Galaxy formation and evolution

In astrophysics, the questions of galaxy formation and evolution are:

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• How, from a homogeneous universe, did we obtain the very inhomogeneous one we live in?
• How did galaxies form?
• How do galaxies change over time?

The formation of galaxies is still one of the most active research areas in astrophysics; and, to some extent, this is also true for galaxy evolution. Some ideas, however, are now widely accepted.

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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.

Related Topics:
Big Bang - Universe - Homogeneous - Cosmic Microwave Background

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The most accepted view is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations by gravitational instability.

Related Topics:
Primordial fluctuations - Gravitational instability

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Recent data strongly suggests that the first galaxies formed as early as 600 million years after the Big Bang, much earlier than astronomers had previously believed. That leaves hardly enough time for the tiny primordial instabilities to grow sufficiently forming protogalaxies into galaxies.

Related Topics:
Primordial - Protogalaxies - Galaxies

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A great deal of the research in this area is focused on components of our

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own Milky Way, since it is the easiest galaxy to observe.

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The observations which must be explained in, or at least not at odds with, a

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theory of galactic evolution, include:

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• the stellar disk is quite thin, dense, and rotates
• the stellar halo is very large, sparse, and does not rotate (or has perhaps even a slight retrograde rotation), with no apparent substructure
• halo stars are typically much older and have much lower metallicities than disk stars (there is a correlation, but there is no absolute connection between these data)
• some astronomers have identified an intermediate population of stars, variously called the "metal weak thick disk", the "intermediate population II", et al. If these are indeed a distinct population, they would be described as metal-poor (but not as poor as the halo stars), old (but not as old as the halo stars), and orbiting very near the disk, in a sort of "puffed-up", thicker disk shape.
• globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods)!
• in each globular cluster, all the stars were born at virtually the same time (except for a few globulars that show multiple epochs of star formation)
• globular clusters with smaller orbits (closer to the galactic center) have orbits which are somewhat flatter (less inclined to the disk), and less eccentric (more circular), while those further out have orbits in all inclinations, and tend to be more eccentric.
• High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).
• and many more interesting data
 
Galaxy formation and evolution: Spiral galaxies ►