Goal: To understand the most energetic stars in the universe, quasars Objectives: 1)To understand Quasars

Quasars Quasars are QUASi-stellAR radio sources They were first observed in the 1950s. They looked like stars on an image. You can tell stars from galaxies because stars appear as a point (and have diffraction patterns if bright) but galaxies are more spread out. However, when they looked at the spectrum, they found something very strange!

These are Quasars (HST)

No known elements! The emission lines matched no known element! Why would this be? A) Quasars are made out of materials unknown in the 1950s. B) They made a mistake with the observations. C) The lines were Doppler shifted by a factor of a few. D) They forgot to take relativity into account.

They are far away! Once it was realized what the emission line difference was they quickly realize that Quasars are very far away! In fact some are on the other side of the observable universe! A red flag should go up now – if they are that far away and we can see them – how bright are they? You have a star which is brighter than any galaxy – how is that possible?

Variable Furthermore Quasars change in brightness! They can do so very quickly (in hours). The shortest a time span an object can fluctuate its brightness is related to its size. The time is the light speed time across the object. So, these are extremely bright objects located in a small area (size of a solar system). How is this possible?

Debate In the 1970s there was a lot of debate over this puzzling matter. Some suggested antimatter (the only way they could think of to convert that much energy). Some suggested that the redshift wasn’t due to distance but due to light coming out of a huge gravitational well. However the gas emitting the light was hot and diffuse (known by the emission lines) – so could not be.

The solution The light doesn’t even come from the “star”. The light is coming from an accretion disk. As gas and dust orbits friction causes the gas and dust to heat up, emit light, and fall slowly down towards the object in the center. If the object in the center is massive you can liberate 10% of the mass energy of the dust and gas as energy (as opposed to 0.7% from fusion).

Not the full story Still, how do we get a region of space the size of a solar system to radiate more light than a galaxy? Also, this light is radiated in all type of light from gamma rays to radio! How is this possible?

Black Holes! Quasars are super massive black holes! Quasars are billions of times the mass of our sun. Oddly enough their density is about the same as our sun. The amount of light they emit is dependant on the rate they accrete matter.

More bizarre news Most quasars are > 5 billion light years away. Very few are < 3 billion. Why do they seem to shut off? Some may have to do with them no longer having gas to eat. Also, as a black hole gets bigger, its density decreases. If the density gets too low it no longer plays with its food, it eats it whole (although black holes are NEVER vacuum cleaners).

Active galaxies The closest to quasars we get in the present is active galaxies. These are galaxies that for some reason have materials falling into the center so that the center is bright. Often times this is a result of a galactic merger. 1% of galaxies are like this.

4.6 billion light years away

The light still travels out from the universe even after the quasar shuts off

Jets

What else we can learn from Quasars Quasars are light bright search lights shining though our universe. They shine not just through space but through time. Since the universe expands, the light from the searchlight expands with time too. This creates what is known as a Lyman-alpha forest (called this because the Hydrogen emission it follows is called the Lyman-alpha line)

How Lyman-alpha forest works Each time the light from the Quasar passes a cloud or galaxy the light from the Quasar is shifted to a different wavelength. The gases in the cloud will emit and absorbed (based on the properties of the cloud or galaxy) at a specific wavelength that is not shifted. So, each cloud adds its fingerprint or signature to the spectrum. This allows us to know what everything is like along the line of sight (and in some way how the gas in the universe has evolved with time).

spectrum

w/o normalization

Conclusion Some of the most distant observable objects are Quasars which are super massive and super bright black holes that are eating large amounts of matter.