Active Galactic Nuclei (AGN) is the highly luminous and compact central part of galaxies that can emit radiations in the whole electromagnetic spectrum. The luminosity of nuclei can overcome total luminosity, coming from all the stars in the host galaxy. So they are also one of the brightest objects in universe. Unlike stars, the origin of high luminosity in AGN is mainly by non-thermal emission processes (The characteristics of the emitted radiation do not depend on the temperature of the source, eg: synchrotron emission, inverse Compton effect, etc).
AGN is powered by mass accretion onto a supermassive Black Hole (BH) with mass in the range of 106-1010 solar mass. The supermassive black hole is surrounded by a gaseous accretion disk with a size of few light days. At a distance of 100 light days from the centre of the AGN, the broad-line region exists with fast-moving gas clouds. In some AGN spectra, broad emission lines are seen which emerges from the broad-line region. At 100 light-years in diameter, an optically thick molecular torus of colder gas exists by surrounding broad line region. The narrow line region exists at a distance of 1000 light-years, consists of small, low-density gas clouds moving at lower velocities than the broad-line region. Narrow emission lines are seen in some AGN spectra produced by this region. In many AGN, radio emission is produced. It is collimated into jets and propagates in a direction that is perpendicular to the plane of the accretion disc.
The broad range of classification includes Radio Loud and Radio quiet AGNs based on radio emission. The primary observational differences between these two categories are:
- The radio-loud AGN produce radio jets and lobes, with the jets being a significant fraction of the total luminosity.
- The radio-loud AGN are associated with elliptical galaxies while the radio-quiets have spiral hosts.
Radio quiet AGNs include Seyfert galaxies, Quasars and LINERs. Seyfert galaxies (identified by Carl Seyfert, 1943) are lower luminosity AGNs with high surface brightness nuclei. Their host galaxy is clearly detectable. A Seyfert galaxy appears like a normal distant spiral galaxy when they are observed through a large telescope. The two distinct subclasses of Seyfert galaxies are Seyfert 1 and Seyfert 2 based on the presence or absence of broad permitted emission lines. Seyfert 1 galaxies have two sets of emission lines. Set of lines, emerges from low density ionized gas (electron density ne≈ 103 -106 cm−3 ), with width of several hundred kilometers per second, referred to as narrow lines. Also, broad lines of widths up to 104 km s −1 in permitted lines only. The absence of broad forbidden-line emission reveals that broad-line gas has high density. Only narrow lines are present in the case of Seyfert 2 spectra.
Quasars are the most luminous sub-class of AGNs. Their spectra are similar to that of Seyfert galaxies, except that (a) stellar absorption features are weak or absent and (b) narrow lines are weaker compared to broad lines as seen in Seyfets. LINERs (low-ionization nuclear emission-line region galaxies) are very low nuclear-luminosity class of AGNs. Spectroscopically they resemble Seyfert 2 galaxies, except that the low ionization lines, eg: [OI ] λ6300 and [NII ] λλ6548,6583 are relatively strong. LINERs are very common.
Roughly 15-20% of AGN are radio-loud. They are subdivided into Blazars and Radio galaxies. Blazars are known for their high variability, emission of extremely high energy gamma rays and jet emission. They are further classified into BL Lacs and FSRQs (Flat spectrum radio quasars). BL Lac objects have weak emission lines or no emission lines and FSRQs are blazars have strong emission lines. In terms of basic phenomenology radio galaxies can be considered as radio-loud Seyferts, with the difference is that they appear to occur in elliptical galaxies. The two types of radio galaxies are Broad-line radio galaxies (BLRGs) and Narrow-line radio galaxies (NLRGs), the radio-loud analogue of Seyfert 1 and Seyfert 2 galaxies.
The appearance of active galactic nuclei depends so strongly on orientation (Unified Model) that our current classification schemes are dominated by random pointing directions instead of more interesting physical properties.
Read the previous article of the same author- THE ’GLOWING’ DUST STORMS ON MARS
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