Quasars

Quasars are one of the most fascinating energetic sources. They constitute a class of active galactic nuclei (AGNs). These cosmic power banks are cosmological in nature, i.e, they are objects in the distant universe (redshift z>1). A defining characteristic of quasars is that they represent a class of AGNs whose emission jet’s axis do not point towards us (i.e, Earth).

Quasars are powered by supermassive black holes ranging in mass from millions to tens of billions and are surrounded by gaseous accretion disks. In a layman’s language, quasars are rotating black holes but their actual constitution is more intricate than what is captured by this simple definition. They appear like super bright spots in the sky. These immensely luminous structures owe their “dazzle” to the gas falling in the accretion disc of the black hole heating up and releasing energy in the form of Electromagnetic radiation.

The discovery of quasars, like many scientific phenomena, was accidental. Scientists, in the 1950s, detected radio wave emissions of some unknown physical source. On analyzing photographic images at visible wavelengths, they resembled faint, star-like points of light. In fact, the term quasar is a contraction of the term ‘quasi-stellar radio source’.

Quasars are located in the centers of galaxies. Most of these host galaxies are merging or strongly interacting galaxies. The observed properties of these AGNs depend on a range of factors like the mass of the black hole, the rate of gas accretion, the orientation of the accretion disc relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy. These observations are made based on high resolution images taken from telescopes like the Hubble space telescope.

The first quasar ever discovered was 3C 273, located in the constellation Virgo. It was identified in 1963 by astronomer Maarten Schmidt.

Classification

Quasars are often classified based on their optical spectra, which can reveal information about their composition, redshift, and distance. The types are:

1. Radio Loud: Quasars with very powerful jets that are strong sources of radio wavelength emission. About 10% of the quasar population falls into this category.

2. Radio Quiet: These are the relatively weaker versions of the radio loud quasars in the sense that they lack powerful jets. Majority of the quasars,i.e, about 90%, falls in this category.