Hey guys, my name is Shubham Kejriwal and
today in this video, I'll be talking to you
guys about interstellar as well as intergalactic
magnetic fields.
So there exists a very small amount of magnetic
field in interstellar as well as intergalactic
space and I was asked a question by Nandini
Jha regarding the consequences of the presence
of such a magnetic field.
This was posted to me as a question in the
previous video, the Q'n'A that I made along
with a few other questions that I plan to
be answering through other videos.
In case you would be interested in such topics,
do hit the thumbs up button and subscribe
to the channel so that I know that you guys
like watching such videos!
I decided to go one step ahead and give you
a brief history and description of all the
research that has been going around from 1982
on the subject. I conducted a whole lot of
research and hopefully would be able to give
you guys information about some of these astrophysical
phenomena which at least I find really interesting.
First of all, let's talk about the ways of
detection of such magnetic fields.
The most prominent way of detection of magnetic
fields in interstellar space is through the
analysis of polarized, ionized interstellar
dust clouds.
So, in our galaxy like many other spiral galaxies,
there exist different dust clouds which could
be made out of supernova remnants, which could
be nebulae or anything else for that matter.
These contain of very small particles which
can at times be charged. When they are charged,
it is called an ionized interstellar dust
cloud.
Now when these charged particles are present
inside a magnetic field, due to the magnetic
field, they get polarized in a certain direction.
Now if unpolarized light from a background
star passes through this polarized interstellar
dust cloud, it also gets polarized.
When telescopes receive this polarized starlight
which passed through the interstellar medium,
we can tell a lot about the characterstics
about the interstellar cloud as well as the
magnetic field inside of which it exists.
Now specifically in the galactic center region,
there is a certain presence of long filament-like
structures called Nonthermal Radio Filaments.
As the name suggests, they do not have thermal
properties but specifically illuminate in
the radio frequencies.
It is predicted that these Nonthermal Radio
Filaments are present in zones of extreme
magnetic fields of upto one milligauss in
the central galactic region.
The mere presence of such nonthermal radio
filaments in the galactic center tells a lot
about the presence of a magnetic field in
the center. They seem to have magnetic field
strengths of about one millgauss which scaled
to parsec long size of these radio filaments
contains energy equivalent to that of 3000-4000
supernova remnants. That is a lot of energy!
Anyways, those are two of the most prominent
evidence of the presence of a magnetic field
in interstellar space. Except for that, the
plancks satellite has already mapped the entire
magnetic field of our galaxy as well as the
surrounding galactic region.
Now let us talk about where exactly these
magnetic fields originate from. There are
hypothesis regarding a seed magnetic field-
a very small amount of magnetic field generated
through different processes that we would
be talking about which can later be amplified
and maintained through dynamos.
One of the hypothesis suggests that during
the electroweak phase transition of the universe
or during the quark hadron field transition
phase of the universe, these magnetic fields
could've originated.
Now because of the intense cosmology related
to this topic, I would rather not go into
the details of it for the sake of this video
and I don't think I myself understand the
whole thing completely.
There would be links to the research papers
which would talk more about these transition
phases in the description down below so in
case you are interested, you can always take
a look.
There are also hypothesis of origins of these
magnetic fields during the inflation period
of the universe.
This was one phase in the evolution of the
universe during which it expanded at very
rapid speeds and a lot of changes took place
in the universe explaining its current condition.
Now to understand this, you would have to
know that each of the four forces that we
now know of, the electromagnetic force, the
strong force, the weak force, as well as the
gravitational force, have their own field
according to the standard model of the universe.
Now, thanks to Quantum Mechanics, each of
these four fields would have certain quantum
fluctuations related to them. What is hypothesized
is that during the inflation period, the quantum
fluctuation of the electromagnetic field amplified,
and once the universe reheated, the electric
field fluctuations damped to zero however,
the magnetic field fluctuations were frozen
into the intergalactic plasma.
This majorly concerns with the magnetic field
generation in the whole of the universe. In
interstellar magnetic field generation, there
are other processes involved.
One major hypothesis is through batteries.
Now different particles floating around in
spiral galaxies have certain charges linked
to them.
However, because the total number of charges
in the galaxy is approximately the same, these
charges cancel out and the whole galaxy is
in turn neutral.
However, one interesting aspect of this is
the fact that the positive as well as the
negative charges carry different masses. Electron
has a different mass as compared to proton.
