grew in strength and intensity under the inexorable influence of self-gravity. Eventually, clouds developed that fragmented into stars. Much of the detailed physics in this schematic of galaxy evolution is now understood.

Edwin Hubble's observations, and subsequent Hubble’s Law led to the idea that the universe is expanding.  Shortly after the Big Bang, the Universe was entirely filled with hydrogen and helium with a few other trace elements like lithium. Thanks to the tiny fluctuations in density of this material, it started to clump together into vast clouds of gas with increasing density. Astronomers believe that the process of galaxy formation was really led by dark matter, which outnumbers regular matter. This invisible material also clumped together, and it attracted regular mass with its gravity, channeling material together into larger and larger collections. And so, the first proto-galaxies were formed. Within these newly formed proto-galaxies, clumps of material gathered together, and eventually created star forming regions, and within these regions the first stars began to form. These stars lived a short and violent life, and seeded the next generations of stars with the material created in their powerful supernovae. These foremost proto-galaxies were gravitationally attracted to one other, and merged together into larger and larger structures, eventually becoming the large spiral galaxies we know today.

Galaxies do not act alone. The distances between galaxies seem large, but the diameters of galaxies are also equally large. Compared to stars, galaxies are relatively closer to one another. They can interact and, more importantly, collide. When galaxies collide, they actually pass through each other - the stars inside don't run into one another because of the enormous interstellar distances. But collisions do tend to distort a galaxy's shape. Computer models show that collisions among spiral galaxies tend to make elliptical ones (so, spiral galaxies probably haven't been involved in any collisions). Scientists predict that as many as half of all galaxies have been involved in some sort of collision.

Gravitational interactions between colliding galaxies can cause several things:

• New waves of star formation
• Supernovae
• Stellar collapses that forms black holes or supermassive black holes in active galaxies

The process of galaxy formation is still going on today. Our Milky Way is expected to collide with the neighboring Galaxy, Andromeda  in the next few billion years, and created an even larger elliptical galaxy. We can see plenty of examples of these largest galaxies elsewhere in the Universe.

Questions to Ponder:

• What is the origin of spiral structure in galaxies?
• What is our galaxy's halo made of, and how was it formed?
• How do we know what parameters to use when simulating the collision of the Milky Way and Andromeda?