Fall – 2011.
Professor: Leonardo Martinez
EVOLUTION OF GALAXIES
1. Epoch of the Galactic Formation
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated thevariety of structures observed in nearby galaxies. It is one of the most active research areas in astrophysics.
After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand.
The most accepted view today is that all the structure we observe today wasformed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, and star clusters and stars. One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe. Hence the distribution of galaxies is closely related to the physics of the early universe.
As theuniverse cooled clumps of dark matter began to condense, and within them gas began to condense. The primordial fluctuations gravitationally attracted gas and dark matter to the denser areas, and thus the seeds that would later become galaxies were formed. These structures constituted the first galaxies. At this point the universe was almost exclusively composed of hydrogen, helium, and dark matter.Soon after the first proto-galaxies formed the hydrogen and helium gas within them began to condense and make the first stars. Thus the first galaxies were then formed. In 2007 the Keck telescope, a team from California Institute of Technology found six star forming galaxies about 13.2 billion light years (light travel distance) away and therefore created when the universe was only 500 millionyears old. The discovery of a galaxy more than 13 billion years old, was reported in January 2011 which existed only 480 million years after the Big Bang.
The universe was very violent in its early epochs, and galaxies grew quickly, evolving by accretion of smaller mass galaxies. The result of this process is left imprinted on the distribution of galaxies in the nearby universe. Galaxies are notisolated objects in space, but rather galaxies in the universe are distributed in a great cosmic web of filaments. The locations where the filaments meet are dense clusters of galaxies, that began as the small fluctuations to the universe. Hence the distribution of galaxies is closely related to the physics of the early universe. Despite its many successes, this picture is not sufficient toexplain the variety of structure we see in galaxies. Galaxies come in a variety of shapes, from round featureless elliptical galaxies to the pancake-flat spiral galaxies.
2. How long after the Big Bang did galaxies begin to form?
The best available measurements as of 2011 suggest that the initial conditions occurred about 13.7 billion years ago. It is convenient to divide the evolution of theuniverse since then into three phases. The very early universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth.
The instant in which the universe is thought to have begun rapidly expanding from an extremely high energy density is known as the Big Bang.
Following this period, in the early universe, the evolution of the universeproceeded in accordance with the tenets of high-energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Matter then continued to aggregate into the first stars and ultimately galaxies, quasars, clusters of galaxies and superclusters formed. There are several...
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