PBS Space Time

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 weve 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.

Weve 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 its 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 its appetite may even be waking up again.

Well 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 thats 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 thats 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 thats what were seeing here - light bounced off extremely high energy electrons within the vast bubbles.

Its 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?

Lets start with when - its 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 thats actually a serious option.

Option number 2 is more terrifying: a temper tantrum from Sagittarius A*.

Lets look at the first scenario.

Believe it or not, its 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 isnt 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 theres 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 arent 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, its 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, theres a problem with this theory too.

With jets alone, theres 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 - theyre 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 Africas 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.

Theyre also likely generated by electrons, but in this case, the electrons are accelerated by magnetic fields and are emitting whats 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 Ways 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 dont need to agonize over any extra gamma rays being generated by this process, because our Earths 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 doesnt mean that were 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.