The inner 600 light years of our galaxy is a maelstrom of cosmic radiation, turbulent swirling gas clouds, intense star formation, supernovae, huge bubbles of radio energy, and of course a giant supermassive black hole. This bustling downtown of the Milky Way is a potential treasure trove of discovery but has been difficult to study as the galaxy’s central regions are obscured by dust and glaring radiation. But a new image of this region with unprecedented detail reveals more than we’ve ever seen before. We find some familiar objects like supernovae but also some mysterious structures – gaseous filaments dozens of light years long channeling electrons at near light speed.
Behold, the galaxy’s centre as never seen before:
The new MeerKAT image of the Galactic centre region is shown with the Galactic plane running horizontally across the image. Many new and previously-known radio features are evident, including supernova remnants, compact star-forming regions, and the large population of mysterious radio filaments. Colours indicate bright radio emission, while fainter emission is shown in greyscale. Credit: I. Heywood, SARAO. Image description: SARAO
Creating this incredible image required the power of the MeerKAT observatory. MeerKAT is located in South Africa which has a vantage point of the galaxy’s centre 25,000 light years away toward the constellation Sagittarius. MeerKAT sees the Universe in radio waves which penetrate interstellar dust normally obscuring this region of space. The image above is the result of 144 hours of telescope observation and 70 terabytes of raw data. Created from 20 separate pointings, the image is a mosaic representing 6 square degrees of sky – the equivalent of 30 full Moons spanning 4 full Moons wide – not just a speck in the night. If your eyes were sensitive enough to radio waves, this is what the the galaxy would look like above you.
The Milky Way is a flat-ish disk of hundreds of billions of stars with a bulging central core. Within the core exists a region called the “Central Molecular Zone” or CMZ with gas densities and turbulence 10-1000 times what you find in outer disk of the Milky Way (where we are located thankfully) and cosmic radiation levels equally higher than the rest of the galaxy. Clusters of young massive stars are being created from fresh gas drawn toward the CMZ. Some of these massive stars have already exploded in these clusters as supernovae.
Another version of the MeerKAT image of the Galactic centre Credit: I. Heywood, SARAO
Much of what we’re observing in the heart of the galaxy are familiar objects – stars exploding and gas flowing – but there are mysterious structures unique to this region we don’t fully understand. Reaching out from the Milky Way’s core are giant magnetic tendrils – like strands or filaments – up to 150 light years in length. They appear in pairs or clusters like “strings on a harp” separated from each other by about 1 AU (the Earth’s distance from the Sun), and act like wires conducting electrons accelerated to near the speed of light.
Numerous filaments spanning the Galactic plane. The filaments are identified in the lower panel “Spectral Index” Credit Heywood et al Figure 9
We’ve seen these filaments before MeerKAT. They were first discovered by Farhad Yusef-Zadeh of Northwestern University in the 1980’s. But MeerKAT’s new image has uncovered 1000 more filaments – 10 times more than previously known. The filaments are difficult to see as they blend into the rest of the other structures and shapes of turbulent gas and star forming regions at the centre of the galaxy.
Because the filaments are magnetic, the flow of electrons along their length creates a unique signature of radiation that can be detected by MeerKAT – Synchrotron radiation. Synchrotron radiation is generated by the motion of high speed electrons through a magnetic field. This unique radiation allows the filaments to be teased out in images from other structures. Software image processing also highlights the edges of the filaments further drawing them out from the background galaxy.
So what are these filaments, what is accelerating electrons along their length, and why are they unique to the centre of the galaxy?
Lurking at the centre of the Milky Way is a supermassive black hole. Black holes come in a few varieties. Supermassives weigh in at millions or even billions of times the mass of our Sun and are typically found in the centres of galaxies. Extending outward from the central region of the Milky Way are two large bubbles of gas emitting radio waves. They are symmetrical, forming an hourglass rising above and below the galactic plane.
This cirrus cloud-like emission from the Galactic centre super bubble dominates this image. This is traversed by a complex of many parallel filaments. The radio bubble nestles against the diffuse Sagittarius A region in the lower centre of the image. The bright dot near the centre of this region is Sagittarius A*, a 4 million solar mass black hole. This image captures the chaotic complexity of the very heart of our Galaxy. Credit: I. Heywood, SARAO Image Description SARAO
MeerKAT found these radio bubbles in 2018 which extend a thousand light years outward from the central region and are thought to be created by a massive outburst from our black hole about 100,000 to a million years ago – powerful enough to send two send the bubbles surging outward from the galaxy’s disk. While large black holes feed, huge amounts of energy are released that could create bubbles like these. (Black holes are messy eaters.)
