How Black Holes power Quasars

With the rise of radio astronomy in the 1950s, astronomers began looking to the skies for any sort of radio waves that they may study. Throughout the 1950s and 1960s they had been receiving hundreds of radio signals with no obvious coordinating counterparts in the visible spectrum that could have emitted such strong and consistent radio signals., and these signals appeared to be stars that were in our very own Milky Way Galaxy.

Contents

Formation Characteristics & types Other AGN After effects Conclusion Bibliography

In 1962, Cyril Hazard, M.B. Mackey and A.J. Shimmins, using a telescope in Australia, were able to pinpoint exactly where these radio sources were coming from. They used occultations of the moon to find the point where it disappeared and then re-emerged. They traced this to an object that appeared to be a star, but was emitting large amounts of radio waves and ultraviolet radiation. This object, 3C 273, is dubbed the first quasar ever found.

Then in 1963 Maarten Schmidt, using the Hale Telescope at Palomar Observatory, in the USA took the spectrum of 3C 273 and noticed that it had a redshift of 16%. This revealed that this star-like object that appeared the same size, distance and shone with as much luminance in the night sky as the stars in our own galaxy was in fact much further away. About 2.3 billion light years away in fact, and the galaxy in which it was residing was moving away from us at 47,000km/s. At the time that was the furthest object that we had ever observed in the night sky. In this article I will be discussing what a quasar is and its different characteristics.

Formation

Quasars are considered active galactic nuclei(AGN). Compact and super luminous objects at the center of a galaxy that are powered by massive black holes and are constantly accumulating matter. Their luminosity is about 10 to 100,000 times greater than that of our Milky Way Galaxies’. The expanded name is quasi-stellar radio source meaning star-like radio source/object, as it appeared to be a star upon its initial discovery. They emit all sorts of radiation from ultraviolet to radio to infrared, etc. Quasars are essentially super massive black holes that grow to such large sizes by feeding on an insane amount of gas. This gas doesn’t just go straight into the black hole, it goes in an orbit around the black hole slowly reaching its center forming a spiral shape around the black hole. This spiral shape is called an accretion disk and it is the one that becomes really hot and emits super bright light. Early on in the universe a lot more galaxies were closer to each other. Quasars can only be formed if an astronomical amount of gas falls into a super massive black hole of a galaxy. Galaxies being closer together allowed for more collisions which in turn fed these supermassive black holes until they became these massive ultra luminous phenomena.

Quasars are so compact that light would take only a couple of days to go from one side to another for the smallest quasars, which is less than 1,000 astronomical units. The biggest might take a couple of light years. An astronomical unit is equivalent to the distance between Earth and the sun, which is about 150 million kilometers in diameter. This may seem like a massive object but compared to the size of its host galaxies it is quite small in comparison. The milky way galaxy for example is 100,000 light years or 946 quadrillion kilometers in diameter. Even though quasars appear small they produce an enormous amount of energy and emit such a radiant light that it reaches seemingly impossible distances.

The earliest known quasars were formed less than a billion years after the big bang and peak quasar activity was said to be around 3 to 4 billion years after the big bang which is now about 10 billion years ago. Quasar activity seems to appear in episodes. One episode lasts around a million years and the total life span of an average quasar would be around 10 to 100 million years. It appears that the more massive the black hole the quicker the lifecycle of the quasar is. After those millions of years of activity, the quasar dies down completely, leaving behind the once dormant black holes.

Characteristics & types

Quasars are the brightest objects in the entire universe, being able to outshine entire galaxies. One quasar can emit energy more powerful than thousands of milky way galaxies combined. When observed quasars exhibit massive redshifts, meaning they are moving from us at a rapid rate. The varying emission lines in their spectra allows for scientists to determine a lot of their physical properties and their surrounding environment.

