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.

Fundamental questions in astrophysisc

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

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

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 was formed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, hierarchically forming superclusters, clusters, galaxies, 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.

The observed components of galaxies (including our own Milky Way) that must be explained in, or at least not be 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).

On the 11th July 2007, using the 10 metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasedena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old [1].

Recent research as a part of the Galactic Zoo project suggests that there is an unexplained parity violation, with a greater proportion of the galaxies rotating in an anticlockwise direction when seen from the Earth[2].

Subcategories of Galactic Astronomy

A standard set of subcategories is used by astronomical journals to split up the subject of Galactic Astronomy:
1. abundances - the study of the location of elements heavier than helium
2. bulge - the study of the bulge around the center of the Milky Way
3. center - the study of the central region of the Milky Way
4. disk - the study of the Milky Way disk (the plane upon which most galactic objects are aligned)
5. evolution - the evolution of the Milky Way
6. formation - the formation of the Milky Way
7. fundamental parameters - the fundamental parameters of the Milky Way (mass, size etc)
8. globular clusters - globular clusters within the Milky Way
9. halo - the large halo around the Milky Way
10. kinematics and dynamics - the motions of stars and clusters
11. nucleus - the region around the black hole at the center of the Milky Way (Sagittarius A*)
12. open clusters and associations - open clusters and associations of stars
13. solar neighbourhood - nearby stars
14. stellar content - numbers and types of stars in the Milky Way
15. structure - the structure (spiral arms etc)