What is a Quasar?
A quasar is not an object as such, but the evidence of one: a supermassive black hole, in the centre of a galaxy. They are a subset of Active Galactic Nuclei (AGNs) and are the most luminous (brighter than 100 billion Suns) and most massive (heavier than 100 million Suns) of them all. They are also quite uncommon, since they take a while to “warm up” and have a short life, finishing once the sufficient fuel, dust and gas for a quasar to occur has been used up on creating such bright events.
Any AGN smaller than 100 million Suns (1 million to 100 million) is then dimmer than 100 billion Suns (1 billion to 100 billion), and is known as a Seyfert galaxy: much more common than their quasar counterparts, comparable to how stars get less common and last for less time the brighter they are (known as “cosmic downsizing”).
This brighter trait of quasars means that they can be seen from a larger distance away, and being less common, appear to exist in the older (further away) universe, with the number of AGNs increasing from 13 billion years ago, before decreasing in density at around 10.5 billion years ago .
Then, after quasars come blazars: AGNs that are pointed directly at Earth. These are much brighter than quasars, since sometimes the light, emitted in jets from AGNs, remains concentrated for hundreds of thousands of light years, whilst usually moving at 99% the speed of light .
How to Find a Quasar
Even in amateur telescopes, the brightest quasars are visible, looking like a star, whilst the host galaxy of the AGN is invisible (click here for a guide on how to observe them). However, some are out of the optical range, since they are too near or far from Earth. The distance changes the colour (or wavelength) of the light due to how wavelength increases with distance, creating a redshift (i.e. the light is redder and so less energetic than bluer light) .
When trying to locate them with large telescopes, professional astronomers use a survey technique, looking across the sky, specifically at the universe when it was less than 3 billion years old, at the place when most quasars are: redshift z=3, being the amount of redshift something has (the greater the number, the greater the redshift), corresponding to how far away from Earth it is (greater redshift means its further away), known as “Hubble’s Law”.
Examples of surveys to do this include the Sloan Digital Sky Survey (SDSS), with its survey to find quasars among other things, called the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). It mapped the sky zone where quasars exist (outlined earlier), finding 500,000 of them when it observed 6,000 square degrees of sky (just over 14.5% of the sky ), showing how common they are at that age of the universe .
The Oldest Quasar Ever Discovered
Discovered at a distance where the light observed was made around 670 million years (5% of its current age ) after the Big Bang , the catchy named quasar "J0313-1806" was discovered, at a brightness 1,000 times that of our own galaxy and a mass 1.6 billion times that of our Sun, using the equivalent of 25 Suns per year (compared the the Milky Way’s rate of the equivalent of 1 Sun per year ) , making the gas flowing out at 20% the speed of light. In fact, this quasar is so old that the black hole it originates from didn’t form from the collapse of a large star, as the common stellar mass black hole do (found nearer to Earth), but by cold hydrogen gas from the early universe collapsing in on itself to form a black hole .
This discovery was only 20 million light years further than the last record for the oldest quasar, but the supermassive black hole it originate from is double the mass .
What Quasars Tell Us
They help us understand the early universe, being some of the only objects that can be detected by telescopes on Earth that far back in the past. They show us how black holes were made in the early universe. As well as this, it can tell us about what is between us and the quasar being observed, showing what the temperatures and compositions of gasses in the early universe were, by studying the spectrograms of light coming from quasars, having passed through all this material .
This means that, as we find more and more quasars going further and further back in time (since they're further and further away in space, and light takes time to travel), we can build a picture of the early universe, and what and how things formed in that unique era in our universe’s history.
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Click here to see an animation by ESO (European Southern Observatory) of one of the most distant quasars in our universe: ULAS J1120+0641
Click here to see the paper on the discovery of J0313-1806
Click here to look at a quasar, 3C 273, in a giant elliptical galaxy in the constellation Virgo (the first quasar to be discovered, discovered in 1960 by Allan Sandage) on ESA's ESASky
“Most distant quasar discovered sheds light on how black holes grow”. Phys.org. Archived from the original on 17th January 2021.