Quasars

We used to believe that the Universe was a quiet place, slowly settling and without many powerful activities taking place. Our belief was rapidly being smashed into pieces as advances in technology with state-of-the-art instruments opened our eyes to the real monstrous Universe. Not long after seeing a glimpse of the vastness of the Cosmos, realization came that nothing has settled yet; in fact we live in a very active and busy Universe. Tremendous effort was set forth to try to understand all of this and yet there are so many things which we still cannot comprehend. Enthrilling objects like massive black holes, powerful active galaxies, overwhelming explosions and immense radiant sources are not uncommon.

Quasars are amongst the most energetic objects in the heavens. When astronomers first noticed them early in the twentieth century at radio frequencies, they suspected them to be some peculiar nearby stars. Later on it was discovered that these strange objects were actually a lot farther away and not stars at all. Quasars get their name from the first observation as being star-like objects, hence they where called quasi-stellar radio sources, in short Quasars. These structures are in fact very active galaxies. Not all QSOs are observed in radio frequencies though, actually there are now more observed in optical and other frequency regimes.

At the cores of these extravagant sources it is believed that there must lie super massive black holes, that are surrounded by a spinning disks of material which are drawn relentlessly into the its gravitational maw. At two opposite outer edges of the core, highly collimated streams of particles can be hurled into space with enormous speeds very close to the speed of light. Optical QSOs do not manifest this feature. How these so-called 'Jets' form, what their properties are and why there are knots within them, no-one can fully answer. Twisting and wrapping of strong magnetic-field lines in the spinning disk cause ions to be accelerated. At the poles, these particles get incredibly collimated by the tightly confined field-lines and shoot outwards at very high speeds. This is the best answer for the streaming radio wave emanating particles coming from the poles which we call jets.

In most cases only on one side a jet has been observed though. The reason for that is because the jet approaching the Earth is relativistically 'beamed' (Pearson and Zensus, 1987). The radio emission received from the jet at one side is always much brighter than that from the other side. This asymmetry is a consequence of flow speeds approaching that of light. A flow moving towards us at almost the speed of light catches up with its own radiation, which thereby appears boosted in intensity. In contrast, the radiation from a flow moving away from us is correspondingly dimmed, making a fainter "counterjet". Radio emission coming from a Quasar and its jets is due to synchrotron radiation, generated by accelerating electrons to ultra relativistic speeds through magnetic fields. This non-thermal emission provides a wealth of information about the properties and morphology of a Quasar.

Not all Quasars are emitting strong radio waves. In fact, most have very weak radio emission that deserves the name radio-quiet. Radio 'loudness' is a measure for the radio strength of a source and is usually parameterized by R, the ratio between centimeter to optical flux densities. This parameter is conventionally defined as R $\equiv$ L$_{5 GHz}$/L$_{440 nm}$. Generally, the ratio R = 10 is chosen as the boundary between the two populations. According to this criterion, only a small fraction of objects $\sim$10% are dubbed radio-loud (Kellermann, 1989; Stocke et al., 1992; Kellermann, 1994). Another criterion is to select the boundary at P$_{6cm}$ $\approx$ 10$^{25}$ W Hz$^{-1}$ sr$^{-1}$ (Miller et al., 1990). This also qualifies only 10%-20% of the objects as radio-loud. To be completely objective, it must be noted that there are some studies that are questioning the used distribution where-after Ho and Peng (2001) recalculated the radio-loudness with a different distribution and claim that at least 60% of the sources count as radio-loud.

Just like Quasars, Radio Galaxies (RGs) can be distincted in the same manner, with the main difference that RGs are less luminous compared to Quasars. Perhaps it's better to say that Quasars are the powered-up versions of RGs. In figure (1) you can see an example of a powerful radio-loud RG.

Figure 1: Cygnus-A

6.5cm

On the left side, a false color image of Cygnus-A (3C405.0), the most powerful Radio Galaxy in our part of the universe is shown. At 700 million light years distance, this double-lobed object is one of the brightest radio sources in our sky. It is receding from us at 16811 km sec$^{-1}$ and corresponding to a redshift of 0.05607. Red in the image represents the regions with the brightest radio emission, while blue shows regions of fainter emission.

Quasars are now known to be the most distant and powerful sources of energy in the Universe. Just now scientists are beginning to unravel the mysteries of this object, but still dazzled by the amount of power shooting out from the core. How the knots are created in the jets, which are like lumpy clouds of gas, is still quite puzzling.