the extreme characteristics that are leaving scientists baffled.
The center of the Milky Way, 27,000
light-years from Earth, is a place of
violence. This innermost region, the
central molecular zone, spans 1,600
light-year and is by all accounts the
most extreme part of our galaxy.
Density, temperature, and turbulent
velocity, a measure of chaotic fluid
motion, are around 1 to two orders of
magnitude higher here than anywhere else
in the galaxy. The cosmic ray energy
density, a proxy for energetic activity,
is 2 to three orders of magnitude
higher.
This region is home to vast complexes of
molecular gas, about 20 million solar
masses worth, dense cosmic clouds,
ionized plasmas, extreme cosmic ray
energy, ultraviolet and x-ray radiation,
and turbulent magnetic fields. It is a
hotbed of cosmic activity from the
formation of stars to exploding
supernova. And let's not forget
Sagittarius a star, the super massive
black hole 4 million times the mass of
our sun at the very center of it all.
These conditions are hugely exciting for
astronomers, but they make the galactic
center notoriously hard to image.
Visible light can't penetrate the dense
clouds of dust and gas. So researchers
turned to other parts of the
electromagnetic spectrum to lift the
veil and reveal the secrets at the heart
of the galaxy.
Radio waves have the longest wavelengths
of the electromagnetic spectrum from a
few millime to hundreds of kilome and
the wavelengths in the range of millime
to tens of meters are ideal for radio
astronomy. They pass through the
obscuring clouds of gas and dust, giving
us a clear view of what lies beneath. In
the early 1980s, Furhad Ysef Zade,
studying for his PhD, was using the very
large array telescope in New Mexico to
produce a radio map of a section of the
galactic center. He was planning to
study star forming regions, but narrow
strips of radio emission were streaking
across the entire survey area right
through the parts he was interested in.
He thought they must be artifacts in the
data or imaging errors, which any
scientist will tell you is highly
annoying. So after much frustration and
no luck resolving the problematic
artifacts, he returned to the VLA to
image again at another frequency. And
that was when his Eureka moment struck.
At 400 a.m. one morning, he was
comparing the two samples taken at
different times using different
wavelengths, and he saw the same
structures in both images.
This was no artifact. This was a very
real finding, something unlike anything
he or anyone else for that matter had
come across before.
Zade was seeing highly ordered
structures where previously only chaos
was thought to exist and they had some
very unusual features. Most striking was
their vast scale. These were continuous
narrow strips of radio emission 50 to
100 light-years long, but only 1 to
three light-years wide, dangling
vertically above and below the central
molecular zone, the most extreme part of
the Milky Way. Some appeared in pairs or
clusters running parallel to each other
like strings on a harp, each separated
by a standard distance of around one
astronomical unit, the distance between
Earth and the Sun. When he cross-cheed
them with the infrared data from that
area, Zarde also discovered they had no
counterpart in that area of the
spectrum. This told him they were
non-therrmal emissions. That is to say
they were not produced by heated gases.
This was corroborated by other
measurements such as spectral index and
polarization which showed that the
filaments were highly magnetic and
emitting synretron radiation. Synrettron
radiation occurs when electrons moving
near the speed of light interact with a
strong magnetic field which beg the
question what on earth or should I say
not on earth was accelerating the
electrons to such speeds. The emissions
along the length of the structures were
continuous ruling out localized events
like star formation or supernova
remnants. So Zade dubbed them
non-therrmal filaments and suggested
they were likely related to galactic
scale phenomena.
His observations didn't correspond to anything else in the known galaxy, and
Zarde had many more questions. Where did
the non-thermal filaments come from?
What was maintaining their linear
structures over such vast distances of
space and time? Why, when clustered,
were they so evenly spaced?
But almost as soon as this startling
discovery was made, the trail started to
go cold. The available telescopes at the
time simply didn't have the sensitivity
needed to provide answers.