As a consequence, if pressure is applied onto
these particles, they would accelerate at
different relative rates and this infact can
give birth to a magnetic field.
One important example of batteries that are
used to explain a lot of processes that go
around in interstellar medium are called Biermann
Batteries. Again, more information about this
would be in the description.
Another method of origin of magnetic field
in the interstellar space is through stars
and active galactic nuclei. The dynamos present
in these stars and AGNs can eject magnetic
field into interstellar space which can be
very high strength however short lived.
That is, by the way, a general trend in interstellar
magnetic field. They are relatively high in
strength of upto a few nanogauss but are short
lived. However, for the origins of intergalactic
magnetic fields, they are very minute in amount,
even smaller than a few nanogauss, however,
they are much long lived.
Now once this magnetic field is generated
there is a need for maintaining and amplifying
this magnetic field and one of the most prominent
hypothesis regarding this process is through
turbulent dynamos.
These are random turbulent motions of stars
and galaxy systems which amplify and maintain
seed magnetic fields that we just talked about.
The two major types of turbulent dynamos are
large scale as well as small scale dynamos.
These are also known as fluctuation and mean-field
dynamos respectively but we would use the
more convenient naming scheme.
The large scale dynamos are the one in which
the scale of the dynamo is much greater than
the turbulent motion that produced it and
the vice versa takes place in small scale
dynamos where the scale of the dynamo is smaller
than the scale of the turbulent flow that
generated it.
Remember the Nonthermal Radio Filaments that
we just talked about? The observation of these
Nonthermal Radio Filaments suggest that they
have magnetic fields of strength equivalent
to about 1 mG or higher. However, other studies
of the same regions suggest that the fields
there could not in any case be greater than
1 microgauss.
That's a difference of a factor of about x1000!
And so, there is a major controversy in the
field strength of magnetic field in the galactic
center where these nonthermal radio filaments
are mostly present.
However, it is hypothesized that both types
of dynamos that we just talked about, the
large scale AND the small scale dynamos can
ensure that these nonthermal radio filaments
can exist in certain regions and the other
regions can have a much weaker magnetic field.
Now the small scale dynamos can produce fields
which are very strong but act on a very small
region. Those could be the regions in which
these Nonthermal Radio Filamnts are present.
However, on a larger scale, the large scale
dynamos can come into play which are much
weaker in strength but again, are spread across
a very large area.
And so, the problem of NRFs is solved! Not
really, there are still discrepancies in how
exactly that works because that doesn't really
match exactly with what the theoretical predictions
are and what we observe them to be, but again,
that's space for you!
And now, finally, after talking about all
of this, let's talk about the consequences
of the presence of magnetic fields in interstellar
as well as intergalactic regions.
So the consequence of magnetic field could
be taken up in different perspectives. If
we talk about Earth, the planet itself has
a very strong magnetic field compared to the
magnetic field of intergalactic or even interstellar
regions. It has a magnetic field which is
at least about a 100,000 times stronger than
the interstellar or the intergalactic magnetic
field and so, I don't think that it can have
a very major significance on to Earth or the
people living inside of it!
However, if we see the consequence of having
a magnetic field elsewhere, we see a lot of
interesting astrophysics happening. For example
in the galactic center region, we see structures
like nonthermal radio filaments that we briefly
talked about in this video which tell us a
lot about how material evolves in the presence
of magnetic field in the galactic center,
Except for that, the presence of magnetic
field in the polarized interstellar gas can
also tell us more about the structure of this
interstellar gas and can help us study supernova
remnants or interstellar dust in a much nicer
way.
This can then help us understand stellar evolution
and star explosions in a much better way than
we currently know of!
This is a very recent field if you ask me,
with interest rising only after 1982 after
many discrepancies were found. Anyways that
is pretty much it for this video, I talked
to you guys about a whole lot of stuff regarding
intergalactic and interstellar space. I hope
you guys found it interesting. I found it
really exciting and interesting to be able
to go through all those research papers and
take a look at all the stuff that people have
been talking about about these types of magnetic
fields!
But yeah, that's pretty much it for this video,
I hope you guys enjoyed watching it. Hit the
thumbs up button if you did, subscribe to
the channel if you want to see more of such
content and well yeah that's pretty much it.
Thank you so much for watching, my name is
Shubham Kejriwal and I will catch you guys
in the next one!
