SKA-VLBI Promises to Revolutionize Mapping of the Milky Way Through Cosmic Masers
Featured image: Artist’s conception of the SKA-Low station array in Western Australia, with a diagram overlay showing maser sources tracing spiral arms across the Milky Way. [Credit: SKA Observatory / CSIRO]
A new study published on arXiv outlines how the Square Kilometre Array’s very long baseline interferometry capabilities, known as SKA-VLBI, will transform our understanding of the Milky Way’s structure by using cosmic masers as precision beacons. The paper, which reviews the current state of Galactic maser astrometry and projects forward to the SKA era, argues that the leap in sensitivity and sky coverage will provide the first complete, model-independent 3D map of the Galaxy.
Cosmic masers are the radio equivalent of lasers. Naturally occurring in star-forming regions and evolved stars, they amplify radio emission at specific frequencies, producing extraordinarily bright and compact sources that can be detected across the Galaxy. Because maser emission arises from regions just tens of astronomical units across, masers serve as nearly ideal point sources for astrometric measurements. When observed with VLBI techniques that link radio telescopes across continents, they yield parallax measurements accurate to a few microarcseconds.
Current maser astrometry programs, notably the Japanese VERA project and the Bar and Spiral Structure Legacy Survey (BeSSeL), have measured approximately 200 masers across the Galaxy. These observations have provided the first direct evidence that the Milky Way is a four-armed spiral, measured the distance to the Galactic Center at 8.2 kiloparsecs, and revealed the kinematic signature of the Galactic bar. But the existing sample is heavily biased toward the Northern Hemisphere. Only one maser has been measured beyond the Galactic Center, leaving the entire fourth quadrant effectively unmapped.
A tenfold increase in sample size
SKA-VLBI will change this dramatically. With its sensitivity, the SKA will be able to detect masers more than 10 times fainter than current arrays, increasing the observable sample from approximately 200 sources to more than 2,000. The SKA’s location in the Southern Hemisphere, at the Murchison Radio-astronomy Observatory in Western Australia, provides access to the Galactic Center and the fourth Galactic quadrant, which are poorly visible from northern arrays.
The leap in astrometric precision is equally significant. Current VLBI arrays achieve approximately 10 to 20 microarcseconds of parallax precision for the brightest masers. SKA-VLBI, combining the SKA’s collecting area with existing VLBI infrastructure, will push this to 1 to 3 microarcseconds. At a distance of 10 kiloparsecs, that corresponds to a distance uncertainty of roughly 5 percent, compared to 20 to 50 percent with current arrays.
This precision, the paper argues, will enable the first truly model-independent geometric reconstruction of the Milky Way’s spiral structure. The key targets are methanol masers at 6.7 and 12.2 GHz, which are exclusively associated with high-mass star-forming regions. Because massive stars form only in spiral arms, methanol masers serve as direct tracers of the Galaxy’s spiral structure. Water masers at 22 GHz and hydroxyl masers at 1.6 GHz will be accessible in later SKA phases.
Outstanding questions
Existing maps of the Milky Way suffer from fundamental ambiguities. It is still debated whether the Galaxy has two dominant spiral arms or four, an uncertainty that propagates into models of star formation, Galactic dynamics, and even the distribution of dark matter. The geometry of the Galactic bar, the nature of the mysterious 3-kiloparsec arm, and the scale length of the stellar disk all remain contested.
The paper projects that a sample of roughly 200 methanol masers would be sufficient to trace spiral arms out to 17 kiloparsecs with an arm width precision of about 0.3 kiloparsecs. For the Galactic bar, where only 15 masers currently have reliable measurements, SKA-VLBI could increase the sample to more than 200 sources, allowing the bar’s three-dimensional morphology and pattern speed to be determined with orders of magnitude better precision.
The bar’s current best-fit parameters, a semimajor axis of 4.0 kiloparsecs and an orientation angle of 36 degrees, carry substantial uncertainties. SKA-VLBI data would pin these down while simultaneously testing whether the bar has a pronounced vertical component, a question with implications for how the bar funnels gas toward the Galactic Center.
Efficiency gains
Beyond sensitivity and sky coverage, SKA-VLBI offers a dramatic improvement in observing efficiency. Current maser astrometry programs require thousands of hours of telescope time spread over multiple years to accumulate the astrometric signal. SKA-VLBI will achieve equivalent precision in roughly 300 hours per source, and its ability to observe multiple masers simultaneously within the same primary beam means that large samples can be built far more quickly.
The paper estimates that a comprehensive survey of 2,000 masers across the entire Galaxy could be completed within a few years of SKA operations, delivering a legacy data set that would serve as the reference frame for Galactic astronomy for decades.
The preprint is available on arXiv under reference 2606.27692.