Two categories of quasars are radio-loud and radio-quiet. Radio-loud quasars emit strong radio waves alongside jets, only around 10% of quasars are known to be radio-loud. A large number of quasars do not emit any radio frequencies unlike previously believed, making them radio-quiet.

Blazers are a category of radio-loud quasars that emit their signals and jets directly towards Earth making it appear extra bright to us.

There are a number of other categories of quasars based on redshift values and the presence or absence of certain emission lines.

Gravitational lensing is when a massive object warps spacetime causing light from a distant source to bend around it, magnifying, distorting or multiplying the images of the background objects. In 1979 the first case of gravitational lensing of quasars was viewed. 2 quasar images, PHL 0957+561 A and B, that were practically identical in physical properties were discovered. This led us to theorizing that the bending of light from intervening galaxies causing more than one path for the light to travel through was a plausibility for multiple other similar quasar systems.

The properties of quasars are extremely similar to those of other active galactic nuclei such as those of seyfert and radio galaxies. Nebulosities which are responsible for reassembling galaxies have been found around low redshift quasars. Using these 2 pieces of information we then know that quasars have a luminosity that over powers the rest of the galaxy’s luminosity, putting it on the most extreme end of active galactic nuclei.

Quasars are capable of forming at the earliest stage of a galaxy formation or after the galaxy has already gotten far into development. Regardless of the time it forms it can cause a variety of things to occur. It can affect the amount of stars that can form within the galaxy and the speed at which the galaxy produces its stars particularly closer to the center of the galaxy as that is where quasars are found. The radiation outflows and jet heat ups that occur as a result of quasars, can and have removed the entire interstellar medium from the host galaxy, leaving it empty and barren.

Other AGN

There are a number of different active galactic nuclei out there. Quasars, as mentioned, are on the extreme end of AGNs. Quasars appear so bright to us because they are releasing jets of light; when these are pointed toward Earth, they are called blazars.. BL Lac, a type of blazer, has very few spectral lines but is shown to have quite a varying amount of luminosity. Seyfert galaxies have their own AGN but are not nearly as active as quasars and do not produce any bright jets. There are 2 types, seyfert type 1 AGN and seyfert type 2 AGN. These 2 are actually the same object that is simply seen at a different angle making it type 1 and 2. Type 1 has both broad and narrow emission lines, because we are looking straight down at the accretion disk. Type 2 on the other hand doesn’t have any strong emission lines as we are looking more at the edge where some of those lines are covered by a thick layer of dust.(Cooper, K.)

Low Ionization Nuclear Emission-line Regions also known as LINERs are found on the opposite end of the spectrum from quasars and blazers as they have very weak AGN activity. Some of these do not even possess an accretion disk, which would mean some aren’t truly AGNs. LINERs are considered some of the least luminous AGNs out there.

After effects

The copious amounts of radiation pouring out from quasars has a lasting and massive effect on its host galaxy. This effect is like that of a solar wind or supernova but many times more powerful. This outflow can trigger massive starbursts by pushing molecular gas clouds together. It can also completely blow away gas from the interstellar medium making it too energetic to collapse to form a star. This can also entirely remove the gas from the galaxy for hundreds of millions of years at a time or possibly forever.

Quasar outflows are also responsible for the clearing out of hundreds or thousands of solar masses of materials and objects in a galaxy annually. The outflows can also have lasting effects on the surrounding area around the host galaxy or other galaxies. High velocity clouds of gas called galactic fountains can be formed by the outflows. It can also have positive effects like the reionization and metal enrichment of the galaxies it touches.

Conclusion

It is believed that almost, or all galaxies once hosted a quasar. All the gas loss due to quasars has impacted the formation of quasars in the current years of our universe. Due to the massive loss of gas and the ever expanding universe keeping galaxies far away, these galaxies no longer collide and so no extra matter can be transferred into these black holes in the first place to even allow for quasars to form once again. The bright quasars that dot our night skies are now far gone and we are simply viewing a magnificent phenomenon that occurred in the earlier days of our universe.