Where did these gigantic space bubbles come from? - Ashkbiz Danehkar
In November 2010, NASA announced
the discovery of a strange,
never-before-seen galactic object:
two gigantic gaseous bubbles,
each emanating an impressive
25,000 light years
from the center of our home galaxy,
the Milky Way.
Inside the structures,
named the Fermi Bubbles,
streams of high energy particles traveling
faster than the surrounding medium,
collide with dust, gas, and light,
to create gamma rays,
the most energetic form of light.
Astronomers were perplexed.
While gamma rays are not
uncommon in space,
radiation of this magnitude had only
been observed in distant galaxies.
And they are typically produced
by large-scale powerful events,
like explosions of supernova.
Yet, compared to our galactic neighbors,
the center of the Milky Way
and the supermassive black hole
that resides within it,
was always thought to have
been relatively calm.
So, what was the powerful event that
created these massive structures?
And is the center of our galaxy
not so sleepy after all?
A clue to answering these questions
came in December 2020,
when astronomers announced
the discovery
of yet another set of radiating spheres.
Entitled the eROSITA bubbles,
these structures are even wider,
extending nearly half the distance of the
entire Milky Way in both directions,
and fully encapsulating the Fermi Bubbles.
They emit soft X-rays, which have
frequencies lower than gamma rays,
but are still highly energetic
forms of light.
Astronomers quickly surmised
that the overlapping bubbles
most likely share a single origin.
And the event that formed them must have
generated a massive amount of energy—
approximately 1 million times
that which the Sun will produce
during its entire lifetime.
Based on the speed of the jets
of energetic electrons within the bubbles,
they calculated the event
most likely took place
less than 3 million years ago.
This is relatively recent compared to the
galaxy’s 13-billion-year lifespan,
and means our early ancestors might have
even witnessed the powerful event,
as a gigantic ball of heated mass
illuminating the night sky.
But what exactly was the powerful event?
Two theories quickly emerged about what
could have created the bubbles
and the high energy particle jets within.
And both are still debated today.
The first theory is that the bubbles
stem from a recent massive burst
of star formation
toward the center of our galaxy.
Newly forming stars produce
a vast outflowing of hot gas,
called stellar winds.
Meanwhile, young massive stars
die quickly,
causing energetic supernova explosions.
Stellar winds combined
with these explosions
can lead to the formation of large-scale
galactic winds.
These galactic winds can push
away the surrounding material,
creating gigantic bubbles.
The second theory is that the structures
are the result of a powerful outburst
from the supermassive black hole
at the center of our galaxy.
Named Sagittarius A*,
this black hole lives
up to its title of supermassive,
as it’s approximately 4 million times
the mass of the Sun.
And scientists have documented similar
jets of energy
emanating from similar black holes
in other distant active galaxies.
These jets are found in active
galactic nuclei known as quasars,
and they're created as dust and gases
rapidly fall into the feeding black hole.
This gathers hot ionized gas
around the vicinity,
which is then ejected from the center
at ultra-fast velocities.
This theory suggests that Sagittarius A*,
which is thought to be relatively quiet,
may have been active relatively recently.
And it begs the question:
will it wake up again?
Scientists use supercomputers
to run what is known as
hydrodynamic numerical simulations,
where different physical conditions that
may have led to bubble formation
are explored.
While several results suggest that
extreme outbursts from Sagittarius A*
likely contributed to the creation
of the bubbles,
it remains to be seen whether past star
formations may have also played a role.
Other simulations show evidence
of other contributing factors,
like the influence of circumgalactic
medium winds from outside our galaxy,
which may explain some of the
bubbles’ unique features.
These computational simulations
will only get more precise
as we continue to launch more sensitive
and dynamic telescopes into space.
But whatever answers we unlock will
undoubtedly lead to more surprises
about our mysterious,
and perhaps not so calm, galaxy.