The “hour glass” radio bubbles discovered by MeerKAT extend vertically above and below the plane of the galaxy. Many magnetised filaments can be seen running parallel to the bubbles. (Adapted from results published in Heywood et al. 2019.).
There have also been outbursts of energy from the supernovae explosions and star formation in this region. It’s possible the filaments are created by the interaction of both the radio bubbles and star formation as the magnetic fields of energized gases expanding from the black hole become entangled with outbursts from stellar formation and explosions. The black hole’s outbursts also accelerate electrons along the length of the filaments themselves.
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If an interaction between the black hole outbursts and supernova explosions creates the filaments, that interaction would explain why the filaments are unique to this part of the galaxy. They also seem to appear only within the radio bubbles themselves. The filaments have also captured cosmic rays – high energy particles created by stars – in their magnetic fields. The particles can be dated essentially leaving a time stamp on the filaments. The cosmic ray time stamp matches the period of outburst from the black hole further leading researchers to believe the filaments are related to the black hole outburst. The MeerKAT images make it possible to catalogue each of the filament clusters to learn more about them and therefore the story of the centre of our galaxy.
Catalogue of newly discovered filaments from MeerKAT. Credit Heywood et al Fig 8
MeerKAT spotted some other incredible things within our galaxy’s busy downtown. The image below shows the spherical supernova explosion (rare for supernovae images which are usually more asymmetrical) and features an unknown object resembling a comet (though not a comet) toward the right which might be streaking past leaving behind a wake.
A rare, almost-perfect spherical supernova remnant that has been discovered at the edge of the MeerKAT mosaic. Numerous compact radio sources are also visible, many of which signpost supermassive black holes at the centres of galaxies far beyond our own. There is also an intriguing tailed radio source visible on the right of the image, which could be an object in our galaxy moving at high speed, leaving a trailing wake. Credit: I. Heywood, SARAO. Image Description by SARAO
But the other image that made me gasp – a “mini spiral.” This mini-spiral below is a structure extending out from the Milky Way’s central black hole thought possibly to be gas spiraling toward the black hole itself!
The upper panel’s glowing area shows a synchrotron radiation halo surrounding the supermassive black hole at the centre of the galaxy. The radiation highlights the flows of gas funneling toward the black hole. As the gas approaches the black hole, it is heated beginning to ionize where electrons are stripped from the gas. The ionized gas appears as a “mini-spiral” in the lower panel within about five light years of the central black hole. The scale on the top panel shows 5pc (parsecs or about 16 light years). In other words, this cloud of ionized gases and synchrotron radiation swirling about our black hole is light years across. Credit Heywood et al Figure 14
In the early 1930’s astronomer Karl Jansky first detected radio emission from the galactic centre. That observation – considered the birth of radio astronomy – was the earliest version of what MeerKAT is observing now. It’s possible some of those radio emissions Jansky detected were the first observation of the black hole at the centre of our galaxy.
Karl Jansky’s radio telescope build to rotate on Ford Model-T wheels. This is the first radio telescope ever created. Image courtesy of NRAO CC by 3.0
We’ve come a ways since Jansky’s radio telescope built literally on Ford Model-T wheels. None of these new views of our galaxy would possible without the MeerKAT facility and the work of lead author Dr. Ian Heywood and research team that created the mosaic image.
Inaugurated in 2018 with 64 antennae spread over 8km of the Northern Cape of South Africa, MeerKAT is the most sensitive radio telescope of its kind. Between facilities like this, and the newly launched JWST, we are in for a front row seat to the unveiling of the Universe as never before.
MeerKAT observatory Credit SARAO
Milky Way centre by the MeerKAT array of 65 radio dishes in South Africa. The image spans 4 times the Moon’s size in the sky. Ian Heywood (Oxford U.), SARAO; Here is a full sized version of the picture that was posted in Astronomy Picture of the Day. Colour processing on the image was done by Juan Carlos Munoz-Mateos (ESO) whose Instagram channel you should definitely check out. Has some of the coolest astro images I’ve seen.