What are Active Galactic Nuclei?

Luckily nowadays you have plenty of resources, starting from Wikipedia, that allow you to obtain information in the field of Active Galaxies. So much has been written about this topic certainly better than I could do. I just want to highlight some issues that can be useful for amateur observers during the observation of these objects.

• Active Galactic Nuclei (AGN) are the central regions of galaxies that have an unusually higher luminosity than the nucleus of a normal galaxy. In most cases, the luminosity exceeds by several times the luminosity of the entire host galaxy.

• This means that, even if we are observing a galaxy, the images in our telescope have a point-like appearance, exactly like stars. Only in some cases of nearby and fainter active galaxies we can appreciate the galactic structure.

• Active Galactic nuclei are among the most energetic sources in the universe. Unfortunately, however, they are also very distant objects and their apparent magnitude, no brighter than 10th, gets more complicated the observation with the amateur instruments.

• AGNs emit across the entire electromagnetic spectrum, from radio waves to gamma rays, and their spectral energy distribution (SED) is very different from that of a normal galaxy. Therefore, even from a spectroscopic point of view, they present interesting peculiarities that can be recorded with an amateur equipment.

• More or less all AGN are objects that present a rapid and marked variability at all wavelengths. The study of this variability in the optical bands represents an exciting field of research for an amateur astronomer. A good photometry is not only a source of personal satisfaction but can lead to collaboration with professional astronomers.