In November of 2020 astronomers unveiled an image of the Milky Way’s violent center similar in importance to the Event Horizon Telescope (EHT) image of Galaxy M87’s gargantuan black hole. The image (above) shows the new view of our galactic center from the Murchison Widefield Array, with the lowest frequencies in red, middle frequencies in green, and the highest frequencies in blue. Huge golden filaments indicate enormous magnetic fields, supernova remnants are visible as little spherical bubbles, and regions of massive star formation show up in blue. The supermassive black hole at the center of our galaxy is hidden in the bright white region in the center.
A radio telescope in the Western Australian outback has captured a spectacular new view of the center of the galaxy in which we live, the Milky Way. The image from the Murchison Widefield Array (MWA) telescope shows what our galaxy would look like if human eyes could see radio waves.
Missing Companions of Sagittarius A*
Understandably, the image did not appear to capture signs of the 10,000 to 20,000 of black holes in a region just six light years wide surrounding the Milky Way’s supermassive black hole, Sgr*A that no one has been able to find. For more than two decades, researchers have searched unsuccessfully for evidence to support a theory that thousands of black holes surround supermassive black holes (SMBHs) at the center of large galaxies.
There are only about five dozen known black holes in the entire Milky Way galaxy, which 100,000 light years wide said Columbia University astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab, who was not involved with the new image, adding that extensive fruitless searches have been made for black holes around Sgr A*, the closest SMBH to Earth and therefore the easiest to study. “There hasn’t been much credible evidence.”
In an email to The Daily Galaxy, Chuck Hailey wrote; “While there is no direct evidence, since isolated black holes are difficult to detect, the existence of about a dozen black holes with stellar companions, detected with X-rays, allows us to infer the existence of a much larger population of these “hidden” black holes, all orbiting within a stones throw (astronomically speaking) of about 3 light years from Sgr A*.”
Powerful Frequency Range
Astrophysicist Dr Natasha Hurley-Walker, from the Curtin University node of the International Center for Radio Astronomy Research (ICRAR), created the images using the Pawsey Supercomputing Center in Perth.
“This new view captures low-frequency radio emission from our galaxy, looking both in fine detail and at larger structures,” she said. “Our images are looking directly at the middle of the Milky Way, towards a region astronomers call the Galactic Centre.”
The data for the research comes from the GaLactic and Extragalactic All-sky MWA survey, or ‘GLEAM’ for short. The survey has a resolution of two arcminutes (about the same as the human eye) and maps the sky using radio waves at frequencies between 72 and 231 MHz (FM radio is near 100 MHz).
“It’s the power of this wide frequency range that makes it possible for us to disentangle different overlapping objects as we look toward the complexity of the Galactic Center,” Dr Hurley-Walker said. “Essentially, different objects have different ‘radio colors’, so we can use them to work out what kind of physics is at play.”
Relics of Massive Stars
Using the images, Dr. Hurley-Walker and her colleagues discovered the remnants of 27 massive stars that exploded in supernovae at the end of their lives. These stars would have been eight or more times more massive than our Sun before their dramatic destruction thousands of years ago. Younger and closer supernova remnants, or those in very dense environments, are easy to spot, and 295 are already known. Unlike other instruments, the MWA can find those which are older, further away, or in very empty environments.
Hurley-Walker said one of the newly-discovered supernova remnants lies in such an empty region of space, far out of the plane of our galaxy, and so despite being quite young, is also very faint. “It’s the remains of a star that died less than 9,000 years ago, meaning the explosion could have been visible to Indigenous people across Australia at that time,” she said.
Ancient Aboriginal Sightings
An expert in cultural astronomy, Associate Professor Duane Hamacher from the University of Melbourne, said some Aboriginal traditions do describe bright new stars appearing in the sky, but we don’t know of any definitive traditions that describe this particular event. “However, now that we know when and where this supernova appeared in the sky, we can collaborate with Indigenous elders to see if any of their traditions describe this cosmic event. If any exist, it would be extremely exciting,” he said.
Dr Hurley-Walker said two of the supernova remnants discovered are quite unusual “orphans”, found in a region of sky where there are no massive stars, which means future searches across other such regions might be more successful than astronomers expected. Other supernova remnants discovered in the research are very old, she said. “This is really exciting for us, because it’s hard to find supernova remnants in this phase of life–they allow us to look further back in time in the Milky Way.”
The MWA telescope is a precursor to the world’s largest radio telescope, the Square Kilometre Array, which is due to be built in Australia and South Africa from 2021. The low-frequency part of the SKA, which will be built at the same site as the MWA, will be thousands of times more sensitive and have much better resolution, so should find the thousands of supernova remnants that formed in the last 100,000 years, even on the other side of the Milky Way.
The images of the Galactic Center can be viewed on the GLEAMoscope.
Publications: ‘New candidate radio supernova remnants detected in the GLEAM survey over 345° < l < 60°, 180° < l < 240°’, published in Publications of the Astronomical Society of Australia (PASA) on November 20th, 2019.
Image credit top of page: Dr Natasha Hurley-Walker (ICRAR/Curtin) and the GLEAM Team