The Milky Way galaxy is relatively calm by the destructive standards of the rest of the
Universe, and compared to its own very violent past.
But just recently we discovered that its violent past was much more recent than we thought
- and could even happen again.
You may be wondering why I'm not floating somewhere off in space.
You'd think it would be the perfect place to weather a global pandemic.
Not so - like many other people, I'm confined to my real world apartment for the duration
- but I promise that won't stop us bringing you the wonders of the cosmos.
Whatever is happening here on Earth, the universe remains awesome.
For example, the center of the Milky Way galaxy.
Not only is it home to an enormous black hole four million times the mass of the Sun, but
it also swarms with smaller black holes, searing hot clouds of gas, massive stars right on
the edge of going supernova, and some of the most energetic radiation in the cosmos.
Yet as extreme as it may sound, the present-day Milky Way is actually relatively calm by the
standards of many other galaxies in the Universe.
Our central black hole, Sagittarius A*, has been in a quiet or “inactive” phase for
as long as we’ve been observing it.
But across the Universe, there are loads of “active” galaxies - ones that harbour
active galactic nuclei, or AGNs, at their centers.
In these, the supermassive black hole at the galactic core is in the process of sucking
down a blazing hot vortex of gas.
The most powerful AGNs are called quasars, and they can shine a thousand times brighter
than their surrounding galaxy.
We’ve talked quasars — and what makes them so awesome — in a previous episode.
Most researchers in the field believe that all supermassive black holes went through
violent AGN phases in the past — and that includes our very own Sagittarius A*.
The Milky Way was probably once an AGN - and it’s just as well for us that this activity
is long past.
Or is it?
According to a series of recent studies, Sag A*’s last feeding binge may have been more
recent that we thought - and it’s appetite may even be waking up again.
We’ll start in 2010, when a team of astronomers from the Harvard-Smithsonian Centre for Astrophysics
were using the Fermi Gamma-ray Space Telescope to look for evidence of dark matter in the
innermost regions of the Milky Way.
See, astronomers believe that when dark matter particles crash into and annihilate each other,
the result could be a fireworks show of high-energy gamma rays.
We have of course covered dark matter and dark energy in detail previously.
Before I get to what the team observed, let me describe what they expected to see.
Dark matter has the property that it tends to spread out diffusely and evenly.
In the case of the Milky Way it should form a vast ball that envelops the galaxy, thickening
slightly towards the center.
Gamma rays produced by dark matter annihilations should have similar structure.
But that is NOT what the team saw.
They found something totally unexpected, and arguably just as amazing.
Instead of a diffuse cloud of gamma rays, they saw a massive pair of high-energy gamma
ray bubbles with sharp edges extending more than 25,000 light years in either direction
above and below the plane of the Milky Way - that’ nearly a third the width of the
entire Milky Way disk — and somehow, no one had ever noticed it before.
We call these the “Fermi Bubbles”.
How does something so massive and so powerful stay hidden for so long?
Well, it turns out that the plane of the Galaxy also glows brightly in gamma rays.
This is mostly from cosmic ray interactions with the interstellar medium.
It goes like this: atomic nuclei - mostly lone protons - can get accelerated to extreme
energies, typically in supernovae or other cataclysmic events.
These “cosmic rays” can then collide with nuclei in the gas between the stars - again,
mostly the protons of hydrogen.
The protons sometimes obliterate each other to form a neutral pion particle plus some other stuff.
The neutral pion will then decay into a pair of gamma rays.
And that’s the glow you see when you look at the disk of the Milky Way - if you have
gamma ray vision.
This gamma ray fog was masking the mysterious bubbles lurking in the background.
Unless you know how to look that is.
The key to finding the Fermi Bubbles was that its gamma rays are even more energetic than
the diffuse emission.
The gamma rays produced by neutral pion decay from these cosmic ray collisions tend to drop
off in intensity towards higher energies.
So, fewer very high energy gamma rays.
On the other hand, the Fermi Bubbles produce a lot more gamma rays at higher energies.
The difference in energy distributions - or in their gamma ray spectra, means that the
diffuse gamma ray background can be cleanly subtracted.
And voila, the Fermi Bubbles shine through.
In the Fermi Bubbles, gamma rays are generated by a high-energy process known as Inverse
Compton Scattering.
Now that’s pretty clear from the shape of the spectrum alone.
