Active Galactic Nuclei (AGN)


The galaxies showing phenomena in their central region that cannot be explained by stellar processes are called active galaxies. Approximately 10% of the nearby galaxies host an active nucleus. The main observational characteristics of AGN are the following: 

1) Bright stellar-like nucleus that emits strong, variable continuum emission (IR, UV, X-ray, sometimes radio). The continuum emission cannot be produced from stars because the energy spectrum in the optical band has the “wrong” shape, increasing towards higher frequencies.

2) We detect strong emission lines from the nucleus in both the optical and UV bands. Sometimes the lines are very broad, suggesting that the gas which emits the line emission can have velocities up to ~104 km/sec.

3) About ~10% of AGN show double radio “lobes” on either side of the galaxy. In some cases “jets” pointing towards the giant lobes are detected in radio, optical and X-ray images. These jets appear to move faster than light!

 

agn_picture.jpg (96976 bytes)

There are many types of AGN. A schematic diagram of their central region is shown on the left (not in actual scale). The various AGN types can be explained by differences in the viewing angle, in the amount of energy that they emit and by the presence or absence of a jet. Today, we believe that their central engine consists of a super massive black hole (with a mass ~106 – 109 times larger than the Solar mass) and a gaseous disk which supplies the black hole with material (with a rate of ~ 0.02 – 20 Solar masses per year).

 

The dynamical energy released as the material falls into the black hole is capable to account for the tremendous energy that is emitted from the active nucleus (~ 1010-1013 times larger than the Solar luminosity). However, there are still many details of the extreme physical mechanisms that operate in the central region of these objects that cannot be understood yet.

 

A list of ongoing projects that are based on Skinakas observations of AGNs are:

 

- Variability of the continuum emission is a defining property of AGN. A major observational project at Skinakas observatory involved the monitoring of 7 “radio-quiet” AGN (i.e. objects which do not have a jet and do not emit strong radio emission) in various optical bands on a daily basis for three weeks. Observatories in China and Japan participated in this project as well. The main objective is to study in detail the optical variations in order to understand their physical origin.

 

- “BL-Lacs” is a particular sub-class of AGN. These objects host a jet of electrons that move at a speed close to the speed of light. They show extreme variability (i.e. large amplitude variations on short time scales) at all wavebands. A large number of BL-Lac objects are currently observed with the 1.3 m telescope at Skinakas Observatory. These observations involve intensive monitoring of an object for a few nights each time in order to study in detail the optical variations on time-scales of ~ hours (“intra-night variations”). These observations will help us to study the extreme physical processes that operate in the relativistic jets of these objects.  

 

- AGN are strong X-ray emitters. The last few years, American/European satellites like ROSAT, Chandra and XMM have detected a large number of sources which emit strongly in the X-ray band). Most of them are probably distant AGN, however, their identify cannot be revealed until we obtain optical observations of these sources. A number of sky fields with unknown X-ray sources are currently being observed at Skinakas Observatory. These observations will help us identify these enigmatic sources and perhaps discover a whole new population of hidden AGN.