Why Are Active Galaxies So Luminous?
In the 1950s, astronomers observed that some external galaxies are extremely luminous in the radio wavelengths. In some cases, they outshine our Milky Way galaxy by hundreds or thousands. Due to the excessive emission of light, these galaxies were termed as active.
The question arises: why are active galaxies so luminous?
The answer is simple: something in the galactic center is hungry.
Most galaxies are not active. A normal galaxy spectrum is a composite spectrum of all the stars in the galaxy. It shows only the strongest absorption lines with the exception of Hα emission lines which indicate star formation.
In basic terms, this means that all the light from a normal galaxy comes from stars.
In an active galaxy, most of the light comes from the gas in the galactic nucleus. This gave rise to the term active galactic nucleus or AGN.
In the past, I was taught that there are three distinct types of active galaxies. However, after researching, I have discovered that the types are controversial. Check that out on Wikipedia. I will just tell you what I know.
There are three types of active galaxies based on observed properties:
- Seyfert galaxies
- Double-lobed radio sources
- Quasars
All three types are very closely related.
Seyfert Galaxies
These are spiral galaxies with extremely radio and x-ray bright nuclei. They constitute about 2% of spiral galaxies and their nucleus energy emission is about 100 times that of the Milky Way. The nucleus size is about 1 AU which is the distance between the Earth and the Sun.
A Seyfert galaxy spectrum has very strong, broad emission lines. These lines suggest the gas in the nucleus is very hot — about 100 million kelvin. The broad lines suggest the gas is rotating rapidly (10,000 km/s) around a massive object in the core of the galaxy.
…sounds like a black hole.
Double-lobed Radio Sources
These are among the brightest radio sources in the sky.
They contain:
- A central compact active galaxy
- A pair of jets emanating from this galaxy
- Lobes that produce synchrotron radiation
- Hot spots in the lobes caused by the collision of the jets with the intergalactic medium
The scale of double-lobed radio sources can be enormous. Officially, the largest known, J1420−0545, is around 4.5 million parsecs across. Speculatively, the largest observed is 5.8 million parsecs across. To put that into perspective, our Local Galaxy Group that contains around two dozen galaxies is about 3 million parsecs across. Read more about the speculation on the European Southern Observatory’s website.
…black holes are known to correlate with jets.
Quasars
Quasars or Quasi-Stellar Objects (QSO) were first detected as extremely strong radio sources and looked like stars. Some didn’t show up in the visible, so we knew they had to be different (quasi) than normal stars.
The optical spectra of quasars were perplexing at first because they consisted of a few broad unidentified emission lines. In 1963, Maarten Schmidt discovered these emission lines are actually highly redshifted lines of hydrogen. However, Doppler shift tells us that this shift implies speeds faster than the speed of light.
It was discovered that these redshifts are actually cosmological redshifts caused by the expansion of the universe. According to Hubble’s Law, these large redshifts suggest that these objects are at immense distances. This implies two things:
- Quasars are extremely luminous. They can be up to 1000 times more luminous than normal galaxies. In fact, they are the most luminous type of active galaxy.
- They are very old. The extreme distance means that we are looking very far back in time when galaxies were more dense in space and interacted with one another more often.
The central “engines” of quasars are small, about the size of our solar system. You may be thinking that is not small. Think of it this way, quasars emit 1000 times the energy that the Milky Way emits from a volume the size of our solar system. Blows my mind.
Back to the original question:
Why are active galaxies so luminous?
It is because they all have a central supermassive black hole that is actively feeding from an accretion disk. Typically, this happens when galaxies interact and knock material towards the black hole. The material heats up and causes the nucleus of the galaxy to be extremely luminous. Often, jets will appear.
Let’s be thankful that our planet is about 8,000 parsecs from our supermassive black hole and closer to the edge of the disk…I am not trying to become food for a black hole. To close, here is an interesting video I found about what it would be like if Earth was near the center of our galaxy.
