How do you map something you can’t see? For astronomers studying the Milky Way, the answer lies in radio waves-the very lowest frequencies that slip past the dust and gas obscuring our galaxy’s heart. Now, in a stunning technical feat that fuses massive datasets, advanced algorithms, and supercomputing power into a single panoramic view, researchers have unveiled the most sensitive, widest-area low-frequency radio map ever made of the galactic plane.
The achievement leverages two major surveys conducted with the Murchison Widefield Array-a precursor to the Square Kilometre Array-located in the radio-quiet desert of Western Australia. One-the GaLactic and Extragalactic All-sky MWA survey (GLEAM)-ran from 2013 to 2015, capturing the “big picture” of the southern sky between 72 and 231 megahertz. A follow-up, GLEAM-X, came after an upgrade in 2018 that had increased the array’s resolution and sensitivity enough to show the fine-scale detail. According to Natasha Hurley-Walker at Curtin University, “These different radio colors allow astronomers like me to disentangle the complicated astrophysics in our galaxy.”
These are very complementary datasets; GLEAM works well on very large-scale, diffuse structures, while GLEAM-X provides the resolution to detect small, bright features. In merging them, without losing either scale, the team used image domain gridding, a computational technique avoiding the expensive convolution kernels of classical gridding. Working directly in the image plane for subsets of data, it efficiently corrects direction-dependent effects-important for low frequencies with shifting distortions due to the ionosphere.
Those ionospheric distortions, induced by irregularities in Earth’s upper atmosphere, can shift the apparent position of radio sources from one night to another. The team used advanced calibration, similar to the SPAM technique, to model and remove these effects. The processing entailed matching thousands of observations across different epochs, which took over one million processing hours on the high-performance systems of the Pawsey Supercomputing Research Centre.
The resultant mosaic covers 95 percent of the Milky Way visible from the southern hemisphere, painting its galactic plane in unprecedented low-frequency detail. The broad frequency range enables “radio colour” mapping: emission from supernova remnants glows orange, brightening at lower frequencies owing to synchrotron radiation from cosmic magnetic fields, while star-forming regions shine blue due to dominated thermal emission from ionized gas. In one glance, astronomers can distinguish between the aftermath of stellar death and the cradles of stellar birth.
Technically, the MWA’s design underpins this achievement. Its 4,096 spider-like dipole antennas, grouped into tiles across several square kilometres, have no moving parts. Instead, they use electronic beamforming to survey vast swaths of sky simultaneously. Each antenna’s low-noise amplifier boosts faint cosmic signals before they are digitized and correlated, producing the raw visibilities that feed into the imaging pipeline. The array’s wide field of view makes it especially adept at capturing both extended structures and rare, transient phenomena.
The scientific payoff is huge. The sensitivity of the map opens the door to locating faint, ancient supernova remnants whose shockwaves have long since dissipated, tracing the distribution of cosmic rays, and studying the interplay of magnetic fields with interstellar matter. It also refines the census of H II regions that is, ionized gas clouds surrounding young, massive stars shedding light on the Milky Way’s current star formation rate. While the MWA and its surveys have pushed low-frequency radio astronomy to new limits, the forthcoming SKA-Low will surpass them, with thousands of times more sensitivity and finer resolution. Lessons from GLEAM and GLEAM-X on calibration, ionospheric correction, and data fusion are directly informing SKA-Low’s design and processing strategies. Until that instrument comes online, this new radio portrait stands as the definitive low-frequency view of our galaxy’s southern expanse, a synthesis of engineering innovation and astrophysical insight.