Extremely energetic electrons interact with lower-energy light, boosting that light to
the much more energetic gamma ray regime.
And so that’s what we’re seeing here - light bounced off extremely
high energy electrons within the vast bubbles.
It’s estimated that the energy contained in this ocean of electrons is equivalent to
that released by 100,000 supernova explosions.
So the questions become - where the bleep did that energy come from, and when was it
released?
Let’s start with “when” - it’s a bit more straightforward.
Based on the speed of the gas within the bubbles -measured at nearly 9000 km/s by the Hubble
Space Telescope - the bubbles must have been growing for a few million years.
Basically it happened yesterday - at least compared to the 10 billion year
age of the Milky Way.
But exactly what happened a few million years ago?
I mentioned an energy of 100,000 supernovae - well that’s actually a serious option.
Option number 2 is more terrifying: a temper tantrum from Sagittarius A*.
Let’s look at the first scenario.
Believe it or not, it’s not so uncommon for astronomers to witness storms of supernova
explosions raging across a galaxy.
Or at least to catch a snapshot of a galaxy in the middle of such an event.
When a galaxy has a lot of fresh gas and when that gas gets a shake - for example, if the
galaxy is shaken by a collision or close interaction with another galaxy - then that gas can form
stars at a really insane rate across the galaxy. We call these events starbursts
and we see them in many other galaxies like M82 and the Antennae Galaxy.
The star formation part of a starburst isn’t energetic enough to produce the Fermi Bubbles,
but starbursts are always accompanied by an enormous number of supernova explosions because
the most massive stars produced in the starburst die - rather explosively - very quickly.
The shock waves created by the supernovae from a starburst in the Milky Way could literally
punch a hole through the interstellar medium, potentially resulting in something like the
massive bubble-shaped cavities we see today.
Well that sounds promising — but there’s a problem.
All of those supernovae would have had to leave remnants behind - neutron stars and
black holes.
The neutron stars should be seen as pulsars, and there just aren’t enough in that region
to account for a starburst of the required magnitude.
OK, so what about option 2?
Could the Fermi Bubbles have been created by activity from our supermassive black hole?
In fact, could Milky Way have been a full-blown AGN just a few million years ago?
To answer that we need to consider how active the black hole would have to have been.
Now A full quasar might devour many millions of times the mass of the Sun over one active
period, which could last for several million years.
A lot of that mass goes into the black hole, but 10% or more is converted into energy in
the form of light before it hits the black hole, which is why quasars shine so bright.
Sagittarius A* would have converted matter to energy with much less efficiency.
Nonetheless, it’s estimated that to energize the Fermi Bubbles, Sag A* would have needed
to devour - a single 50 solar mass star that happened to wander too close.
So it would have been less of a feast and more of a snack followed by a burp.
Definitely not enough to turn Sag A* into a quasar.
But even that single relatively small event could have driven jets in the directions of
those bubbles.
That all sound good in terms of our energy budget.
Unfortunately, there’s a problem with this theory too.
With jets alone, there’s no way to carve out structure quite like these bubbles from
the interstellar medium.
AGN jets that get trapped within their galaxies tend to form more wedge-like shaped structures
with internal structure - for example hotspots.
But the Fermi Bubbles are … well, bubbly, and they have relatively uniform intensity - they’re
much smoother than we would be expected from a jet.
So what - two nice theories and neither of them work?
Actually, both of them work - if they work together.
Here is our best thinking on the most likely scenario.
A mini AGN phase is triggered either by an influx of gas or by a random massive star
getting too close to the black hole.
A pair of powerful jets and/or winds blast out from the Galactic Centre, and any remaining
gas along the path of these outflows would be compressed by shocks, provoking a sudden
burst of star formation like we described previously.
This starburst and the subsequent supernova barrage smooths out the energy in the bubbles.
AGN activity and starburst activity are very commonly seen together - in fact M82 that
we saw previously harbours an AGN in its core.
The energy from those outflows will eventually shut down the star formation, but in the early
phase it can help kick it off.
OK, so problem solved?
Well maybe - this is still just our best guess.
But new studies are unlocking the secrets of the galactic core.
In a result published last year in Nature, a team of astronomers using South Africa’s
MeerKAT telescope array discovered a handful of never-before-seen radio structures throughout
the Galaxy — including two bubble-shaped structures extending above and below the Galactic
plane.
