Galaxy formation and evolution
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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.
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 is that all the structure we observe today was formed as a consequence of the growth 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 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).
Spiral galaxies
Spiral galaxies 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, gravity 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 the event would disrupt the delicate structure of the disk. 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, after the initials of the authors of that paper, Olin Eggen, Donald Lynden-Bell, and Allan Sandage) describes a single (relatively) rapid monolithic collapse, with the halo forming first, followed by the disk. Another view published in 1978 (known as SZ for its authors, Leonard Searle and Robert Zinn) 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 satellite 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 (the Andromeda Galaxy) 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.
Further examples of satellite dwarf galaxies that are in the process of merging with the Milky Way are the Canis Major Dwarf Galaxy, discovered in 2003 and thought to be responsible for the Monoceros Ring, and the Virgo Stellar Stream, discovered in 2005.
Elliptical galaxies
Giant elliptical galaxies are probably formed by mergers on a grander scale. In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currently approaching each other at high speed. Since we cannot determine the speed of M31 perpendicular to the line from us to it, we do not know if it will collide with the Milky Way. If the two galaxies do meet they will pass through each other, with gravity distorting both galaxies severely and ejecting some gas, dust and stars into intergalactic space. They will travel apart, slow down, and then again be drawn towards each other, and again collide. Eventually both galaxies will have merged completely, streams of gas and dust will be flying through the space near the newly formed giant elliptical galaxy. Out of the gas ejected from the merger, new globular clusters and maybe even new dwarf galaxies may form and become the halo of the elliptical. The globulars from both M31 and the Milky Way will also form part of the halo; globulars are so tightly held together that they are largely immune to large scale galactic interactions. On the stellar scale, little will happen. If anybody is around to watch the merger, it will be a slow, but magnificent event, with the sight of a distorted M31 spectacularly spanning the entire sky. M31 is actually already distorted: the edges are warped. This is probably because of interactions with its own galactic companions, as well as possible mergers with dwarf spheroidal galaxies in the recent past - the remnants of which are still visible in the disk populations.
In our epoch, large concentrations of galaxies ( clusters and superclusters) are still assembling. This "bottom-up" picture is referred to as hierarchical structure formation (similar to the SZ picture of galaxy formation, on a larger scale).
While we have learned a great deal about ours and other galaxies, the most fundamental questions about formation and evolution remain only tentatively answered.