The Universe is filled with galaxies of various different types. Some large some small,
some round, some elliptical. Radio galaxies are one type where an active galaxy ejects material
from jets which is thought to form into lobes much further out. Centaurus A is a classic example of
this type of galaxy. But as our telescopes have improved we have been able to see these types of
galaxies much more clearly and it has revealed that some of these lobes can extend incredible
distances. Some as much as 62times the size of our Milky Way. This is normally explained as
the active galaxy having a supermassive blackhole which is ejecting material at a much faster rate.
But a new discovery of a radio galaxy which is nearly 160 times the size of our Milky Way
is asking problematic questions for which there are no answers. Let's dive in and find out more...
At the heart of a galaxy lies the Active Galactic Nucleus or AGN.
Inactive galaxies produce jets which are mostly observed in radio
wavelengths. These types of galaxies are referred to as Radio Galaxies.
Where these jets extend on the megaparsec scale they are called giant radio galaxies. These jets
can stretch millions of times the size of the central galaxy and form hotspots at the end.
These giant radio galaxies are so large that they are comparable to the size of galaxy clusters!
For context, 30 Milky Way galaxies in a line would be equivalent to a megaparsec.
Explaining how these colossal beasts form is not that simple. Since their discovery in 1974
different generations of astronomers have been working to solve this mystery.
By 2016 less than 300 Giant Radio Galaxies had been discovered.
The project Search and Analysis of Giant Radio Galaxies (SAGAN) has so far identified 820.
These Studies have provided three alternative mainstream explanations
for the enormous size of these giant radio galaxies.
1. The host galaxy supports a massive black hole compared to other normal radio galaxies.
SAGAN's analysis of the radio galaxies reveals no direct correlation between the size of the AGN
in comparison to the size of the lobes 2. The giant radio galaxies formed in a more
sparse environment, where the jets can cruise to relativistic speeds unhindered to grow to be very
very large. Here the SAGAN analysis reveals that only 10% were located in dense clusters.
3. The size of the giant radio galaxies is due to their long
ages which has allowed them to grow so large. This model cannot explain
the continued fuel source for the AGN over this prolonged period of time.
The SAGAN results, therefore, show that no single factor can explain these giant radio galaxies.
A new find pushes these limits even further. With a new giant radio galaxy stretching a staggering
16.3 million light-years across. This is about 160 times the width of our Galaxy
and four times larger than the previous title holder which spanned only 3.9 million light-years.
At the centre of these large lobes sits a very ordinary elliptical galaxy. Its mass
is estimated to be roughly 240 million times the mass of the Sun which is half that of our Milky
Way. The AGN in this galaxy is also on the small side compared to most radio galaxies. It is also
not a very luminous galaxy. This presents a rather large problem as this means that
very massive galaxies or AGNs are not necessary to grow large giants and neither is high radio power.
This leaves astronomers stumped but they are investigating some potential explanations.
The first is that it exists in a much less dense region in space allowing it to grow
larger and another explanation is that it exists inside a filament of the cosmic web.
Let's just take a step back here. Anthony Peratt's concept of galaxy formation
involved two Birkeland currents twisting around each other and at a certain point,
a double layer formed. The material between the filaments would then be compressed into the AGN.
The outer edge of each filament would also become excited causing it to light up forming
what appears as the lobes. At the same time, a bridge would form extending out from the AGN. In
this model no material is actually ejected along these, they are not jets but just appear as jets.
If you are interested in learning more about Peratt's model I will link some
videos I produced that outline his model in much more detail. Can we use this model
to explain what we are seeing here? The first question is whether the intergalactic filaments
are all the same size? Could two large filaments be responsible for creating this large separation?
The problem I see with this is that if the filaments are larger then there is more material
being compressed towards the centre and therefore the host galaxy would be larger. This is not what
we see. It is just the lobes that seem to be much larger compared to other radio galaxies.
An alternative possibility is that the galaxy is not as far away as they believe
making the span of the lobes much smaller but this would equally make the galaxy much smaller too.
Neither of these are particularly useful but picking up one of the mainstream astronomer's
points maybe we also need to consider the material in between the filaments. In Peratt's
model the galaxy forms from the intergalactic material between the filaments being compressed.
As the astronomers have already noted the majority of these giant radio galaxies seem to occur
in less dense regions of space. This would mean that the central galaxy would be smaller
and less energetic. In which case either of the two
previously mentioned scenarios might explain the formation.
So we either have larger filaments width in these less dense regions or we have standard filament
width but a smaller than normal galaxy at a closer distance than the redshift would indicate.
Hannes Alfven felt that the lobes in radio galaxies were caused by a double layer
further out which were part of a galactic circuit which powered the galaxy. In this theory,
the galaxy would have jets that pushed material outwards along its axis but a significant distance
from the galaxy a double layer would form. This would cause electrons to be accelerated in one
direction and ions in the other. The electrons would be scattered more due to their much smaller
mass and would be responsible for the radio noise in the lobes that we detect. So is it possible
that the double layer formed much further out compared to other galaxies? There are certain
conditions that can cause a double layer to form, these include magnetic fields, electric fields,
differences in drift velocities, etc. I have done a separate series exploring the details
of double layers which I will link down below in the description. Is it possible that in this case
the ejected material remained as a coherent evenly distributed stream for a much longer
distance compared to other galaxies? Could the lower density space have helped here? Could they
have formed further out and the lower density also allowed the electron cloud to grow larger as well?
One thought that I keep coming back to is that part of this problem is perspective.
The assumption that it is large is based on the distance measurements through the redshift and
our predetermined idea of what a galaxy should look like. A simple comparison of the lobes sizes
scaled to the same size shows that the lobes are actually all very similar in terms of shape and
size once we scale them. The biggest outlier is Hercules A but this may be because we are seeing
finer detail in the surrounding cloud that we simply cannot see in the other galaxies. We may
also not be viewing all the galaxies side-on and some are very likely at different angles which
would alter the size of one lobe compared to the other and also the distance from the host galaxy.