Milky Way and stellar echoes caught in the outback

The scorched red-Earth 800km north and slightly east of Perth is a part of the world that is exceptionally quiet. That’s on purpose and designed to create a shielding sphere from the outside world. The traditional owners, the Wajarri people, have lived in harmony with these lands for tens of thousands of years.

No mobile phones are allowed here, nor are there any visitors. The vast 520 km diameter zone is free of most human and electromagnetic interference. It’s like the world we live in, the tasks we complete, people we talk to, tv shows we watch and the way we go about in our day-to-day activities stop at the edge of this special zone. For good reason.  

Nestled in the core of this region is one of the world’s newest astrophysical observatories – spread across the land with an array of telescope dishes and insect-like dipole antennae. These electromagnetic ears are listening and watching the deepest realms of our Galaxy, using highly sensitive equipment, to probe the unknowns about our Universe’s history.

Suddenly, a signal arrives. It’s faint, and it’s not like similar signals of its nature. It’s located in a different part of the sky than what was expected. As the colossal volumes of data stream into the receivers, down through the optical cables and out to the supercomputing facility – an image starts to take shape.

It’s a shell-like structure. A sphere of radio waves, fuelled by a rapidly expanding shockwave – generated by the end of the life of a massive star, that died some 9,000 years ago – when the ancestors of the Wajarri people walked through these lands.

At that moment, 9,000 years ago – a star went supernova – outshining the entire galaxy for a brief moment in time. It’s an echo, now the subject of study of scientists at the Murchison Widefield Array.

The Murchison Widefield Array (MWA) telescope has taken a remarkable view of the centre of our Galaxy, the Milky Way Galactic Plane. The image itself shows gigantic golden filaments representing astronomically-sized magnetic fields and is peppered with spherical bubble-like shells – the remaining echoes of once cataclysmic massive stellar explosions known as supernovae.

The findings and data from the survey come from the GaLactic and Extragalactic All-sky MWA survey, also known as ‘GLEAM’. 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 Centre,” said Astrophysicist Dr Natasha Hurley-Walker, from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), who created the images using the Pawsey Supercomputing Centre in Perth.

“Essentially, different objects have different 'radio colours', so we can use them to work out what kind of physics is at play.”

Three papers have been release by the research collaboration that outline the observational data, analysis and results from the survey. The first paper (Hurley-Walker et al. 2019a), provides data from the GLEAM survey for a large portion of the Galactic Plane, across 20 frequency bands ranging from 72 – 231 MHz, in addition to specifying 22,097 components from a 60 MHz bandwidth in a compact source catalogue.

The second paper (Hurley-Walker et al. 2019b), utilises the latest data from the GLEAM survey and combines it with the space-based Widefield Infrared Survey Explorer (WISE) telescope, following up on 101 supernovae remnant (SNR) candidates to confirm if they are the final remains of massive stellar explosions or if they can be reclassified as giant molecular clouds of neutral hydrogen.

Lastly, the third paper (Hurley-Walker et al. 2019c) describes the discovery of 27 new supernovae remnants using the data of the GLEAM survey from the MWA telescope – including the discovery of the lowest surface brightness supernova remnant ever detected.

The findings across all three papers were produced as part of a larger global collaboration. Researchers from a number of Australian Universities and institutions were involved, and included:

• International Centre for Radio Astronomy Research (ICRAR)
• ARC Centre for Excellence for All-sky Astrophysics (CAASTRO)
• University of Technology (Sydney)
• ARC Centre for Excellence for All-sky Astrophysics in 3 Dimensions (ASTRO 3D)
• University of Sydney
• Western Sydney University
• University of Western Australian; and
• University of Melbourne

The new image of the Galaxy taken by the MWA, shows what our galaxy would look like if human eyes could see radio waves.

“This new view captures low-frequency radio emission from our galaxy, looking both in fine detail and at larger structures,” said Dr. Hurley-Walker.

“Our images are looking directly at the middle of the Milky Way, towards a region astronomers call the Galactic Centre.”

In this latest study, the Galactic Plane was divided into two regions – the inner-Galaxy and the outer-Galaxy as part of the survey and imaged, which produced a resulting catalogue of 22,097 identified radio sources detected across 2,670 degrees-squared.

The survey also (in assistance with WISE) looked into 101 potential SNR candidates that were proposed in the region, and established that 10 are definitely SNRs, two might be SNRs and five can be reclassified as ionised hydrogen (HII) regions. Of the remaining, two are potential candidates but require further research and the remaining 82 candidates were not detected in this data.

Lastly, the survey detected 26 new SNRs using the data from the GLEAM survey, including the lowest surface-brightness SNR ever detected. Accumulating on this, the survey also discovered two SNRs with Galactic longitude range of 220-degrees to 240-degrees, which would require follow up observations of the pulsars within them to further enhance our understanding of how SNRs that are located far from star forming regions.

Supernova remnants are the expanding shell of materials and energy, from a supernova explosion (either when a massive star collapses or when a white dwarf star accretes enough material to trigger a thermonuclear explosion).

Ejected energy from the blast expels much of the materials into the surrounding interstellar medium at localised supersonic speeds (roughly 10% the speed of light or about 30,000km/s). This causes a shock wave to form ahead of the blasted materials which heats up the plasma in the path of the shock wave to millions of Kelvin.

