The Milky Way’s Greatest Hits — Now in Low Frequency!
Introduction
Ever wondered what our Galaxy sounds like if you tuned your cosmic radio to the lowest frequencies? The Galactic and Extragalactic All-Sky Murchison Widefield Array Survey Extended (GLEAM-X) does exactly that — and this third release zooms in on the Galactic Plane. Using the Murchison Widefield Array (MWA) in Western Australia, astronomers combined data from two observation phases to create a map of the southern Milky Way between 72 MHz and 231 MHz.
That’s like listening to the deep bass of the Universe. The result? A panorama rich in supernova remnants, H II regions, pulsars, and faint diffuse emission. GLEAM-X III isn’t just another survey — it’s an upgrade to our Galactic sense of hearing, turning the invisible hum of the cosmos into a full-scale radio symphony.
Observations
The MWA doesn’t swing around like optical telescopes — it simply lets the sky drift overhead in what’s called drift-scan mode. GLEAM (Phase I) ran over 28 nights from 2013–2015, while GLEAM-X (Phase II) observed for 113 nights from 2018–2020.
The frequency range (72–231 MHz) was divided into five sub-bands, with each pointing lasting about two minutes per band. GLEAM-X introduced longer baselines, up to 5 km, doubling the resolution to about 45″–2′.
In other words, the telescope went from “slightly fuzzy” to “HD vision.” Combining GLEAM’s deep sensitivity with GLEAM-X’s sharper resolution gave astronomers a perfect duo: deep, wide, and beautifully detailed.
Data Reduction
Raw cosmic noise isn’t pretty. The GLEAM-X team calibrated, flagged, and imaged thousands of two-minute snapshots, corrected positions, and then stitched them together into an enormous, seamless Galactic map.
Calibration and Flagging
Before the concert, you tune your instrument — and radio astronomy is no different. Each observation was calibrated using nearby bright sources to fix amplitude and phase errors.
When no calibrator was nearby, in-field calibration using a model sky did the job. Radio Frequency Interference (RFI) was the main villain here, especially at low frequencies, so bad channels were flagged and zapped.
After this cosmic spring cleaning, dodgy snapshots were discarded, and the brightest off-axis sources were suppressed to prevent ghostly echoes from haunting the final map.
Initial Snapshot Imaging
Once the data were calibrated, every two-minute block became a quick snapshot of the sky. Using WSClean, the team created wide images down to the 10% primary beam level, tweaking weighting parameters for both sharpness and sensitivity.
These weren’t the final art pieces — just quality checks. If a snapshot looked like a melted Dalí painting due to ionospheric blurring, it got tossed. The good ones became the backbone of the full mosaic.
Astrometric Calibration
The Earth’s ionosphere loves to misbehave, bending radio waves and making stars appear to “wiggle.” To fix this, the team cross-checked source positions against trusted catalogs like SUMSS and NVSS.
Using Image-Domain Gridding (IDG), they corrected distortions directly during imaging. The result was sub-arcsecond precision — sharp enough to confidently cross-match with optical and infrared surveys.
In short, they tamed the ionosphere — a feat that would make any radio astronomer proud.
Joint Deconvolution
Here’s where the real wizardry happens. Rather than stitching images after processing, GLEAM and GLEAM-X data were deconvolved together — like mixing two audio tracks in perfect sync.
This joint deconvolution recovered both tiny point sources and sprawling diffuse structures, solving the notorious “missing short-spacing” problem in interferometry.
The process captured scales from 45″ to 15°, revealing everything from bright pulsars to ancient supernova shells.
Mosaicking
After imaging, the team faced the cosmic jigsaw puzzle: combining thousands of snapshots into massive mosaics. Each image tile was re-projected, flux-corrected, and assembled into Galactic mosaics covering ℓ = 233° → 44° and |b| < 11°.
Twenty narrow-band mosaics and one wide-band image (170–231 MHz) were created. Fluxes were rescaled to match GLEAM’s reference, and overlaps were noise-weighted to avoid seams.
The result is nothing short of breathtaking — a smooth, high-resolution radio portrait of the Milky Way, preserving both faint filaments and compact structures in one colossal frame.
Noise Levels
Background noise averaged 3–6 mJy beam⁻¹, rising near the Galactic center (because, well, it’s busy there). Even so, these are among the cleanest low-frequency maps ever produced.
Catalogue
From the wide-band images, the team extracted nearly 98,000 sources — the Milky Way’s own “radio playlist.” Over 92,000 had reliable spectra that could be modeled precisely.
Spectral Fitting
To understand how bright each source is across frequencies, astronomers fitted the flux density ( S_\nu ) using a power-law model:
\[S_\nu = S_0 \left(\frac{\nu}{\nu_0}\right)^{\alpha}\]where ( \alpha ) is the spectral index — negative for non-thermal (synchrotron) sources and near zero for thermal ones.
When the curve bent, a curved power-law was used. About 92,787 sources passed the test, each with its own radio “fingerprint” — from steep-spectrum relics to flat-spectrum nebulae.
Astrometry
GLEAM-X’s positions are accurate to ≈ 1 arcsecond, which is like measuring the width of a coin from a kilometer away — precise enough for seamless cross-matching across wavelengths.
Reliability and Completeness
How trustworthy is the catalogue? The team planted thousands of fake sources in their data to test recovery. For strong detections (≥ 7σ), false positives were under 1%.
Completeness varied slightly with Galactic noise: the crowded Galactic center was trickier, while outer regions were pristine.
In short — if GLEAM-X says it’s there, it’s really there.
Comparison with GLEAM-GP
Think of GLEAM-GP as the early demo and GLEAM-X III as the remastered version. With longer baselines and cleaner processing, the new survey drastically reduces artefacts and confusion.
Bright sources are crisp, diffuse emission pops out beautifully, and even tangled regions now make sense.
Where GLEAM once blurred structures together, GLEAM-X pulls them apart — revealing the hidden anatomy of the Galactic Plane in glorious detail.
Science Applications
Supernova Remnants
Supernova remnants (SNRs) — the ghosts of exploded stars — shine brightly in the GLEAM-X maps. Their steep synchrotron spectra make them ideal low-frequency targets.
GLEAM-X helps uncover faint, old remnants that previous surveys missed, finally addressing the “missing SNR” problem. It’s cosmic archaeology at 150 MHz.
H II Regions
If SNRs are the funerals of stars, H II regions are their birth announcements. These ionised gas bubbles around newborn stars glow via free-free emission.
GLEAM-X’s spectral range allows easy distinction between flat-spectrum H II regions and steep-spectrum synchrotron emitters. Some regions even absorb background radiation, appearing as dark cavities in the radio sky.
This helps scientists trace star formation and understand how massive stars sculpt the interstellar medium. It’s like ultrasound imaging for the Milky Way.
Planetary Nebulae
Tiny but mighty, planetary nebulae appear as flat-spectrum dots — the final whispers of dying Sun-like stars. GLEAM-X captures these with surprising clarity, enriching models of stellar evolution and ionisation dynamics.
Pulsars
GLEAM-X also picks up the faint hum of pulsars. By cross-matching with known catalogs, astronomers found dozens of detections — and potential new candidates.
Their steep spectra make them stand out, turning the Galactic map into a beacon hunt for spinning neutron stars.
Final Remarks
GLEAM-X III is the most detailed low-frequency survey of the southern Milky Way ever produced. It bridges old and new radio astronomy, setting the stage for the upcoming SKA-Low era.
If the Galaxy had an album, this would be the remastered, lossless version — bass, brilliance, and beauty included.