ACTIVE GALAXIES

The term "active galaxy" refers to a galaxy that shows a huge internal production of energy, which is usually located in its nuclear region. For this reason, they are also called "active galactic nuclei". Active galaxies are divided in several classes, according to their appearance, to the amount of emitted energy and to their spectrum. The Seyfert galaxies, the quasars and the radiogalaxies are the most important classes.
 

Active galaxies emit an enormous luminosity, hundreds or thousands of times larger than a normal galaxy. This emission cannot be explained as energy produced just by their stars.
The intensity of light emitted in the several spectral bands, and the presence or absence of spectral lines, are different from those of normal galaxies. Active galaxies are often strong sources in "unusual" spectral bands, that is the radio and X bands.
Finally, the luminosity of many of these galaxies varies on very short timescales, a few days or even hours. If stars were the main light source, this evidence could not be explained.
These facts lead to think that there must be another light source, besides the stellar one. This should be a violent mechanism, whose nature is still uncertain, and that acts in a very small region of the galaxy. Most of the light emitted by active galactic nuclei comes from central regions, whose size is a few light-hours or light-days. In order to understand how small is the emitting nucleus, you may think that the distance between the Sun and Pluto is 5.2 light-hours, while the the diameter of a typical galaxy is 100,000 light-years or more! This means that the source of an active nucleus can be as small as the solar system.

The hot gaseous disk in the center of the active galaxy M87. The shape of the disk is a spiral, and it rotates so fast that perhaps it is spun up by a massive black hole  hidden in the nucleus. The mass of this black hole was calculated as 3 billion times that of the Sun. The stars contained in the galaxy are not enough to create the large gravitational field, which is required in order to make the gas rotate so fast. M87 is a giant elliptical galaxy, visible in the Virgo constellation. (HST)

How is all this energy generated? In these years, the active galactic nuclei have been studied in more and more detail using more and more powerful instruments (like radio interferometers), but their nature is still uncertain. Along the years, several theories have been proposed to explain the emission (a strong stellar emission, the explosion of a huge number of supernovae, etc.). All of them have been discarded due to observational evidence. The currently strongest idea is that the central engine of active nuclei is an extremely massive black hole. Its mass should be 10 million to 1 billion that of our Sun, and it should be concentrated in a very small space (the radius of a black hole is equal to about 3 km per solar mass). According to this model, the surrounding matter (stars, gas, dust) forms a thick disk, donut-shaped. The matter feeds the black hole, it falls on it and emits intense radiation. Then it is swallowed by the central "monster", and is converted into electromagnetic radiation at a very high efficiency. This phenomenon is called accretion. In order to obtain the energy emitted by one of these galaxies, 2 10³⁰ Kg of matter are required (which is as much as the solar mass). If the black hole is rotating, it is able to accelerate the nearby gas, and to expel it out from the nucleus, as a couple of jets. The jets would be aligned with the rotation axis. For example, the radio lobes of radiogalaxies could be explained in such a way. Jets are a much common phenomenon in active galactic nuclei.
After the idea of a central black hole was accepted, it was also proposed that the same mechanism could generate the great variety of the active galaxies. According to the so called unified model, if one changes the mass of the black hole, or the morphology of the host galaxy, or the viewing angle, then one obtains a radiogalaxy or a Seyfert one.

M87 is a E1 elliptical galaxy in the Virgo constellation. It is one of the galaxies that have the largest number of globular clusters (several thousands), and it is very bright. It was identified as the strong radio source Virgo A, and it is also very bright in the X band. It is an active galaxy, probably hosting a central black hole. (SEDS)

Seyfert galaxies owe their name to the German astronomer that discovered them, in 1943. They are spiral galaxies, and their nucleus is very bright. They are 100 times brighter than our Galaxy, but the luminosity comes from a very small central region. Seyfert galaxies are 2-3% of all galaxies. They emit mostly in the infrared, and strong emission lines are present in their spectrum. Thus, hot ionized gas must be present in their nucleus. Their emission is anisotropic, that is the intensity changes versus the direction. This is probably due to a dust "donut" surrounding the central engine.
 
 

Another type of active galaxies are the Lacaertides. The name comes from the first galaxy of this type that was observed, BL Lacerti. They have a starlike appearance, but a faint luminous halo can be seen around the nucleus, which reveals the presence of the galaxy. Their optical spectrum is flat. The emitted power does not depend on the frequency, so it cannot be generated by stars. The spectrum shows almost no lines. They are highly variable, of a factor of 100, on timescales of a few days or hours. These peculiarities make them some of the more mysterious objects of the Universe. The commonly accepted explanation is that they are active nuclei observed exactly along the direction of their jets.

The active galaxy Centaurus A in a X ray image. (HEASARC)

Radiogalaxies are strong radio sources, and jets detaching form the nucleus are often observed. The jets extend for hundreds of thousands of light years, and they form radio emitting lobes.
The quasars are the most luminous and distant objects of our Universe, and are one of the most exciting research fields in modern Astrophysics.

Centaurus A and M87 are among the most famous active galaxies.

Plot of the gas speed in the center of the active galaxy M87. The speeds have been measured from the blue or the red shift of its spectrum. At the edge of the disc, the speed is of the order of 550 km/s. It is so large that only a very strong gravitational field in the galactic center can explain it. This must be a black hole. (HST)

Centaurus A is a very bright radiosource. It is a very bright elliptical galaxy, and a dark band of gas and dust can be seen across its surface. This band rotates very fast, and it seems that it is the remnant of a galactic cannibalism between a spiral and and elliptical galaxy. They merged due to the gravitational interaction.
Centaurus A is a very strong emitter in the radio band; the emission comes from two giant lobes, which extend for two and a half light years, perpendicular to the dark band. The galaxy is also emitting energy as X rays, two times larger than that emitted in the radio band. The X emission varies on timescales of a few days. The nucleus is emitting collimated plasma jets, and also gamma rays.
M87 is an elliptical galaxy, whose size is 40,000 light years. It is located in the center of the Virgo cluster of galaxies. The other galaxies of the cluster rotate around it. A jet of matter originates from the M87 nucleus, and it extends for 5,000 light years in optical light. It can be seen out to 8,000 light years in the radio band. The speed of the stars in the galactic nucleus rises steeply towards the center, and this lead to the hypotheses of a central black hole.