There are three main types of SNRs:

• Shell-like, spherical ring structures expanding away from the central event
• Composite structures – in which the shell contains a central pulsar wind nebula, filling the ring with emissions
• Thermal composite structures – where detection of a central thermal x-ray is observed to be surrounded by a radio shell

Galactic cosmic rays are thought to be generated from SNRs who provide highly energised shock fronts that can generate high-energy cosmic rays. Some of these cosmic rays have been observed to collide with Earth’s upper atmosphere, creating a shower of decayed high-energy particles that rain down to Earth.

SNRs are important to galaxy evolution – in that they allow opportunity for heavy elements nucleosynthesised inside the cores of massive stars to be distributed across the galaxy (and thus becoming a part of future generation of stars) as well as providing the energy to heat up the Interstellar Medium.

SNRs emit in a range of wavelengths from radio waves  to Gamma-rays, observed by terrestrial and space-based telescopes – but mostly have been detected in radio waves. In the forward shock region of the blast wave, cosmic rays and magnetic fields enable synchrotron emissions to be generated – which are detectable at radio frequencies with telescopes like the MWA.

The MWA telescope is a low-frequency radio array instrument, with an operating frequency range of 70 – 300 MHz. It is made up of spider-like antennae, each with 16 dipoles and arranged in a 4 x 4 configuration, sitting atop a 4m x 4m mesh ground plate. Since beginning its operations in 2013, the MWA has achieved some big goals for the science community, including:

• Constraining the limits of the first-ever traceable Fast Radio Burst (FRB)
• Discovering new ionospheric structures in Earth’s atmosphere
• Attributed to the global collaboration of the first observation of a binary neutron star system
• Creation of 300,000 galaxies catalogue
• First radio panorama of the universe

The MWA telescope was built to be a scientifically flexible instrument, with four primary science goals:

• Detection of the Epoch of Re-ionisation (EoR)
• Galactic and Extra-Galactic Science (GEG)
• Time domain astrophysics
• Solar, heliospheric and ionospheric  (SHI) science

In October 2019, a paper (Beardsley et al.) outlined the results from the first phase of the MWA telescope – including how 60+ science programs had logged 20,000 observing hours and produced 146 papers to date. Adding to this, the paper also highlights the second phase opportunities, further enhancing the telescope as a multi-purpose instrument – including the introduction of a rapid-response triggering system which arms the telescope with the ability to quickly observe transient events triggered from electromagnetic or multi-messenger signals.

The GLEAM survey looks at the sky from Declination of 30-degrees south – which is roughly everything south of an imaginary line drawn through the town of Eneabba, north of Perth and across South Australia and into New South Wales, intersecting roughly with Grafton.

To conduct GLEAM surveys, researchers divide the sky into seven strips in Declination, with a single strip covered in one night – accounting for roughly 8 to 10 hours of observational time. The survey further divides the observation into five separate frequencies, scanning 120 seconds in each band, allowing all frequencies to be covered over a 10-minute setting. This usually produces 108 seconds of useable data from the survey.

GLEAM’s operating frequency range is 74 – 231 MHz, with a sensitivity of 6 – 10 milli-Janskies per beam. The angular resolution is roughly 100 arcseconds and the survey covers an enormous 30,000 square degrees of the sky.

One of the key advantages to the MWA is that it can locate SNRs who are older, further away or in very empty environments – unlike the easy to spot younger-types found in denser environments.

Dr 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.

This raises an interesting scenario – did Indigenous communities who lived in Australia at the time witness a supernova event?

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.

Throughout history, supernovae events have been recorded by civilisations around the world, dating back to the earliest record in 185 C.E. – where Chinese astronomers recorded the appearance of a bright star in the sky that faded over the period of eight months.

Interestingly enough, the Vela SNR and pulsar, would have formed roughly 10,000 – 20,000 years ago, and given its location in the sky – southern hemisphere observers would have witnessed the event, although no firm documented evidence of this yet exists.

Professor Hamacher’s work has also established that Indigenous Australians did indeed observe the stars, having recorded the variability of red giant stars Betelgeuse, Aldebaran and Antares through traditional song lines and lore.

The importance of this latest research is that it tells us about the structure of the inner Galactic Plane in great detail, with a focus on the objects and events that are creating radio emissions in the region – which is often unable to be observed in optical telescopes because of the large volumes of dust within our line of site, between us and the Milky Way core.

Adding to this, the historical context of our own Galaxy is explored – especially how older, massive stars spread elements harnessed in their cores across the Milky Way providing a narration of the evolution of our region of the universe.

One wonders if the Wajarri people did look up 9,000 years ago to note the bright new star in the sky that would slowly fade in time. We look up now and see its remnant of this cataclysmic event expanding into the interstellar medium, and learn more about why and how stars explode, and what contributed to the evolution of the Galaxy we occupy.

The quiet zone is not so quiet after all. Celestial noise, galactic fireworks and a roaring Milky Way core continue to rain down data on the scorched red Earth region, cut off from the rest of the world. The Universe continues to drip information to us, building small, incremental steps of knowledge as we march forward in our advancements as a species.

And we are here to learn all of its secrets.