Over the next 35 years, only a handful
of other vertical non-therrmal filaments
were revealed and categorized. Some were
even given enigmatic names like the
snake, pelican, and bent harp. Sadly,
there wasn't enough data to make any
great leaps forward in understanding.
Well, not until 2022. and Miacat's
mindblowing image.
The MICAT radio telescope at the South
Africa radio astronomy observatory or
saro is comprised of 64 interlin
antennas each with a 13.5 m diameter
parabolic dish spread over 8 km of radio
silent zone built over 4 years. The full
array was inaugurated in 2018.
Its location in the southern hemisphere
is perfect for imaging the center of the
Milky Way thanks to our sun's axle tilt
relative to its own position in the
galaxy. So, Miaat has a direct line of
sight into the CMZ and the galactic
center. Over the course of 3 years, an
international team led by Dr. Ian
Haywood and including Zarde, now
professor at Northwestern University,
directed Mia Cat to a 6.5 square degree
portion of the galaxy, a section of the
sky around 30 full moons wide with
Sagittarius A star right in the middle
using Lband radio frequencies of 856 to
1,712 MHz, equivalent to wavelengths of
18 to 35 cm. They split this area into a
20part mosaic, directing the telescope
to survey each tile in turn over a total
of 144 hours on target. This was the
first time Mircat's full array was used
with 60 to 62 dishes sampling the sky at
any one time. After generating 70
terabytes of raw data, the equivalent to
700 hours of 4K YouTube content, the
team then had to process it. That was no
mean feat. Given the complexity of the
environment, they needed to put the data
through a highpass filter using a method
called difference of Gaussians. This is
a commonly used edge smoothing technique
to remove background noise and enhance
the visibility of fine structures,
especially important for visualizing
non-therrmal filaments. And this is the
result.
More like a work of art than a
scientific study, it captures a wealth
of features. Some are wellnown like
Sagittarius A star seen in the central
saturated area here and clearer views of
previously known supernova remnants and
star forming regions. This here is a
supernova remnant. To its left is a
runaway pulsar, the mouse and up on the
right one of the longest and most famous
non-therrmal filaments, the snake.
As noted by the team, one of the most
startling discoveries was the sheer
number of filaments apparent in the
image, an order of magnitude greater
than all previously known, most of which
had never been seen before.
This was gamechanging for Zard and his
colleagues. Now we finally see the big
picture. A panoramic view filled with an
abundance of filaments. He said this is
a watershed in furthering our
understanding of these structures. There
was finally enough data to carry out
meaningful population studies. They set
to work carrying out statistical
analysis of the filaments. This work
published in the astrophysical journal
letters not only further categorizes the
filaments but gives tantalizing clues to
their origin. The new data confirmed
that all of them are magnetized.
In fact, the team found that the
magnetic field was significantly greater
in some cases up to 10 to 100 times
stronger than typical galactic magnetic
fields. The new analysis also confirmed
that synretron radiation is a defining
characteristic.
Interestingly, the MICAT data revealed
that there is a steepening with galactic
latitude. In other words, the filaments
appear to cool as they stretch away from
the galactic plane. This gives us a clue
as to their possible origin. The
electrons further away from the galactic
plane could be older, implying that the
filaments are related to past activity
of Sagittarius a star. And there was
another clue that suggested this too.
Enormous structures known as radio
bubbles. First discovered by Haywood
Zade and the Miacat team in 2019, these
huge radio emmitting structures
stretched symmetrically above and below
the galactic plane, forming an hourglass
shape thousands of light years across.
They are thought to have been created by
a phenomenal outburst from Sagittarius a
star about 100,000 to a million years
ago. An event powerful enough to leave
such a scar on the galaxy could have
been vast quantities of gas and dust
falling into the black hole or a huge
and sudden burst in star formation close
by. An incident like this would have
triggered an intense outburst of energy
and whipped up galactic winds, driving
gas and cosmic rays violently away from
the galactic center, stretching and
amplifying magnetic field lines in its
wake, creating those bubbles and
non-therrmal filaments. What's more,
strong magnetic fields, which as we now
know are a confirmed characteristic of
all filaments, capture cosmic rays. And
the great thing is we can date them.
