Active Galactic Nuclei  – by Albert Lim (2000)

The Universe contains countless number of galaxies and most are normal. A small number however have very bright nuclei - so bright in fact that the nuclei brightness actually outshines the billions of stars within the galaxy. These are known as Active Galactic Nuclei or AGNs for short. AGNs emit huge amounts of radiation across the electromagnetic bandwidth from radio, through optical and X-rays to Gamma-rays as well as high speed particles - they can therefore be studied at all wavelengths. They are also known to be extremely variable with brightness changes on the order of 10 billion times or more the brightness of our Sun within an hour ! Because their brightness change is so dramatic, astronomers know that the AGNs must be small by galactic terms - perhaps one to two times the size of the Earth’s orbit. That means the energies within these small regions must be truly enormous - making AGNs one of the most powerful objects in the Universe. Astronomers currently believe that the most probable candidates of AGNs are black holes. Quasars are thought to be distant AGNs. While radio galaxies, seyferts, quasars, blazars are all AGNs, about 10% of them are observed with radio jets. Astronomers now think that many of the apparent differences between AGN types are due to our different viewing orientation with respect to the AGN’s disk.

An example of an AGN is Markarian 421 located in the constellation of Ursa Major at RA 11h 04m 25.6s and Declination +38d 12m 47sbased on 2000 co-ordinates.
Markarian 421 (Mkn 421) is a BL Lacertae object and is one of the few AGNs which have been measured across a wide range of wavelengths, with numerous observations obviously conducted from space. The montage below shows Mkn 421 from radio to extreme gamma energies. The broad band spectrum shown is distinctly separate into 2 parts. Both parts are believe to be related to each other through the interaction of radiation with high energy particles either close to a central black hole or in a relativistically moving jet of particles along our line of sight. The curves on the graph represent measurements of the spectra at bright and faint levels.

Fig 10  :   Broad band spectrum of AGN -  Makarian  421 and their respective images. From top left  1) VLA  - Radio map showing region less than 5 minutes square from Very Large Array at 20cm wavelength taken from the FIRST sky-survey project. 2) HST -  Optical image 35 arc minutes field showing surrounding host galaxy UGC6132 taken with Hubble’s Planetary CCD Camera. 3) UIT - Ultra violet photo taken at 1,500 Angstrom wavelength and exposure of 1430 seconds through the Shuttle-based Ultraviolet Imaging Telescope during Astro-2 mission. 4) EUVE - 1275 seconds exposure in the range of 44 to 240 Angstroms covering 1 degree square from all-sky survey of Extreme Ultraviolet Explorer. 5) ROSAT - X-ray image covering 40 arcminutes square from ROSAT’s Position Sensitive Proportional Counter (PSPC) over 14 hours of observation time in May 1992. 6) EGRET - EGRET image from the Compton GammaRay Observatory covering 10 degrees at 1 degree resolution. 7) Cerenkov - from Mt. Hopkins atmospheric pair telescope - J. Buckley, Washington University