Just like the Fermi Bubbles, these radio bubbles seem to originate from our central black hole
and extend in opposite directions above and below.
They’re also likely generated by electrons, but in this case, the electrons are accelerated
by magnetic fields and are emitting what’s known as synchrotron radiation.
The biggest difference, however, is that these particular structures only extend 1400 light
years above and below the plane of the Galaxy — leading astronomers to believe that they
might be younger, more recent versions of the much-larger Fermi Bubbles.
Although the energy required to power these radio bubbles is several thousand times lower
than that required for the Fermi Bubbles, evidence points to similar explanations in
both cases — some combination of accretion onto the Milky Way’s central black hole
and a flurry of star formation egging each other on.
This was a relatively weak event, but it was also relatively recent.
So do we need to be worried about it happening again?
Well we probably don’t need to agonize over any extra gamma rays being generated by this process,
because our Earth’s atmosphere absorbs most of the gamma ray radiation that comes our way.
However researchers believe that the starburst and accompanying supernova cascade that may
have helped forged the Fermi Bubbles would have been visible to the naked eye for our
early hominid ancestors.
The night sky several million years ago may have been much brighter than we ever could
have imagined.
We may have to wait until something like this happens again to know for sure.
That could take a while.
Or will it?
In a paper posted last month, a team of astronomers presented evidence that the X-ray activity
of our central black hole has been increasing over the past four years.
While this probably doesn’t mean that we’re going to witness the creation of a newborn
Fermi Bubbles in the near future, we may be able to get new insights into the all-too
frequent tantrums of Sagittarius A*, and their surprisingly large influence on the Milky
Way - a not-so-inactive galactic denizen of space time.
Hey Everyone.
Before we do comments, if you're looking for more science content to help get you through
all that's going on.
PBS has a lot of great shows including Antarctic Extremes.
It’s a journey to Earth’s most remote laboratories to see how science is done in
Antartica.
It’s out now on PBS's new science channel Terra and there's a link in the description.
And speaking of Terra, last week we delved into the science and non-science of how we determine the
age of the earth.
First up, several people were offended that I didn’t actually mention Clair Patterson,
who was the first person to use uranium-lead dating with zircon crystals to calculate the age of the Earth.
Now we did give a review of the method, but you guys are right that we should have mentioned
Clair – who was not only the first to get the right age for the earth, significantly
advance this radiometric dating technique, but he also discovered the disastrous lead levels
in the environment due to industrial use of lead, and campaigned tirelessly against the
latter.
Theo RixLux wants to know more about how cracks in a crystal can lead to a linear correlation
in the concordance diagram.
Sure thing, Theo.
So in a perfectly intact crystal, the ratios of lead-206 to uranium-238 on the Y-axis to
lead-207 to U-235 on the X-axis follow a curved line.
The U-238 has a shorter half life so first it decays quickly in the first couple of billion
years, then the U-235 dominates the changes at later times, leading to the classing concordia
curve.
A number of you quoted Good Omens, which itself has a very precise claim for the age of the Earth.
Now I think I’ll just read the quote, because how’m I gonna beat Terry Pratchett and Neil
Gaiman for wit?
" Current theories on the creation of the Universe state that, if it was created at
all and didn't just start, as it were, unofficially, it came into being between ten and twenty
thousand million years ago.
By the same token the earth itself is generally supposed to be about four and a half thousand
million years old.
These dates are incorrect.
Medieval Jewish scholars put the date of the Creation at 3760 B.C. Greek Orthodox theologians
put Creation as far back as 5508 B.C.
These suggestions are also incorrect.
Archbishop James Usher (1580-1656) published Annales Veteris et Novi Testamenti in 1654,
which suggested that the Heaven and the Earth were created in 4004 B.C.
One of his aides took the calculation further, and was able to announce triumphantly that
the Earth was created on Sunday the 21th of October, 4004 B.C., at exactly 9:00 A.M.,
because God liked to get work done early in the morning while he was feeling fresh.
This too was incorrect.
By almost a quarter of an hour."
Tasha Montgomery points out that this makes Earth a Libra. Seriously Tasha
Have you learned nothing on this show.
A Libra? You forgot to take into account the precesion of the equinoxes.
So accounting for our axil procession, I think that makes Earth a Taurus, which really explains so much.