GLEAM-X Radio Survey Maps Milky Way Stars Birth and Death

Low-Frequency Radio Astronomy and the Invisible Galaxy

The Murchison Widefield Array and Multi-Frequency "Radio Colors"

1. Instrumental Capabilities and Wide-Field Imaging

   The Murchison Widefield Array (MWA), located at Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-Astronomy Observatory on Wajarri Yamaji Country in Western Australia, is ideally suited for low-frequency radio surveys of the Milky Way. The MWA consists of 4,096 dipole antennas arranged across an area of approximately 1.5 × 1.5 kilometers, operating at frequencies from roughly 70 to 300 megahertz. Unlike conventional telescopes with single primary mirrors, the MWA employs aperture synthesis—combining signals from many small antennas spread across a wide area to achieve angular resolution equivalent to a much larger telescope. The instrument's wide field of view enables observation of large regions of the sky simultaneously, making surveys of extended structures like the Galactic plane feasible on reasonable timescales. Between 2013 and 2014, the MWA conducted the original GLEAM (GaLactic and Extragalactic All-sky MWA) survey, observing the entire sky south of declination +30 degrees. Between 2018 and 2020, the extended MWA configuration was used to conduct GLEAM-X observations, providing higher resolution and greater sensitivity than the original GLEAM survey. The two surveys, combined and processed, yielded the new Galactic Plane image released in 2025.

2. Multi-Wavelength "Radio Colors" and Data Processing

   The revolutionary aspect of the new GLEAM-X Galactic Plane image is its representation of the sky in "radio colors." Rather than observing at a single radio frequency, the MWA observations span multiple frequency bands across the 72-303 MHz range. The research team processed the data to create separate maps at three representative frequencies: 72-103 MHz (rendered as red), 103-134 MHz (green), and 139-170 MHz (blue). These three frequency bands are chosen to represent distinct physical processes and reveal different astrophysical sources. Low-frequency radio emission, particularly below 150 MHz, is dominantly produced by synchrotron radiation—the radiation emitted when high-energy electrons spiral in magnetic fields. This process is characteristic of supernova remnants and other energetic phenomena. By observing at multiple frequencies and analyzing how the brightness changes across frequencies, astronomers can determine the physical properties of astrophysical sources, such as their magnetic field strengths and the energies of accelerated particles. Creating the full-sky, full-frequency image required approximately 1 million CPU hours of computational processing at the Pawsey Supercomputing Research Centre. Silvia Mantovanini, the lead researcher responsible for the image processing, spent 18 months constructing the calibrated, combined dataset and producing the final visualizations. The computational challenge reflects the immense data volume involved in modern radio surveys—the GLEAM-X survey alone comprises more than 40,000 individual observations requiring over 20 million CPU-hours to process completely.

3. The Breakthrough Image: Revealing Birth and Death

   The resulting image is, in a word, stunning. The Milky Way appears as a vibrant, colorful ribbon stretching across the sky, with distinct features visible at low frequencies. Most striking is the color pattern: much of the Galactic plane appears in reddish hues, indicating prominent low-frequency radio emission. Superposed on this background are distinct features: large red circles of various sizes delineating the expanding shells of supernova remnants—the remnants of massive stars that exploded at the end of their lives billions of years ago. Smaller blue regions dot the Galactic plane, marking stellar nurseries—dense clouds of gas where new stars are actively forming and recently formed stars emit intense radiation. This visual separation of stellar death (red) and stellar birth (blue) is a distinctive feature of low-frequency radio observations, impossible to achieve at shorter wavelengths. As Silvia Mantovanini explained: "You can clearly identify remnants of exploded stars, represented by large red circles. The smaller blue regions indicate stellar nurseries where new stars are actively forming." The image thus provides an immediate, visually compelling representation of two key astrophysical processes occurring throughout the galaxy: the catastrophic death of massive stars in supernovae and the nurturing of new stellar generations in molecular clouds.

Analysis I: Supernova Remnants and Galactic Death

1. Mapping the Remnants of Dead Stars

   Supernova remnants (SNRs) represent the final gaseous echo of stellar death. When a massive star reaches the end of its life and explodes in a supernova, it ejects material at speeds reaching 10,000 kilometers per second—roughly 3% the speed of light. This expanding shell of hot, ionized gas collides with the surrounding interstellar medium, creating shock waves that accelerate particles to extreme energies and create intense magnetic fields. These energetic particles and fields produce synchrotron radiation—precisely the low-frequency radio emission that GLEAM-X observes. The expanding shells of supernova remnants appear as distinctive circular or shell-like features in radio maps, recognizable by their morphology and radio colors. The GLEAM-X Galactic Plane image contains numerous such features. Although hundreds of supernova remnants have been catalogued over decades, the team estimates thousands remain undiscovered. One reason is sensitivity: many older, fainter remnants were too faint for previous surveys to detect. Another is that many remnants have large angular sizes on the sky—they span areas of the sky comparable to several full moons. Such large structures are difficult to identify against the background Galactic radio emission using previous smaller-field-of-view instruments. The GLEAM-X survey, with its combination of wide field of view, deep sensitivity, and low-frequency coverage, is ideally suited to detect these large, faint remnants. "Although hundreds of supernova remnants have been discovered so far, astronomers suspect that thousands more are waiting to be found," the research team noted. "The discovery of these 'missing' SNRs, and hence a characterisation of the full SNR population, is crucial for understanding the production and energy density of Galactic cosmic rays and the overall energy budget of the interstellar medium."

