Previous Paper Questions:

1. How do you differentiate a Nebula from a Galaxy?

2. What are the distinguishing features of a galaxy and active galaxy?

3. Distinguish between radio quiet and radio loud galaxies.

4. Describe the origin and evolution of galaxies. Discuss the violent activities in galaxies.

5. How will you define and differentiate the Quasars and microquasars?

GALAXIES AND THEIR EVOLUTION AND ORIGIN

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million (10⁸) stars to giants with one hundred trillion (10¹⁴) stars, each orbiting its galaxy's center of mass.

Galaxies are categorized as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers.

All galaxies began forming at about the same time approximately 13 billion years ago. Models for galaxy formation have been of two basic types: "top-down" and "bottom-up".

TOP-DOWN MODEL

The "top-down" model on the origin of the galaxies says that they formed from huge gas clouds larger than the resulting galaxy. The clouds began collapsing because their internal gravity was strong enough to overcome the pressure in the cloud. If the gas cloud was slowly rotating, then the collapsing gas cloud formed most of its stars before the cloud could flatten into a disk. The result was an elliptical galaxy. If the gas cloud was rotating faster, then the collapsing gas cloud formed a disk before most of the stars were made. The result was a spiral galaxy.

A more recent variation of the "top-down" model says that there were extremely large gas clouds that fragmented into smaller clouds. Each of the smaller clouds then formed a galaxy. This explains why galaxies are grouped in clusters and even clusters of galaxy clusters (superclusters). However, the model predicts a very long time for the collapse of the super-large clouds and fragmentation into individual galaxy clouds. There should still be galaxies forming today.

Figure : Top-Down Model

BOTTOM-UP MODEL

The "bottom-up" model builds galaxies from the merging of smaller clumps about the size of a million solar masses (the sizes of the globular clusters). These clumps would have been able to start collapsing when the universe was still very young. Then galaxies would be drawn into clusters and clusters into superclusters by their mutual gravity. This model predicts that there should be many more small galaxies than large galaxies---that is observed to be true. The dwarf irregular galaxies may be from cloud fragments that did not get incorporated into larger galaxies. Also, the galaxy clusters and superclusters should still be in the process of forming---observations suggest this to be true, as well.

Figure : Bottom-Up Model

Other types of galaxies

Active galaxy

A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN). A significant portion of the total energy output from the galaxy is emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. Seyfert galaxies or quasars, are classified depending on the luminosity, are active galaxies that emit high-energy radiation in the form of x-rays.

Seyfert galaxy

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses but unlike quasars, their host galaxies are clearly detectable. Seyfert galaxies account for about 10% of all galaxies. Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, the luminosity of their cores is equivalent to the luminosity of whole galaxies the size of the Milky Way.

Quasar

Quasars or quasi-stellar radio sources are the most energetic and distant members of active galactic nuclei. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies. Their luminosity can be 100 times greater than that of the Milky Way.

The Difference between Radio-Loud and Radio-Quiet Active Galaxies

The recent development of unified theories of active galactic nuclei (AGN) has indicated that there are two physically distinct classes of these objects - radio-loud and radio-quiet. The primary observational distinctions between the two types are: (1) The radio-loud objects produce large scale radio jets and lobes, with the kinetic power of the jets being a significant fraction of the total bolometric luminosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects are associated with elliptical galaxies which have undergone recent mergers, while the radio-quiets prefer spiral hosts. (3) The space density of the radio-louds at a given optical luminosity is ≈ 10 times lower than that of the radio-quiets. Despite these differences, the (probably) thermal emissions from the AGN (continua and lines from X-ray to infrared wavelengths) are quite similar in the two classes of object.

What is a microquasar.

A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star. The matter being pulled from the companion star forms an accretion disk around the black hole. This accretion disk may become so hot, due to friction, that it begins to emit X-rays. The disk also projects narrow streams or "jets" of subatomic particles at near-light speed, generating a strong radio wave emission.