Milky Way

Since we are a part of Milky Way, it is difficult to construct an external view. However, through various methods, we have determined the size of our galaxy, and our place in it.

Structure

1. Centre : The centre of the Milky Way lies in the direction of Sagittarius, about 8.5 KPC (kilo parsec) away. In 1915, Shapley attempted to figure out the location of the centre of the galaxy by mapping out the positions of globular clusters.

2. Bulge : The bulge lies in the centre of the galaxy. It is about 6 KPC across, and extends above and below the disc. There is very little dust. Thus the bulge consists mainly of old stars.

3. Disc : Most of the stars in the Milky way are located in a disc about 2 KPC, and about 40 KPC across. The disc is composed primarily of hot, young stars and contains lots of dust and gases.

4. The halo is primarily made up of globular clusters, called stellar halo. It is roughly spherical in shape. A spherical extension of stellar halo is called 'dark matter halo'. Both stellar and dark matter halos are centered on the centre of the galaxy, and is about 30-40 KPC in radius. There is very little dust and gas in the halo. The stellar halo is primarily composed of old stars. Thus, it has even fewer metals than that of the sun. Milky Way is a spiral barred galaxy. Spiral arms are filled with gas and dust, and therefore are regions of intense star formation. There are lots of young, hot (O and B type) stars in the arms of spiral galaxies, making them brighter and bluer than the surrounding regions. Spiral arms are actually density waves. Spiral galaxies are supposed to be supported by rotation. Thus Milky Way possesses an angular momentum.

Formation of the Milky Way

• Galaxies are thought to be formed when gas clouds of higher than average density in the universe collapse under the force of their own gravity.

• After the end of the collapse the clouds settles into a quasi stationary state.

• Evolving star return chemically enriched gas to the interstellar space, where it is mixed with remaining unprocessed gas and star formation continues. Evolution continues at a slower pace. • There are two competing pictures of how galaxy formation proceeds: monolithical and hierarchical

Monolithic Collapse Model

In the monolithic collapse model, it is assumed that large galaxies form coherently in the collapse of a massive cloud containing the bulk of the material constituting the galaxy.

According to this model, material in the Milky Way had a spherical distribution, and the first generation of stars was born in globular clusters. These globular clusters were left behind in the halo of the Milky Way as matter collapsed into a thin disc. With no new material left in the halo, the only new star formation begins in the disc of the Milky Way. These new stars have higher metallicity, being made from the remnants of earlier mass stars. Recent observation shows:

1. The sun has metal absorption lines. This consisted with the monolithic model. 2. Not all globular clusters have the same age. This fact contradicts monolithic collapse model, as it is expected that all globular cluster stars would only have formed before the collapse of material in the current disc of the Milky Way. 3. The presence of dark matter in the halo. This result doesn't explicitly contradict the monolithic collapse model. The rapid collapse of the stellar halo, followed by a more gradual build up of the disc fits with the picture.

The monolithic collapse model is proposed by Eggen, Bell –Lynden and Sandage in the year 1962 also known as ELS model. According to this model, a galaxy forms from a single gas cloud that collapse, fragment and form stars.

Hierarchy Model

According to this model, most of the stars form in much smaller clouds, which later agglomerate together to form the galaxies we now observe.

During 1970s and 1980s, it is realized that the Big bang would grow through gravity and merge to form larger mass objects. Our understanding is that there are many more low mass fluctuations than large mass fluctuations of 10^6-10^8 solar masses were much more common than those of 10^12 solar masses.

According to this model, it is considered that the Milky Way grew from fragments of mass 10^6 to 10^8 solar masses. Initially these fragments were in isolation, forming stars and globular clusters. They have their own metal enrichment and star formation histories. Then they merge.

In this model, the inner regions of the growing Spherion where the density of matter was greater, evolution would be fastest. This produces things like metal –rich, old bulges. Collisions and tidal interactions between merging fragments would disrupt some globular clusters and left some intact. In this model, the disrupted systems would have led to the present distribution of field halo stars, while leaving intact globular clusters distributed throughout the spheroid. Through many random mergers, there should be no net rotation of object in the halo.

Model predicts that some proto-galactic fragments should still be out there. This explains significant number of small galaxies orbiting the Milky Way and nearby Andromeda. These are surviving proto-galactic fragments. Some are merging, like the Sagittarius dwarf.

This model claims that the elliptical galaxies can form from gas rich spiral galaxy mergers.

Galaxy Merger

The galaxy mergers can occur when two or more galaxies collide. They are the most violent type of galaxy interaction. Galaxy merger do not involve stars or the system of stars in actual collision due to the vast distance between the stars in the most circumstances. The gravitational interaction between galaxies and the friction between the gas and the dust have the major effects on the galaxies involved. The exact effect of such mergers depends on a wide variety of parameters such as collision angles, speeds, relative size, and composition. This domain is an extremely active area of research.There is some generally accepted results, as follows:

1. When one of the galaxy is significantly larger than the other, the larger will often 'eat ' the smaller, absorbing most of its gas and stars with little other major effects on the larger galaxy. Milky Way is thought to be currently absorbing smaller galaxies, such as Canis Major Dwarf Galaxy, and possibly the Magellanic clouds. The 'Virgo Stellar stream' is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.

2. If two spiral galaxies that are approximately the same size collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanism and often include a stage in which there are 'active galactic nuclei' . This is thought to be the driving force behind many quasars. The final result is an elliptical galaxy.

It is expected that the Milky Way and Andromeda galaxy will probably collide in about 4.5 billion years. If these galaxies merged, the result would be an elliptical galaxy.

Galaxy and general feature and evolution

A galaxy is a gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component called dark matter. A galaxy typically contains 10^7 to 10^11 stars, orbiting around the centre of gravity of the galaxy.

There are different physical structures of the galaxies and are broadly categorized into three, i.e, elliptical, spiral and irregular galaxies. Known properties of these three different structures are given below: Galaxy Type Spiral & barred spiral Elliptical Irregular Mass(solar mass) 10^9-10^11 10^5-10^13 10^8-10^10 Luminosity 10^8-10^10 10^5-10^11 10^7-10^9 Diameter 5-250 1-205 1-10 Stellar Population Halo:Population II Population II Population I Disk:Population I Relative Abundances 77% 20% 3%

The density of matter may be very different in different galaxies and in different part of the same galaxy. Thus, evolution of a galaxy will be the result of a processes occurring on vastly different time and energy scales. No generally accepted comprehensive 'theory of evolution' exists as yet.

- It is estimated that there could be 10^11 galaxies in the observable Universe. - The space between galaxies is known as intergalactic space, is usually filled with tenuous plasma having an average density of less than 1 atom per cubic meter. - Spiral galaxies (dark galaxies) are supported by rotation whereas ellipticalls are supported by random velocity dispersion. - The most massive galaxies in the sky are giant elliptical galaxies. The main driving force for the evolution of elliptical galaxies is mergers of smaller galaxies. These mergers can be extremely violent galaxies often collide at speeds of 50m/s. - During a merger, a typical galaxy can make thousands of solar masses of new stars each year, which is large compared to our galaxy which makes around 10 new stars each year. - The majority of giant galaxies contain super massive black hole in their centers, ranging in mass from millions to billions of times the mass of our sun. - The majority of mass in galaxies is made up of dark matter, a substance that interacts by means of gravity. These substances do not interact through electromagnetic radiation.

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