2. Physical Properties and Age Estimates

   By measuring the brightness of supernova remnants at multiple radio frequencies, astronomers can determine physical properties. The spectral index—how the brightness changes as a function of frequency—depends on the magnetic field strength and the energy distribution of accelerated particles. By measuring spectral indices, researchers constrain the physical state of the remnant. Additionally, if a pulsar (the neutron star remnant of the supernova's progenitor massive star) can be identified within or near the supernova remnant, the pulsar's age can be used to estimate the age of the supernova remnant itself. The pulsar characteristic age, derived from the pulsar's spin-down rate, provides an estimate of the remnant's age. The GLEAM-X team identified several particularly young and interesting remnants. One candidate, designated G 0.1-9.7, appears to be younger than 10,000 years old—a remarkably young remnant in terms of galactic timescales. Such young remnants are crucial for understanding the current supernova rate in the Milky Way and the ongoing enrichment of the interstellar medium with heavy elements produced during supernova nucleosynthesis. The inventory of supernova remnants and their ages provides crucial constraints on the history of massive star explosions in our galaxy and the current rates of cosmic ray production.

3. Contribution to Galactic Cosmic Rays and Energy Budget

   Supernova remnants are believed to be the primary sources of cosmic rays—high-energy particles that constantly bombard Earth and all objects in space. The shock waves in expanding supernova remnants accelerate particles through a process called diffusive shock acceleration, potentially to extreme relativistic energies. Understanding the population of supernova remnants—their numbers, ages, sizes, and energetic properties—is essential for understanding the cosmic ray production rate and the energy density contributed by cosmic rays to the interstellar medium. The interstellar medium, the tenuous mixture of gas and dust pervading the galaxy, maintains a complex energy balance involving thermal energy (temperature), magnetic field energy, cosmic ray energy, and radiation energy. Each component contributes meaningfully to the total energy budget. Cosmic rays, accelerated by supernova remnants, carry substantial energy. By inventorying the supernova remnant population and measuring the particle energies therein, researchers constrain the cosmic ray contribution to the galactic energy budget. This has implications for understanding galactic dynamics, the heating and ionization of the interstellar medium, and the feedback effects of stellar explosions on galactic evolution.

Analysis II: Stellar Nurseries and the Birth of Stars

1. Identifying Star-Forming Regions in Radio Light

   The small blue regions in the GLEAM-X image mark stellar nurseries—dense concentrations of gas and dust where new stars are actively forming. How do young stars and star-forming regions produce low-frequency radio emission detectable by GLEAM-X? The primary source is ionized hydrogen gas. Young massive stars, once they ignite nuclear fusion, produce intense ultraviolet radiation that ionizes surrounding hydrogen atoms in nearby gas clouds. These ionized regions, called HII regions (H-two regions, denoting ionized hydrogen), emit radio radiation through free-free emission—radiation produced when free electrons interact with ionized hydrogen nuclei (protons). The free-free emission spectrum depends on the temperature and density of the ionized gas but produces characteristic emission at low radio frequencies. By identifying compact HII regions in radio maps, astronomers can locate sites of active star formation. "The smaller blue regions indicate stellar nurseries where new stars are actively forming," noted Mantovanini. These regions range from relatively small (a few light-years across) compact HII regions surrounding individual massive stars, to giant HII complexes spanning hundreds of light-years and containing multiple massive stars and clusters of younger stars. By cataloguing the HII regions detected in the GLEAM-X image, astronomers create a census of current star formation throughout the Milky Way. This census reveals where stars are being born, how frequently, and in what environments.

2. Recent Stellar Populations and Star Formation Timescales

   Young stars, particularly massive ones, are short-lived by cosmic standards. The most massive stars, burning their nuclear fuel at prodigious rates, exhaust their hydrogen cores in only a few million years—a blink of an eye compared to the galactic age of 10-13 billion years. Detecting young stars and star-forming regions in the GLEAM-X image thus reveals the galaxy's current star formation activity, the sites of recent stellar birth, and the young stellar populations that energize their surroundings. Star formation is not uniform throughout the galaxy; it concentrates in particular regions. Spiral arms, the grand spiral structure visible in external spiral galaxies like Andromeda, are regions of enhanced star formation due to the compression of gas as material orbits through the density wave. The GLEAM-X image reveals not only current star formation but also the fossilized remnants of earlier star formation—the supernova remnants of stars born millions of years ago. By studying the spatial distribution and ages of both stellar nurseries (current star formation) and supernova remnants (past star formation), astronomers can trace the history of star formation in the Milky Way and understand how it varies with location and time.

