Home > Galaxy formation and evolution
In astrophysics, the questions of galaxy formation and evolution are:- 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.
A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space Telescope true-color image of the Cartwheel Galaxy .
After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Background Radiation, the fluctuations of which are less than one part in one hundred thousand.
The most accepted view is that all the structure we observe today was formed as a consequence of the growing of primordial fluctuations by gravitational instability .
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.
A great deal of the research in this area is focused on components of our
own Milky Way, since it is the easiest galaxy to observe.
The observations which must be explained in, or at least not at odds with, a
theory of galactic evolution, include:
- 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 inclinedInclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planet's equator or the ecliptic), stated in degrees. 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 Cloud s, clouds of neutralBeing neutral means that something is balanced between two or more opposites. The term has many uses in science and otherwise. In physics, a neutral particle or object has zero electric charge. An atom with a positive or negative charge is called an ion. hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca 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
1 Spiral galaxies
Spiral galaxiesA spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. It contains about 100 billion stars, 30 million light-years away toward t cannot be built up by mergers of already existing smaller galaxies. When galaxies collide, the individual stars barely notice. The stars themselves never collide with each other because of the enormous distances between them, compared to their size. So when galaxies collide, they actually simply pass through each other, but the gravitational effects disrupts their structure as this happens. As they separate, gravityThis article covers the physics of gravitation. See also gravity (disambiguation). Gravitation is the tendency of masses to move toward each other. The first mathematical formulation of the theory of gravitation was made by Sir Isaac Newton and proved ast slows them down and, if they are gravitationally bound, will eventually bring them back together for another collision. After several collisions their individual structures are so changed, with many stars mixed up between them, that we identify the result as a single merged object. So after a merger, most of the stars originally belonging to both galaxies remain to form the new merged galaxy (a small fraction will have been thrown out entirely). If either galaxy were a spiral before the merger, the violence of event would disrupt the delicate structure of the diskIn geometry, a disk is an n dimensional region, in Euclidean space, bounded by an n minus;1)-dimensional hypersphere. The everyday usage corresponds to n 2, namely the inside of a circle in the plane A representative disk is three- dimensional volume elem. The existing stars cannot afterwards change their orbits to form a new disk. The stellar disk must essentially form in place; a dense rotating disk of gas forms first, then stars are born inside it.
The earliest modern theory of the formation of our galaxy (known by astronomers as ELS, the initials of the authors of that paper) describes a single (relatively) rapid monolithic collapse, with the halo forming first, followed by the disk.
Another view published some years later (known as SZ) describes a more gradual process, with smaller units collapsing first, then later merging to form the larger components.
An even more recent idea is that significant portions of the stellar halo could be stellar debris from destroyed dwarf galaxies and globular clusters that once orbited the Milky Way. The halo would then be a "new"er component made of "recycled" old parts!
In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies. Rapid technological progress in computers have allowed much better simulations of galaxies, and improved observational technologies have provided much more data about distant galaxies undergoing
merger events. After the discovery in 1994 that our own Milky Way has a satelliteFor other uses, please see Satellite (disambiguation A satellite is an object that orbits another object (known as its primary . The term is often used to describe an artificial satellite (as opposed to natural satellites, or moons). Because all objects e galaxy (the Sagittarius Dwarf Elliptical Galaxy, or SagDEG) which is currently gradually being ripped up and "eaten" by the Milky Way, it is thought these kinds of events may be quite common in the evolution of large galaxies. The Magellanic Clouds are satellite galaxies of the Milky Way that will almost certainly share the same fate as the SagDEG. A merger with a fairly large satellite galaxy could explain why M31 appears to have a double core.
The SagDEG is orbiting our galaxy at almost a right angle to the disk. It is currently passing through the disk; stars are being stripped off of it with each pass and joining the halo of our galaxy. Eventually, only the core of SagDEG will exist. Although it will have the same mass as a large globular cluster like Omega Centauri and G1, it will appear rather different, as it has far lower surface density due to the presence of substantial amounts of dark matter, while globular clusters appear, mysteriously, to contain very little dark matter.