Those detected in the filaments by a
mircat match the proposed period of the
Sagittarius A star outburst considered
responsible for the radio bubbles. In
other words, they are the same age.
The position and capabilities of Mircat,
alongside the same highpass filtering
used to resolve the non-therrmal radio
filaments, not only revealed these
bubbles in astonishing detail, but
showed almost all of the filaments are
confined within them. This close
physical association adds even more
weight to the argument that the same
energetic event created them. Something
powerful enough to create the bubbles
would certainly be able to accelerate
electrons to near the speed of light
with the stretch magnetic field lines
channeling them to produce the
filament's signature synretron emission.
With this hypothesis in mind, Zard and
the team described the formation of
non-therrmofilaments as magnetized
streamers in a cosmic raydriven wind. It
certainly paints a compelling picture
for the possible origin of the
filaments, but it is by no means
conclusive as even the authors
themselves attest. Other theories are
being worked on. With a mystery this
tantalizing, other astronomers have been
studying the filaments, too. But this
single image is still the one that's
told us the most. Zard wasn't kidding
when he said it was a watershed moment.
But with so many unanswered questions,
some going back 40 years, where does
that leave us? Are non-therrmal radios
merely a galactic curiosity?
Not by any means. They are a riddle
wrapped in a mystery inside an enigma
and could shed light on one of the
biggest unanswered questions out there.
How super massive black holes regulate
star formation within a galaxy.
Scientists know that the active centers
of galaxies must transfer energy and
matter into interstellar space through a
process called galactic feedback. If
they didn't, star formation would run
away unchecked, using up a galaxy's gas
and dust faster than observations tell
us. But how this feedback happens is
unknown. Mircat's detailed imagery of
non-therrmal filaments and the radio
bubbles provides us with compelling
evidence that this outflow of energy
could happen in discrete but powerful
outbursts and this is something that has
been seen before.
Fermy bubbles discovered by NASA's Fermy
gammaray telescope in 2010 are even
bigger hourglass shaped configurations
spanning a total of 50,000 lightyear.
These mindbogglingly massive structures
colored magenta in this image are
thought to be millions of years old.
Likely caused by a violent outburst from
Sagittarius A star which calculation
suggests had the energy of 100,000
supernova.
This is much more powerful and ancient
than the event proposed to have made the
filaments and radio bubbles. But
together they paint a picture of
intermittent outbursts from deep within
the heart of our galaxy. Both have the
potential to regulate star formation,
ensuring that the Milky Way doesn't
suffer from burnout. As scientists
continue to unravel the mysteries of
non-therrmof filaments and tackle the
big questions about how the universe
works, the trail doesn't seem to be
going cold again anytime soon. Since the
first full array image, Mircat has found
more of these mystery strands in other
galaxies with very similar properties to
the ones we see in the Milky Way. Their
very existence elsewhere suggests a
common underlying mechanism that alludes
to their role in fundamental galactic
processes.
To conclusively piece together the whole
picture will require another step change
in imaging resolution. And hopefully
that's not too far off as Mircat already
awarded by the Royal Astronomical
Society for its spectacular observations
in radioastronomy was built with
longerterm goals in mind namely to be
incorporated into the square kilometer
array. With a total collecting area of
one square km, it will be 50 times more
sensitive than any other radio
instrument in existence. and it's
expected to be fully constructed by
2028. Keeping an eye on developments in
other parts of the electromagnetic
spectrum will be important too. Zard
believes that the next breakthrough will
come from gamma ray telescopes. Imaging
at higher frequencies results in higher
resolution imagery which has potential
to show us whether the filaments, the
radio bubbles that contain them, and the
vast fermy bubbles are connected.
There's an elegance in order rising out
of chaos and observing non-therrmal
filaments streaming out through the
cosmic winds certainly fits that notion.
So, keep watching this space. And with
images and phenomena this spectacular, I
certainly have no problem doing that.