3. Implications for Stellar Population Modeling and Galactic Chemical Evolution

   The spatial distribution of star-forming regions revealed by GLEAM-X has profound implications for understanding the Milky Way's structure and chemical evolution. Massive stars, born in the young stellar nurseries visible in the radio image, rapidly evolve through their nuclear burning phases and end their lives in supernovae. These supernovae enrich the interstellar medium with heavy elements—carbon, oxygen, silicon, iron, and beyond—produced during the stars' lives through nuclear burning. The enriched gas subsequently collapses to form new generations of stars with higher heavy-element content. This process, galactic chemical evolution, has been occurring since the Milky Way's formation 10-13 billion years ago. By understanding current star formation rates and locations, and comparing to the distribution of supernova remnants (which mark where stars have died and enriched the medium), astronomers constrain models of galactic chemical evolution. The detailed spatial information provided by the GLEAM-X survey, combined with distance estimates and age determinations, will enable unprecedented tests of chemical evolution models and provide insights into how the Milky Way has evolved from a nearly metal-free system at early times to the metal-enriched, chemically diverse system we observe today.

Discussion: A Catalog of 98,000 Sources and Future Discoveries

1. The Significance of the 98,000-Source Catalog

   Beyond the visually striking image of the Milky Way, the GLEAM-X Galactic Plane survey has produced a comprehensive catalog of approximately 98,000 distinct radio sources detected across the Southern Galactic Plane. This catalog encompasses a diverse population of astrophysical objects: pulsars, the rapidly spinning neutron stars that emit beamed radiation; supernova remnants, the expanding shells of stellar explosion debris; compact HII regions, the ionized gas surrounding newly born massive stars; planetary nebulae, the glowing shells of gas ejected by evolved stars; and extragalactic sources—distant galaxies and quasars whose radio emission reaches us from across the universe. The mere existence of a catalog of nearly 100,000 sources underscores the immense richness of the radio universe and the Milky Way's complexity. Each source represents a distinct astrophysical object or phenomenon, each with its own unique properties and story. Together, the 98,000 sources provide an inventory of the radio-emitting population in our galaxy. Future analysis of this catalog will enable statistical studies of these populations—their distribution, their physical properties, and their roles in galactic structure and evolution. Such population statistics are impossible to derive from sporadic observations of individual sources; they require large, comprehensive catalogs like the one produced by GLEAM-X.

2. Pulsar Science and Radio Emission Mechanisms

   Among the 98,000 sources detected by GLEAM-X are hundreds of pulsars. Pulsars are rapidly rotating neutron stars, the ultra-dense remnants of massive star supernovae. A typical neutron star is as massive as the Sun but compressed into a sphere merely 20 kilometers in diameter—so dense that a teaspoon of neutron star material would weigh as much as a mountain. As the neutron star rotates, its magnetic field rotates with it, producing beamed pulses of radio radiation. If the beam sweeps across Earth, we observe pulses as regularly spaced as a cosmic clock. By measuring pulsar properties at different radio frequencies through multi-frequency observations like GLEAM-X, astronomers can study how pulsars produce radio waves, what physical mechanisms generate the emission, and how the emission depends on pulsar properties like spin rate and magnetic field. "By measuring the brightness of pulsars at different GLEAM-X frequencies, astronomers hope to gain a deeper understanding of how these enigmatic objects emit radio waves and where they exist within our Galaxy," the team noted. The 106 pulsar detections below 400 MHz represent first-time detections of known pulsars at these low frequencies, providing new data to constrain pulsar emission models. Additionally, GLEAM-X may have detected previously undiscovered pulsars, identified as sources with flux variations characteristic of pulsars. Future targeted observations with larger radio telescopes will confirm these discoveries.

3. Looking Forward: The Square Kilometre Array and Next-Generation Radio Astronomy

   The GLEAM-X image represents a major milestone in radio astronomy, but it is not the endpoint of progress. The next generation of radio telescopes promises even greater capabilities. The Square Kilometre Array (SKA) Observatory, currently under construction with components in Australia, South Africa, and other locations, will consist of thousands of individual antennas spread over continental distances. The SKA-Low array, to be constructed in Western Australia on Wajarri Yamaji Country, will operate at frequencies and sensitivities overlapping with MWA. Yet the SKA will achieve sensitivity and resolution dramatically surpassing the MWA. "Only the world's largest radio telescope, the SKA Observatory's SKA-Low telescope, set to be completed in the next decade on Wajarri Yamaji Country in Western Australia, will have the capacity to surpass this image in terms of sensitivity and resolution," concluded Associate Professor Hurley-Walker. The SKA will enable detection of fainter, smaller sources, and will provide a 10-year window in the next decade to observe dramatic changes in galactic dynamics, star formation, and stellar evolution. The GLEAM-X survey, by pioneering techniques of large-scale low-frequency radio surveys and demonstrating the scientific value of comprehensive catalogs, has laid crucial groundwork for the SKA era. The lessons learned in processing GLEAM-X data—1 million CPU hours of computation for a single image—inform planning for the dramatically larger data volumes that the SKA will produce. The GLEAM-X image is thus both a spectacular achievement in itself and a stepping stone toward even more ambitious future observations of the Milky Way and the universe beyond.

Conclusion: The Living Radio Galaxy